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11: Secondary Growth - Biology

11: Secondary Growth - Biology


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Learning Objectives

Content Objectives

  • Learn the sequence of development of meristems and tissues in secondary growth
  • Understand the tradeoffs (costs and benefits) of secondary growth

Skill Objectives

  • Identify anatomical features of woody stems
  • Determine the age of a tree based on annual growth rings
  • 11.1: Formative Questions
  • 11.2: Introduction
    In secondary growth, primary tissues and residual meristematic tissues produce secondary meristems, which then produce secondary tissues. Whereas primary tissues allow for vertical growth, secondary tissues allow for lateral growth: they allow stems and roots to become wider.
  • 11.3: Secondary Tissues in the Root
    In roots, the formation of both secondary meristems involves the pericycle. The pericycle and some residual procambium join together to form the vascular cambium, a secondary meristem that produces vascular tissue. The other secondary meristem, the cork cambium, is initially formed solely from the pericycle. Each of these secondary meristems divides in two directions to form a different secondary tissue to the inside and outside of the meristematic layer, respective to the center of the plant.
  • 11.4: Flowcharts of Development
  • 11.5: Secondary Tissues in the Shoot
    Though the secondary tissues are all the same, the sequence of secondary meristem development in shoots is a bit different than in roots. Shoots have no pericycle, so the secondary meristems must be formed from different tissues. In shoots, the vascular cambium is formed from residual procambium within the vascular bundles (fascicular cambium) joined by tissue in the pith rays (interfascicular cambium).
  • 11.6: Dendrochronology
    Dendrochronology is a process used to determine the order and timing (chronology) of events using information in tree rings (dendro- refers to trees). These rings form in response to environmental conditions, storing information on climate and historic occurrence of fires in an area. Essentially, you bore a hole into a living tree and extract a thin section column of the wood. This becomes your reference specimen, as you date each annual ring, tracing back from the present date.
  • 11.7: Summative Questions

Difference between Dicot Root and Monocot Root | Plants

2. The epiblema, the cortex and even the endodermis are peeled off and replaced by cork.

3. Older root has a covering of cork.

4. Endodermis is less thickened and casparian strips are more prominent.

5. Passage cells are generally absent in endodermis.

6. Pericycle produces lateral roots, cork cambium and part of the vascular cambium.

7. The number of xylem and phloem bundles varies from 2-5 or sometimes 8.

8. Xylem vessels are generally angular.

9. Conjunctive tissue is parenchymatous.

10. Conjunctive parenchyma forms the cambium.

11. Secondary growth takes place with the help of vascular cambium and cork cambium.

12. Pith is either absent or very small.

Difference # Monocot Root:

2. Cork is not formed. The cortex and the endodermis persist. Only the epiblema is peeled off.

3. Older root has a covering of exodermis.

4. Casparian strips are visible only in young root. The endodermal cells later become highly thickened.

5. Thin walled passage cells generally occur in the endodermis opposite the protoxylem point.


Chapter 11 - Secondary xylem

Most of the major taxa of vascular plants produce secondary xylem derived from the vascular cambium. Pteridophytes (except some extinct taxa), most monocotyledons, and a few species of largely aquatic dicotyledons, however, produce only primary vascular tissues. In woody plants secondary xylem comprises the bulk of the tissue in the stems and roots. It is the most important supporting tissue in arborescent dicotyledons and most gymnosperms, and the major tissue for the transport of water and essential minerals in woody plants. Secondary xylem is a complex tissue that consists not only of non-living supporting and conducting cells but also of important living components (rays and axial wood parenchyma) which, with those in the secondary phloem, comprise a three-dimensional symplastic pathway through which photosynthate and other essential molecular substances are transported thoughout the secondary tissues of the plant (Chaffey and Barlow, 2001 see pp. 206–207 for more detail). Additional increments of this tissue are added during each growing season (usually annually), but in older regions of most woody species only the outer increments are functional in transport although the number of increments that remain functional varies greatly among different species. Older increments gradually become plugged by the deposition in them of waste metabolites such as resins, tannins, and in some species by the formation of tyloses (balloon-like extensions of axial or ray parenchyma cells into adjacent conducting cells). The inner non-functional secondary xylem is called heartwood, the outer functional secondary xylem, sapwood.


Anatomy of Anomalous Dicot Stems | Botany

In this article we will discuss about the anatomy of various anomalous dicot stems: 1. Bougainvillea – Stem 2. Salvadora – Stem 3. Achyranthes- Stem 4. Chenopodium – Stem 5. Boerhaavia – Stem 6. Leptadenia – Stem 7. Mirabilis – Stem 8. Amaranthus – Stem 9. Nyctanthes – Stem 10. Bignonia – Stem.

1. Anatomy of Bougainvillea – Stem: A Dicot with Successive Rings of Bundles (Family – Nyctaginaceae):

It is circular in outline and exhibits following tissues from outside with in:

1. Single layered epidermis consists of compactly arranged thin walled cells and is covered by thick cuticle.

2. It bears many multicellular hairs when young.

3. In the stem showing secondary growth, cork cambium is present which cuts cork towards outer side and secondary cortex towards inner side.

4. It consists of collenchyma, parenchyma and endodermis.

5. Collenchyma in young stems is present in patches but in old stems it remains in the form of a continuous ring of few layers, present just below the epidermis.

6. Next to the collenchyma is situated the region of parenchyma consisting of many thin walled, oval to spherical cells with intercellular spaces. Sometimes they develop chlorophyll.

7. Endodermis is the innermost layer of the cortex the cells of which are barrel shaped and contain starch grains. In old stems, however, it is not a conspicuous layer.

8. It is represented by thick, sclerenchymatous stone cells forming a discontinuous layer.

9. It consists of phloem and xylem.

10. In the young stem are present many vascular bundles arranged in ring. Bundles are conjoint, collateral, open and endarch. Many medullary bundles are also present.

11. The old stem shows secondary growth.

12. Just below the pericycle are present the patches of primary phloem.

13. Secondary phloem is present inner to the primary phloem.

14. Phloem consists of sieve tubes, companion cells and phloem parenchyma. Phloem fibres are absent.

15. Cambium is present in between secondary phloem and secondary xylem.

16. Secondary xylem, which forms the major part of section, consists of tracheids, vessels, fibres and prosenchyma.

17. Primary xylem is present near the pith facing its protoxylem towards the centre of stem.

18. Many groups of secondary phloem are embedded in the region of secondary xylem and called interxylary phloem or included phloem.

19. Many conjoint, collateral, open and endarch bundles are present in the pith. These are called medullary bundles.

20. It is parenchymatous and the cells are rounded with intercellular spaces.

Anomalous secondary growth is due to the formation of successive rings of collateral vascular bundles. These bundles get embedded in the thick prosenchyma and their phloem appears as included or interxylary phloem. Medullary bundles are innermost secondary bundles.

(a) 1. Presence of vessels in the xylem. (Angiosperms)

(b) 1. Vascular bundles are conjoint, collateral, open and endarch. (Stem)

(c) 1. Multicellular epidermal hairs.

2. Vascular bundles are arranged in ring.

3. Presence of cambium. (Dicot)

Interxylary phloem and medullary bundles are present.

2. Anatomy of Salvadora – Stem (Family – Salvadorace):

T.S. reveals the following tissues from outside within:

1. It is the outermost layer with barrel shaped cells. Cells are covered by a thick cuticle.

2. It consists of parenchymatous hypodermis, few layers of chlorenchyma and an innermost layer of endodermis.

3. Hypodermis is generally thin walled, parenchymatous, but sometimes 2-3 layers of collenchyma are seen.

4. Chlorenchyma is present inner to the hypodermis. It is 3-5 cells deep and cells are filled with chloroplasts.

5. Few layers of parenchyma are also present below the chlorenchyma. The cells contain intercellular spaces.

6. Endodermis is the innermost layer of cortex made up of barrel shaped cells which contain starch grains.

7. It is a discontinuous layer present in the form of patches consisting of many widely spaced strands of thick walled fibres.

8. Vascular bundles are conjoint, collateral, open and endarch.

9. Vascular system composed of primary phloem, secondary phloem, cambium, secondary xylem, primary xylem and included phloem.

10. Primary phloem is crushed and found in the form of patches.

11. Secondary phloem is present just outside the cambium in the form of a ring.

12. Cambium strip consists of rectangular cells arranged in radial rows.

13. Secondary xylem forms a complete cylinder. It is represented by wide vessels and xylem parenchyma.

14. Many medullary rays traverse the secondary xylem.

15. Wide vessels of metaxylem and narrow protoxylem vessels can be observed in the primary xylem present near the pith.

16. Secondary xylem is interrupted by many groups of thin walled phloem representing included phloem, interxylary phloem or phloem islands.

17. It is well developed, thin-walled, parenchymatous and present in the centre.

How Does Included Phloem Develop?

Included phloem develops due to the irregular activity of cambium. Some cambium cells which arc normally responsible for the formation of secondary xylem form secondary phloem. Other adjacent cambium ceils keep on normally producing secondary xylem. After sometime the same abnormally behaving cambium cells start to behave normally, and thus many islands of phloem are formed.

(a) 1. Presence of vessels in the xylem. (Angiosperms)

(b) 1. Vascular bundles are conjoint, collateral, open and endarch. (Stem)

(c) 1. Vascular bundles in ring.

2. Presence of secondary growth.

3. Well developed pith. (Dicot)

Formation of included phloem abnormality of Salvadora.

3. Anatomy of Achyranthes- Stem (Family – Amaranthaceae):

T.S. is wavy in outline with ridges and furrows, and reveals the following tissues from outside with­in:

1. Single – layered epidermis consists of many tubular cells and covered externally by thick cuticle.

2. From some cells arise multicellular hairs.

3. It is well differentiated into collenchyma, chlorenchyma, parenchyma and endodermis.

4. Collenchyma is present only below the ridges and its breadth and depth varies below different ridges.

5. Chlorenchyma is present below the furrows.

6. Parenchymatous cortex is located below collenchyma and chlorenchyma. It is two to three layers deep and cells contain intercellular spaces.

7. Endodermis is the innermost layer of cortex, consisting of elongated cells which lack casparian thickenings. In the old stem endodermis is not clear.

8. It is represented by groups of sclerenchymatous cells situated just outside the vascular tissue.

9. In the young stem, vascular bundles are conjoint, collateral, open and endarch but old stems show secondary growth.

10. In the old stem, the vascular system consists of primary phloem, secondary phloem, cambium, secondary xylem, conjunctive tissue, included phloem and primary xylem

11. Primary phloem is crushed and obliterated.

12. Secondary phloem is present in the form of a complete ring and Consists of sieve tubes, companion cells and phloem parenchyma.

13. A cambial strip is present in between secondary xylem and secondary phloem.

14. Secondary xylem and conjunctive tissue are undistinguishable. Large xylem vessels are very clear in thick walled conjunctive tissue.

15. In the conjunctive tissue (prosenchyma) are present groups of included phloem or interxylary phloem.

16. Primary xylem is present near the pith.

17. It is well developed and parenchymatous.

18. Two medullary bundles are present in pith facing their xylem to each other. They are conjoint, collateral, open and endarch.

Secondary Growth and Medullary Bundles:

In the pericycle region, extrastelar cambium strips develop which produce secondary vascular bundles. Cambium produces the conjunctive tissue. Secondary vascular bundles and conjunctive tissue are present without any sharp limits. So phloem of the secondary vascular bundles appears in the form of patches. This phloem is the included phloem. Medullary bundles are leaf traces.

(a) 1. Presence of vessels in the xylem. (Angiosperms)

(b) 1. Vascular bundles are conjoint, collateral, open and endarch (Stem)

(c) 1. Vascular bundles in ring.

2. Presence of secondary growth.

3. Well-developed pith. (Dicot)

4. Anatomy of Chenopodium – Stem (Family-Chenopodiaceae):

T. S. is rounded in outline and reveals the following tissues from outside with -in:

1. It is composed of a single layer of tangentially elongated cells.

2. A thick cuticle is present on the outer walls.

3. It is well-differentiated and composed of collenchyma and parenchyma. Sometimes a few- layered chlorenchyma is also present in between.

4. Collenchyma is present below epidermis in the form of 3-4 layers.

5. Parenchyma is present inner to collenchyma. It is well developed and contains many intercellular spaces.

6. Endodermis delimits cortex from vascular region. The cells are barrel shaped, filled with starch grains and lack casparian strips.

7. Next to the endodermis is present the parenchymatous pericycle.

8. It consists of well developed conjunctive tissue and secondary vascular bundles, the phloem of which appears like included phloem.

9. Conjunctive tissue is very well developed and consists of thick and lignified cells. It is in very close association-ship with xylem.

10. Vascular bundles are present in a ring and remain embedded in the conjunctive, tissue.

11. These vascular- bundles are formed by extrastelar cambium and thus secondary in origin.

12. Each vascular bundle is conjoint, collateral and endarch.

13. The phloem of these bundles remains embedded in the conjunctive tissue giving the appearance of included phloem or interxylary phloem.

14. It is well developed, and consists of thin walled, polygonal parenchymatous cells with large intercellular spaces.

15. In the pith are embedded many conjoint, collateral, open and endarch vascular bundles.

16. These bundles are primary vascular bundles of the stem and not the leaf-trace bundles.

Secondary growth in Chenopodium is abnormal and takes place by the formation of extrastelar cambium in the pericyclc. The first ring of the cambium is a continuous ring. It first produces a small amount of thin walled ground tissue which pushes the primary vascular bundles towards centre which now appear as medullary bundles. This cambium later on produces secondary vascular bundles and conjunctive tissue which is thick walled.

Such cambial rings are produced in the pericycle successively, and the ring external to the phloem burries the latter in the conjunctive tissue. This phloem gives the appearance of included phloem or interxylary phloem.

(a) 1. Presence of vessels in the xylem. (Angiosperms)

(b) 1. Vascular bundles are conjoint, collateral open and endarch.

2. Cortex is well-differentiated. (Stem)

(c) 1. Vascular bundles are arranged in ring.

2. Well-developed secondary growth and well defined pith. (Dicotyledones)

5. Anatomy of Boerhaavia – Stem (Family – Nyctaginaceae):

T.S. is circular in outline and reveals the following tissues from outside within:

1. Single layered epidermis consists of small, radially elongated cells.

2. Multicellular epidermal hairs arise from some cells.

3. A thick cuticle is present on the epidermis.

4. Some stomata are also present.

5. It is well differentiated and consists of few layered collenchymatous hypodermis followed by chlorenchyma.

6. Collenchyma is 3 to 4 cells deep, but generally near stomata it is only one layered.

7. Chlorenchyma is present inner to collenchyma in the form of 3 to 7 layers.

8. Chlorenchymatous cells are thin walled, oval, full of chloroplasts and enclose many intercellular spaces.

9. Endodermis is clearly developed and made up of many, tubular, thick-walled cells.

10. Inner to the endodermis is present parenchymatous pericycle but at some places it is represented by isolated patches of sclerenchyma.

11. Vascular bundles are present in three rings. In the innermost ring are present two large bundles in the middle ring the number ranges from 6 to 14 while the outermost ring consists of 15 to 20 vascular bundles.

12. Vascular bundles of innermost and middle rings are medullary bundles.

13. Vascular bundles are conjoint, collateral and endarch.

14. Two vascular bundles of the innermost ring arc large, oval and lie opposite to each other with their xylem facing towards centre and phloem outwards.

15. Middle ring consists of 6-14 small vascular bundles.

16. Vascular bundles of inner and middle rings may show a little secondary growth.

17. Phloem consists of sieve tubes, companion cells and phloem parenchyma while the xylem consists of vessels, tracheids and xylem parenchyma.

18. Outermost ring of the vascular bundles contain inter-fascicular cambium which is absent in other two rings.

19. Cambium develops secondarily from the pericycle and becomes active. It cuts secondary phloem towards outer side and secondary xylem towards inner side. Due to these changes the primary phloem becomes crushed and present next to pericycle. Primary xylem is situated near the pith.

20. Interfascicular cambium also soon becomes active and cuts internally the row of cells which become thick walled and lignified and are known as conjunctive tissue.

21. It is well developed, parenchymatous and present in the centre.

(a) 1. Presence of vessel in the xylem. (Angiosperms)

(b) 1. Cortex is well-differentiated.

2. Vascular bundles are conjoint, collateral, open and endarch. (Stem)

(c) 1. Vascular bundles are present in ring.

2. Well-developed secondary growth.

3. Well-defined pith. (Dicotyledones)

6. Anatomy of Leptadenia – Stem (Family – Asclepiadaceae):

It is circular in outline and reveals the following tissues from outside with-in:

1. Outermost, single-layered epidermis consists of many barrel-shaped cells arranged compactly.

2. The cells are covered externally by thick cuticle.

3. It consists of hypodermis, chlorenchyma and endodermis.

4. Hypodermis follows epidermis and consists of thin walled, parenchymatous cells arranged in one to three layers.

5. Chlorenchymatous layers (3 to 6 or more) are present inner to the hypodermis. The cells are filled with chloroplasts and show many intercellular spaces.

6. Endodermis is the innermost layer of cortex consisting of barrel shaped cells, filled with starch grains. The cells lack characteristic casparian strips.

7. A big zone of pericycle is present below the endodermis consisting of parenchymatous cells.

8. It is interrupted by the sclerenchymatous patches at certain intervals.

9. It consists of primary phloem, secondary phloem, cambium, secondary xylem, interxylary phloem, primary xylem and intraxylary phloem.

10. A ring of vascular bundles is present in the primary state which are conjoint, bicollateral, open and endarch.

11. Primary phloem is present in patches at certain places.

12. Secondary phloem ring is present inner to pericycle and consists of sieve tubes, companion cells and phloem parenchyma with no phloem fibre.

13. Cambium consists of thin-walled, brick shaped, actively dividing cells arranged in one layer but later on new cells are cut off and it becomes multilayered.

14. Secondary xylem zone consists of xylem vessels, tracheids and xylem parenchyma. The zone is traversed by many secondary medullary rays.

15. Many patches of interxylary phloem or included phloem are present in the secondary xylem cylinder. Their development is centripetal.

16. Primary xylem consists of protoxylem and metaxylem. The protoxylem is endarch and present near the pith.

17. Intraxylary phloem is present in the form of patches at the periphery of the pith.

18. It is thin walled and parenchymatous.

Abnormal Secondary Growth:

Due to the irregular activity of the cambium at certain places, the secondary phloem is formed towards inner side instead of secondary xylem. Other adjacent cambium cells are normally producing secondary xylem towards inner side.

After some time the cambium resumes its normal activity and thus forms many patches of secondary phloem in the secondary xylem. These are called interxylary phloem or included phloem patches Internal or intraxylary phloem is the primary structure of the primary bicollateral bundles.

(a) 1. Presence of vessels in the xylem.

2. Vessels have perforated end walls with scalariform and regularly arranged holes. (Angiosperms)

(b) 1. Conjoint, collateral, open and endarch vascular bundles. (Stem)

(c) 1. Vascular bundles in a ring.

2. Presence of cambium. (Dicotyledones)

7. Anatomy of Mirabilis – Stem (Family – Nyctaginaceae):

It is rectangular in outline and reveals the following tissues from outside within:

1. Single-layered epidermis consists of compact, rectangular cells, the outer face of which is covered by a thin cuticle.

2. From some cells arise multicellular hairs.

3. A broad cortical region is present below the epidermis consisting of collenchyma, parenchyma and endodermis.

4. Collenchymatous cells are thickened at their corners and oval or polygonal in shape. The region is 3-5 cells deep.

5. Parenchyma is present below the collenchyma, the cells of which are spherical or polygonal in shape with many intercellular spaces.

6. Parenchymatous region is three to many cells deep and cells are generally filled with many chloroplasts.

7. Endodermis is the innermost layer of the cortex consisting of barrel shaped cells.

8. Endodeimal cells lack casparian strips and filled with starch grains.

9. It is represented by few parenchymatous layers below the endodermis.

10. It is composed of primary phloem, secondary phloem, cambium, secondary xylem, conjunctive tissue and primary xylem. Many medullary bundles are also present

11. Vascular bundles are conjoint, collateral, open and endarch.

12. Primary phloem is crushed and occurs in patches.

13. A complete ring of few cells thick secondary phloem occurs just outside the cambium.

14. Cambium is present in the form of a strip in between the xylem and phloem.

15. Secondary xylem is present inner to the cambium, and consists of tracheids, vessels, fibres and xylem parenchyma

16. A continuous cylinder of conjunctive tissue (prosenchyma) is present which is composed of thick and lignified cells.

17. Near the pith are situated groups of primary xylem just opposite to the location of primary phloem.

18. Many medullary bundles are scattered in the pith which are ‘leaf traces’.

19. Medullary bundles in the pith, present towards its periphery, are smaller while that of the central position are larger in size.

20. It is parenchymatous. The cells are oval with many intercellular spaces.

Anomalous secondary growth occurs in the form of succession of rings of vascular bundles. De Bary (1884) was of the opinion that a de novo, extrastelar cambium ring arises in the pericycle but according to Mahcahwari (1930) separate strips of interfascicular cambium develop in the medullary rays between the outer rings of normal bundles. It forms a complete cambium ring by joining with the strips of fascicular cambium.

(a) 1. Presence of vessels in the xylem. (Angiosperms)

(b) 1. Presence of well developed cambium and multicellular hairs.

2. Vascular bundles are conjoint, collateral, open and endarch. (Stem)

(c) 1. Vascular bundles in ring.

2. Presence of secondary growth. (Dicotyledones)

8. Anatomy of Amaranthus – Stem (Family – Amaranthaceae):

Outline of T. S. shows many ridges and furrows, and reveals the following tissues from out side with in:

1. It consists of single layer of barrel shaped cells covered externally by thick cuticle.

2. Lateral and inner walls are thin.

3. It is well differentiated into collenchyma and chlorenchyma.

4. Collenchyma is present just below the epidermis. It is more prominent below ridges. Comers of the cells are thick and the cells are-oval or polygonal in shape.

5. Chlorenchyma is present inner to collenchyma. Thin walled cells are spherical to oval in shape, filled with chloroplasts and contain many intercellular spaces.

6. Endodermis is poorly developed and sometimes absent. The cells are elongated and lack casparian strips.

7. It consists of few layers of thin walled, compactly arranged cells. It becomes sclerenchymatous in older stems.

8. Vascular system consists of primary phloem, secondary phloem, cambium, secondary xylem and primary xylem. Many medullary bundles are present in pith.

9. Vascular bundles are conjoint, collateral, open and endarch.

10. Primary phloem is crushed and present in patches.

11. Secondary phloem is present in the form of a complete ring which consists of sieve tubes, companion cells and phloem parenchyma.

12. Cambium is distinct and present in one to many layers located in between phloem and xylem.

13. Secondary xylem remains embedded in conjunctive tissue and consists of proto-and metaxylem vessels and abundant parenchyma.

14. Conjunctive tissue is present in abundance and consists of thick walled and lignified cells.

15. Primary xyslem is present near the pith facing its protoxylem towards centre.

16. Many scattered medullary bundles are present in the pith.

17. Each medullary bundle is conjoint, collateral and endarch, with the cambium either feebly developed and functionless or absent.

18. It is parenchymatous and cells show some intercellular spaces.

In the pericycle region the outer primary bundles become meristematic and develop few layered cambium. This cambium cuts collateral vascular bundles towards inner side consisting of secondary phloem and secondary xylem. Cambium also cuts many layered parenchymatous conjunctive tissue which becomes lignified and thick walled. This vascular bundle lies completely embedded in conjunctive tissue.

(a) 1. Presence of vessels in the xylem. (Angiosperms)

(b) 1. Cortex is well differentiated.

2. Vascular bundles are conjoint, collateral, open and endarch. (Stem)

(c) 1. Vascular bundles are present in a ring.

2. Well developed secondary growth. (Dicotyledones)

9. Anatomy of Nyctanthes – Stem (Family – Oleaceae):

The outline of T.S. appears quadrangular and reveals the following tissues from outside with-in:

1. Single-layered epidermis consists of rectangular cells.

2. A thick uninterrupted cuticle is present on the epidermis.

3. Many multicellular hairs are present.

4. It is differentiated into collenchyma and parenchyma.

5. Collenchyma is several cells deep below the four protruded comers while only few layers deep at the other places just beneath the epidermis.

6. Parenchyma is present below the collenc­hyma. Many intercellular spaces are present. The region extends upto the vascular tissue.

7. Four vascular bundles are present in the cortex, situated one each in each protruded bulge.

8. Each conical bundle faces its pointed xylem end towards outer side, i.e., epidermis, and is conjoint, collateral, open and exarch.

9. These bundles may show secondary growth at maturity.

11. It is in the form of sclerenchymatous patches.

12. It consists of primary phloem, secondary phloem, cambium, secondary xylem and primary xylem.

13. Primary phloem is crushed and irregularly present in patches below pericycle.

14. Secondary phloem is present in the form of a continuous ring and consists of sieve tubes, companion cells and phloem parenchyma

15. Cambium is one to three cells thick continuous layer present in between phloem and xylem.

16. Secondary xylem is present just inner to the cambial ring and consists mainly of thick walled wood parenchyma and fibres. Tracheids and vessels are also present

17. Primary xylem is situated just near the pith facing its protoxylem towards the centre.

18. It is thin walled and parenchymatous.

Abnormality in Nyctanthes ii the presence of cortical bundles, which are inversely oriented, 4 in number and never directly connected with the main axial ring of the vascular cylinder. These are leaf trace bundles.

Cortical bundles have also been reported in some other families such as Casuarinaceae (Casuarina), Umbelliferae (Eryngium), Papilionaccae (Latkyrus marytimus). Mclastomaccac, Rutaccae, etc.

(a) 1. Presence of vessels in the xylem. (Angiosperms)

1. Presence of multicellular hairs.

2. Vascular bundles are conjoint, collateral, open and endarch. (Stem)

(c) 1. Vascular bundles in ring.

2. Presence of cambium. (Dicotyledones)

10. Anatomy of Bignonia – Stem (Family – Bignoniaceae):

T.S. shows many ridges and furrows and reveals the following tissues from outside with-in:

1. Single-layered epidermis consists of rectangular cells.

2. A thick cuticle is present.

3. A few multicellidar hairs are also arising from some cells.

4. Ii is well-differentiated into collenchyma and parenchyma.

5. Collenchyma is present below the epidermis in the ridges in young stem but at maturity there develops sclerenchyma.

6. Parenchyma is present below the sclerenchyma or collenchyma in the ridges and directly below the epidermis in the grooves.

7. In old stem cortex consists of cork, cork cambium and cortex.

8. Endodermis is undistinguishable from cortical cells. The cells lack casparian strips.

9. It is in sclerenchymatous patches.

10. It phloem, secondary phloem, xylem and primary xylem.

11. Four longitudinal furrows of secondary phloem arc present which are wedged in between the secondary xylem cylinder.

12. Vascular bundles are conjoint, collateral, open and endarch.

13. Primary phloem is crushed and present in small patches.

14. Secondary phloem is in the form of a ring which remains intruded into the secondary xylem at four places.

15. Intruded furrows (four) of secondary phloem are arranged in the form of a cross.

16. In Bignonia unguis-catae, bars of sclerenchyma are present in the furrows of secondary phloem.

17. Cambium is single layered, present in between xylem and phloem and bent towards inner side along the furrows of secondary phloem.

18. Secondary xylem consists of vessels, tracheids, fibres and xylem parenchyma.

19. Due to the intrusion of the phloem at four places, secondary xylem is ridged and furrowed at four places.

20. Primary xylem is present close to the pith facing its protoxylem towards the centre. Its location is just opposite to the patches of primary phloem.

21. It is thin walled and parenchymatous.

Formation of four furrows of secondary phloem in the secondary xylem is due to the abnormal functioning of cambium which was behaving normally sometimes earlier.

At four or more places cambium produces less amount of secondary xylem towards inter side and large amount of secondary phloem towards outer side. Thus four wedges of secondary phloem are formed. They intrude into the secondary xylem and so xylem cylinder appears ridged and furrowed.


Secondary Growth in Gymnosperms

Certain Gymnosperms show typical secondary growth. They are tall, perennial, woody genera, like Pinus. Typically, the secondary wood in most Gymnosperms are manoxylic (non-compact wood with a large amount of parenchyma, large pith and the cortex mixed with less tracheids) or pycnoxylic (wood with a large amount of xylem tracheids or wood and a small amount of cortex and pith with little parenchyma).

  1. The vascular cambium contributes more secondary xylem than secondary phloem. Xylem is typically devoid of vessels. There are distinct late wood and early wood formations contributing to the development of prominent annual rings. An annual growth ring is composed of the early wood and late wood formed in a single year.
  2. The periderm layer is also significantly prominent. The cork cells may be empty and highly suberised. The cork cambium, which may be one or two cells thick, with prominent nuclei, cut off phelloderm or secondary cortex below them.

Fig: Secondary Growth in Gymnosperms.


Anatomy of Flowering Plants

Types of meristems
The meristems may be classified on the basis of their mode of origin, position etc.:
(i) According to origin and development : On the basis of origin, meristematic tissues are of three types:
(a) Promeristem or Primordial meristem : The promeristem originates from embryo and therefore, called primordial or embryonic meristem. It is present in the regions where an organ or a part of plant body is initiated. A group of initial cells that lay down the foundation of an organ or a plant part, is called promeristem. This group consists of limited number of cells, which divide repeatedly to give rise to primary meristem. The promeristem gives rise to all other meristems including the primary meristem.
(b) Primary Meristem: A primary meristem originates from promeristem and retains its meristematic activity. It is located in the apices of roots, stems and the leaf primordial, e.g., Apical meristem and Intercalary meristem.
(c) Secondary Meristem: The secondary meristems are so called because they originate from permanent cells. E.g., phellogen or cork cambium, is an important example of secondary meristem. The secondary meristems produce secondary tissues in the plant body and add new cells for effective protection and repair. They occur in mature regions of roots and shoots.

(ii) According to position : On the basis of their position in the plant body meristems are classified into three categories :

Various meristamatic tissue
(a) Apical meristem : This meristem is located at the growing apices of main, as well as lateral shoots and roots. These cells are responsible for linear growth of an organ. Position of apical cells may either be strictly terminal or subterminal.
(b) Intercalary meristem : These are the portions of apical meristems which are separated from the apex during the growth of axis and formation of permanent tissues. (e.g., Mint), base of internode (e.g., stem of many monocots viz., Wheat, Grasses, Pteridophyets like Equisetum). It occur between mature tissues.
(c) Lateral meristem : These meristems occur laterally in the axis, parallel to the sides of stems and roots. The cambium of vascular bundles (Fascicular, interfascicular and extrastelar cambium) and the cork cambium or phellogen belong to this category and are found in dicotyledons and gymnosperms resulting in an increase in their diameter.

Differences between apical and lateral meristems

S.No. Apical meristem Lateral meristem
1. It occurs at the apex of stem, root and their branches. It is found in lateral position parallel to circumference of the organs.
2. It is a primary meristem. It is secondary meristem, except intra-fascicular cambium.
3. Cells divide in different planes. Cells divide in one plane patricianly both on the outer and inner sides.
4. It produces primary tissues. It gives rise to secondary tissues.
5. It brings about growth in length. It causes growth in girth and thickness.

Structure and Organisation of Apical Meristem
(i) Vegetative shoot apex: Shoot apex was first recognized by Wolff (1759). Shoot apex is derived from meristem present in the plumule of embryo and occurs at the tip of stem and its branches as terminal bud. It also occurs in the inactive state in the axils of leaves as lateral buds. The tip of the shoot apex is dome-shaped and from it flanks, at the base of the dome, one or more leaf primordia. This continues throughout the vegetative phase. Many theories have been put forward to explain shoot apex, such as :

(a) Apical Cell theory: This theory was proposed by Nageli (1858). According to this theory, shoot apical meristem consists of single apical cell. This theory is applicable in case of higher algae, bryophytes and in many pteridophytes but not in higher plants (i.e., gymnosperms and angiosperms).

(b) Histogen theory: It was proposed by Hanstein (1870). According to this theory, the shoot apical meristem consists of three distinct meristematic zones or layers (or histogens).

  • Dermatogen: Outermost layer and it forms epidermis and epidermal tissue system.
  • Periblem: It is the middle layer that gives rise to cortex and endodermis.
  • Plerome: Innermost layer forms pith and stele.

(c) Tunica corpus theory: This theory was proposed by Schmidt (1924). According to this theory, the shoot apex consists of two distinct zones.

  • Tunica: It is mostly single layered and forms epidermis. The cells of tunica are smaller than corpus. The tunica shows only anticlinal division and is responsible for surface growth.
  • Corpus: It represents the central core with larger cells. Corpus shows divisions in all planes and it is responsible for volume growth.

(ii) Root apex: A group of initial cells, present at the subterminal region of the growing root tip, which is protected by a root cap is called root apical meristem or root apex. It is embryonic in origin and formed from the radicle part of embryo. However, in adventitious roots it is produced from derivatives of root apex. The root apex differs from shoot apex as it is short and more or less uniform due to complete absence of lateral appendages (leaves and branches) and differentiation of nodes and internodes. According to Hanstein (1870) root apex of most of the dicotyledons also consists of three meristematic zones – plerome, periblem and dermatogen (fourth meristem calyptrogen to form root cap). Regarding the apical organisation of root following theories have been put forward.
(a) Korper -Kappe theory: It was proposed by Schuepp (1917). This theory is comparable with the tunica and corpus theory of shoot apex. Korper means body and Kappe means cap.

(b) Quiescent centre theory: It was proposed by Clowes (1961). According to him, in addition to actively dividing cells, a zone of inactive cells are present in the central part of the root apex, called quiescent centre. The cells in this region have light cytoplasm, small nuclei, lower concentration of DNA, RNA and protein. These cells also contain fewer number of mitochondria, less endoplasmic reticulum and small dictyosomes.

Permanent Tissues
Permanent tissues are made up of mature cells which develops due to division and differentiation in meristematic tissues. The cells of these tissues are either living or dead, thin-walled or thick-walled. Permanent tissues are of three types :

PERMANENT TISSUES

2. SIMPLE TISSUES
Simple tissues are a group of cells which are all alike in origin, form and function. They are further grouped under three categories :
Parenchyma
Parenchyma is most simple and unspecialized tissue which is concerned mainly with the vegetative activities of the plant. The main characteristics of parenchyma cells are :
(a) The cells are thin-walled and soft.

Parenchyma in T.S.
(b) The cells usually are living and possess a distinct nucleus.
(c) The cells contain well-developed intercellular spaces amongst them.
(d) The cytoplasm is vacuolated and cell wall is made up of cellulose.
(e) The shape may be oval, spherical, cylindrical, rectangular or stellate (star shaped) as in leaf petioles of banana, canna and some hydrophytes.
(f) This tissue is generally present in almost all the organs of plants, i.e., roots, stems, leaves, flowers, fruits and seeds.
(g) If they enclose large air spaces they are called as aerenchyma if they develop chlorophyll, they are called as chlorenchyma and if they are elongated cells with tapering ends, they are called as prosenchyma.

Functions: They perform the following functions :

  • Storage of food materials. g., Carrot, Beetroot etc.
  • Chlorenchyma helps in photosynthesis.
  • Aerenchyma helps in floating of the aquatic plants (Hydrophytes) and also help in gaseous exchange during respiration and photosynthesis, g., Hydrilla.
  • In turgid state they give rigidity to the plant organ.
  • In emergency they behave like meristematic cells and help in healing of the various plant injuries.
  • Sometimes they store secretory substances (ergastic substance) such as tannins, resins and gums and they called as idioblasts.

Collenchyma
It is the tissue of primary body. Occurs in layer below epidermis in dicotyledon stem. Present as a continuous layer or in patches.

  • The cells of this tissue contain protoplasm and are living.
  • The cell walls are thickened at the corners and are made up of cellulose, hemicellulose and pectin.
  • They are compactly arranged cells, oval, spherical or polygonal in outline.
  • No intercellular spaces are present.
  • The tissue is extensible and have capacity to expand.
  • These cells assimilate food when they contain chloroplast.
  • They provide mechanical strength to younger part where xylem is less developed, such as young stem, petiole of leaf etc.
  • They are usually absent in monocots stems and in roots.

Differences between parenchyma and collenchyma

S. No. Parenchyma Collenchyma
1. This is found everywhere in the plant body. It is found in hypodermis of dicot stem and at the leaf base.
2. It can be both primary and secondary tissue. It is usually a primary tissue.
3. Cells are isodiametric. Cells are elongated.
4. It provides turgidity to softer tissues. It provides mechanical strength as well as elasticity to soft structures.
5. This can dedifferentiate rapidly to form meristematic cells. Cells dedifferentiate only rarely.

Sclerenchyma
The main features of sclerenchyma are :

  • It consist of thick-walled dead cells.
  • The cells vary in shape, size and origin.
  • They possess hard and extremely thick secondary walls due to uniform deposition of lignin.
  • In the beginning, the cells are living and have protoplasm but due to deposition of impermeable secondary walls they become dead.

Types of sclerenchyma: They are of two types :

(a) Sclerenchymatous fibres :

  • These are greatly elongated and tapering at both the ends.
  • The fully developed fibre cells are always dead. They are polygonal in transverse section and walls are highly lignified.
  • Intercellular spaces are absent and lumen is highly obliterated. The walls show simple and oblique pits.

  • They provide mechanical strength to the plant.
  • Some of the longest fibre yielding plants are Linumusitatissimum (Flax or Alsi), Corchorus, Cannabis, etc.
  • The fibres are present in hypodermis of monocot stem, in pericycle of many dicots, in secondary wood and vascular bundle sheath in monocot stems.

Types of Fibres

  • On the basis of source and economic use, fibres are of following types:
  • Surface fibres: Fibres obtained from seed coat (testa) of cotton and mesocarp of coconut (coir of commerce) are surface fibres. Cotton fibres are of two types, lint and fuzz. The lint is longer and economically useful while the fuzz is shorter and is not useful.
  • Bast fibres: Fibres obtained from phloem and pericycle are bast or bass fibres. They are most exploited economically, e.g., Jute, Flax, Hemp.
  • Xylary or wood fibres: Fibres associated with xylem are called xylary fibres. Two types of xylary fibres are recognized, libriform fibres and fibre tracheids. Libriform fibres are longer, thick walled and with simple pits while fibre tracheids are shorter, less thickened usually with bordered pits.

(b) Stone cells or Sclereids:

  • They are lignified, extremely thick walled so that the lumen of the cells is almost obliterated. Cells may be spherical, oval, cylindrical, T-shaped or even stellate.
  • They are generally found in hard parts of the plant, e.g., endocarp of Walnut and Coconut.
  • They form part of seed coat in some members of leguminosae. The sclereids provide mechanical support and hardness to the soft parts. Sclereids may be :
    Brachysclereids or stone cells : These are small and more or less isodiametric in shape. They occur in the cortex, pith, phloem, and pulp of fruits (e.g., Pyrus ).
    Macrosclereids or rod cells: These are rod-shaped elongated sclereids usually found in the leaves, cortex of stem and outer seed coats. E.g.,Legumes.
  • Osteosclereids: These are bone or barrel-shaped sclereids dilated at their ends. e.g., leaf of Hakea.
  • Astrosclereids or stellate cells : These are star-shaped sclereids with extreme lobes or arms, e.g., leaf of Nymphaea.
  • Trichosclereids or internal hairs : These are hair-like sclereids found in the intercellular spaces in the leaves and stem of some hydrophytes. E.g., Fig leaf

3. COMPLEX TISSUES
A group of more than one type of cells having common origin and working together as a unit, is called complex permanent tissue. The important complex tissues in vascular plants are : xylem and phloem. Both these tissues together are called as vascular tissue.
Xylem
The term xylem was introduced by Nageli (1858). Xylem is a conducting tissue which conducts water and mineral nutrients upwards from the roots to the leaves.

It also provides mechanical strength to plant parts. On the basis of origin xylem is of two types

Types of Xylem

  • Primary xylem : It is derived from procambium during primary growth. It consists of protoxylem and metaxylem.
  • Secondary xylem : It is formed from vascular cambium during secondary growth.

Xylem Cells
Xylem is composed of four types of cells
(a) Tracheids: The tracheids are elongated tube like cells with tapering or rounded or oval ends with hard and lignified walls.

  • All the vascular plants have tracheids in their xylem.
  • The walls are not much thickened.
  • The cells are without protoplast and are dead on maturity. The tracheids of secondary xylem have fewer sides and are more sharply angular than the tracheids of primary xylem.
  • The cell cavity or lumen of a tracheid is large and without any contents. Tracheids possess bordered pits. Maximum bordered pits are formed in gymnospermous tracheids.
  • They also possess various kinds of thickenings, e.g., annular, spiral, scalariform, reticulate or pitted tracheids.
  • The main function of tracheids is to conduct water and minerals from the root to the leaf. They also provide strength and mechanical support to the plant.

(b) Xylem Vessels: Vessels are rows of elongated tube-like cells, placed end to end with their end walls dissolved.

  • Vessels are multicellular with wide lumen.
  • The vessels may be classified into several types according to the thickening developed in their wall. They may be annular, spiral, scalariform, reticulate or pitted.
  • Vessels are absent in pteridophytes and gymnosperms (except Ephedra, Gnetum, Selaginella, Pteridium).
  • In angiosperms (porous wood) vessels are always present (Vessels are absent in family – Winteraceae and Trochodendraceae of Angiosperm i.e., Wintera, Trochodendron).
  • Vessels along with tracheids form the main tissue of xylem vascular bundles of the angiosperms and help in conduction.
  • It also provides mechanical support to the plant.
  • Vessel members are interconnected through perforations in their common wall.

(c) Wood (xylem) parenchyma:

  • These are the living parenchymatous cells, associated with xylem therefore known as wood parenchyma.
  • They serve as storage of reserve food and also help in conduction of water upwards through tracheids and vessels.
  • Their cell wall is cellulosic, radial conduction of water takes place through ray parenchyma.
  • Radial conduction of water takes place through ray parenchyma.

(d) Wood (xylem) fibres:

  • The long, slender, pointed, dead and sclerenchymatous cells found associated with xylem are termed wood fibres.
  • They possess mostly thickened walls and few small pits. These pits are found abundantly in woody dicotyledons.
  • They provide mechanical strength to xylem and various other organs of plant body.

Phloem (blast)
Term “Phloem” was given by Nageli. Its main function is the transport of organic food materials from leaves to stem and roots in a downward direction. On the basis of origin phloem is of two types:
Types of Phloem
(a) Primary phloem: It is formed by procambium during primary growth.

  • It may or may not show differentiation into protophloem (consists of sieve elements and parenchyma) and metaphloem (develop after protophloem and consists of sieve elements, parenchyma and fiber).
  • During the primary growth the protophloem elements are crushed by the surrounding tissues and disappear. This process is known as obliteration
  • Primary phloem consists of sieve elements, parenchyma and fibre.

(b) Secondary Phloem: It is produced during secondary growth by vascular cambium.
It consists of the following elements :

  • Sieve elements
  • Companion cells
  • Phloem parenchyma
  • Phloem fibres or bast fibres

(1) Sieve element
(i) They are long tube-like cells placed end to end, forming a continuous channel in the plant parts.
(ii) Their cell wall is made up of cellulose.
(iii) Their transverse wall is perforated like a normal sieve and hence they are called as sieve tubes.
(iv) Nucleus is not found in these cells.
(v) Each sieve tube has a lining of cytoplasm near its periphery.
(vi) Their main function is to translocate the food material from one part to the other.

(2) Companion cell

(i) They are thin-walled cells which are associated with sieve tubes.
(ii) They are more or less elongated.
(iii) They are connected with the sieve tube through sieve pore.

Phloem tissues

(iv) They contain nucleus and are therefore, living in nature. This nucleus controls functioning of sieve tube.
(v) They are not found in pteridophytes and gymnosperms but are always present in angiosperms.
(vi) Companion cells helps in maintaining pressure gradient in sieve tube.

(3) Phloem parenchyma : The parenchyma associated with the phloem is called phloem parenchyma. The cells are elongated with rounded ends and possess cellulosic cell walls. These cells are living and store food reserves in the form of starch and fats. They are present in pteridophytes and most of dicotyledonous angiosperms. They are absent in monocots.

(4) Phloem or Bast fibres: The sclerenchymatous fibres associated with the phloem are called as phloem fibres. These are also known as bast fibres. The fibres are elongated lignified cells with simple pits. The ends of these cells may be pointed, needle like or blunt. They are non-living cells that provide mechanical support to the organs.

Differences between Xylem and Phloem

SECRETORY TISSUES
These tissue perform special function in plants, e.g., secretion of resins gum, oil and latex.
These tissues are of two types :
(1) Laticiferous tissues
(2) Glandular tissues
Laticiferous tissues
They are made up of thin walled, elongated, branched and multinucleate (coenocytic) structures that contain colourless, milky or yellow coloured juice called latex. These occur irregularly distributed in the mass of parenchymatous cells. latex is contained inside the laticiferous tissue which is of two types :
(i) Latex cell: A laticiferous cell is a very highly branched cell with long slender processes, ramifying in all directions in the ground tissue of the organ.

  • They do not fuse and form a network.
  • Plants having such tissues are called simple or non-articulated laticifers. e.g., Calotropis (Asclepiadaceae) Nerium, Vinca (Apocyanaceae), Euphorbia (Euphorbiaceae), Ficus (Moraceae).

(ii) Latex vessels : They are formed due to fusion of cells and form network like structure in all directions. At maturity, they form a highly ramifying system of channels full of latex inside the organ. Plants having such tissues are called compound or articulated laticifers. e.g., Argemone, Papaver (Papaveraceae), Sonchus (Compositae), Hevea, Manihot (Euphorbiaceae).
Glandular tissue
This is a highly specialized tissue consisting of glands, discharging diverse functions, including secretory and excretory. Glands may be external or internal.
(i) External glands : They generally occur on the epidermis of stem and leaves as glandular hair as in Plumbago and Boerhaavia, stinging hair secrete poisonous substance in Urtica, nectar secreting glands in flowers or leaves, e.g., Rutaceae and Euphorbiaceae. Digestive enzyme secreting glands in insectivorous plants, e.g., Drosera (Sundew), Nepenthes (Pitcher plant).
(ii) Internal glands: These are present internally and are of several types, e.g., oil glands in Citrus and Eucalyptus, resinous ducts in Pinus, mucilage canals in Cycas. Water secreting glands (hydathodes) in Colocasia (present at the tip of leaves), Tropaeoleum (along margin), etc. The glands which secrete essential oil are called osmophores (osmotrophs).

5. HYDATHODES

  • They are the water exuding structures of plants inhabiting the humid tropics.
  • These hydathodes are found in the leaves of many angiosperms and are located on leaf margins (e.g., Tropaeolum) or at the tip (e.g.. Colocasia).
  • The cells of the mesophyll adjacent to the vascular bundle proliferate to give rise to epithem. The cells of the epithem are thin walled, elongated with dense cytoplasm and deficient in chloroplasts. They have a well-developed system of intercellular spaces and are in close contact with terminal tracheary elements.
  • Overlying the epithem are present two guard cells. The water that moves out of the tracheids under conditions of high root pressure and humidity is ultimately discharged through the terminal pore of the hydathode.

6. THE TISSUE SYSTEM
The various types of tissues present in the body of a plant perform different functions. Several tissues may collectively perform the same function. A collection of tissues performing the same general function is known as a “Tissue System”. There are three major tissue systems present in plants.
(1) Epidermal tissue system
(2) Ground or fundamental tissue system
(3) Vascular or conducting tissue system
Epidermal tissue system
The tissues of this system originate from the outermost layer of apical meristem. It forms the outermost covering of various plant organs which remains in direct contact with the environment.
Epidermis: Epidermis is composed of single layer of cells.

  • These cells vary in their shape and size and form a continuous layer interrupted by stomata. In some cases epidermis may be multilayered, e.g., Ficus, Nerium, Peperomia, Begonia etc.
  • The epidermal cells are living, parenchymatous, and compactly arranged without intercellular spaces.
  • Certain epidermal cells of some plants or plant parts are differentiated into variety of cell types:

(a) In aerial roots, the multiple epidermal cells are modified into velamen, which absorb water from the atmosphere, e. g., Orchids.
(b) Some of the cells in the leaves of grasses are comparatively very large, called bulliform or motor cells. They are hygroscopic in nature, thin-walled and contain big central vacuoles filled with water. They play an important role in the folding and unfolding of leaves. These cells develops from modification of epidermal cell and vein.
(c) Some members of Gramineae and Cyperaceae possess two types of epidermal cells : the long cells and the short cells. The short cells may be cork cells or silica cells.

♦ Cuticle and Wax: In aerial parts of the plant epidermis is covered by cuticle. The epidermal cells secrete a waxy substance called cutin, which forms a layer of variable thickness (the cuticle) within and on the outer surface of its all walls. It helps in reducing the loss of water by evaporation. Other substances deposited on the cuticle surface may be oil, resin, silicon and salts (cystoliths are crystals of calcium carbonate or calcium oxalate, e.g., Ficus. Druse and Raphides, e.g., Pistia). Thick cuticle is found in leaves of dry habita plants.

♦ Stomata: Stomata are minute apertures in the epidermis. Each aperture is bounded by two kidney shaped cells, called guard cells. In xerophytes, the stomata are sunken in grooves due to which rate of transpiration is greatly reduced (e.g. Nerium). Usually there is a large air cavity below each aperture, it is called substomatal cavity. Guard cells are surrounded by subsidiary cells or accessory cells which differ morphologically from the other epidermal cells. In monocots, e.g., Doob, Maize guard cells are dumb bell shape.

  • The outer wall of guard cell is thin and the inner wall is thick .
  • Chloroplast is present in guard cell, which helps in stomatal movement.
  • Stomata aperture, guard cells and subsidiary cells together is refered as stomatal aperture.

♦ Trichomes : These are epidermal outgrowths present temporarily or permanently on almost all plant parts. They may be unicellular or multicellular, vary in size and shape in different species. They may be of different types: stellate hair, glandular hair, short glandular hair, urticating hair and stinging hair. The trichomes serve for checking excess loss of water and for protection.

♦ Root hairs : They are enlargements of special epiblema cells called trichoblasts and occurs in a particular zone of young root called root hair zone. They are specialised to absorb water from soil. They also hold soil particles.
Ground or Fundamental tissue system
Ground tissue system includes all the tissues of plant body except epidermal tissue system and vascular tissues. It forms the bulk of plant body. This tissue system mainly originates from ground meristem. The ground tissue constitute the following parts :
(i) Cortex: It lies between epidermis and the pericycle. The cortex is distinct in dicotyledons but not in monocotyledons, where there is no clear demarcation between cortex and pith. It is further differentiated into
(a) Hypodermis: It is collenchymatous in dicot stem and sclerenchymatous in monocot stem. It provides strength.
(b) General cortex: It consists of parenchymatous cells. Its main function is storage of food.
(c) Endodermis (Starch sheath): It is mostly single layered and is made up of parenchymatous, barrel shaped compactly arranged cells.

  • The inner or transverse wall of endodermal cells have Casparian strips which has deposition of suberin.
  • In roots, thick walled endodermal cells are interrupted by thin walled cells just outside the protoxylem patches. These thin walled endodermal cells are called passage cells.
  • Endodermis with characteristic casparian bands is absent in woody dicot stem, monocot stem and leaves of angiosperms.
  • The young stems of angiosperms show a layer with abundant starch deposition. This layer occurs in the position where endodermis would have been situated which is called as starch sheath.
  • Endodermis behave as water tight dam to check the loss of water and air dam to check the entry of air in xylem elements.
  • Endodermis is internal protective tissue.

(ii) Pericycle: It is a single layered or multilayered cylinder of thin-walled or thick-walled cells present between the endodermis and vascular tissues.
♦ In some cases, the pericycle is made up of many layers of sclerenchymatous cells (Cucurbita stem) or in the form of alternating bands of thin-walled and thick-walled cells (Sunflower stem).
♦ In monocot, the pericycle is made up of thin-walled parenchymatous cells which later on gives rise to lateral roots.
♦ In dicot roots the cork cambium originates in the pericycle which results in the formation of periderm.
♦ Pericycle also gives rise to a part of vascular cambium in dicot roots.

(iii) Pith or medulla: It occupies the central part in dicot stem, and monocot roots.

  • It is mostly made up of parenchymatous cells.
  • In dicot roots pith is completely obliterated by the metaxylem elements.
  • In dicot stem, the pith cells between the vascular bundles become radially elongated and known as primary medullary rays or pith rays. They help in lateral translocation.

Vascular tissue system
The central cylinder of the shoot or root surrounded by cortex is called stele.

  • The varying number of vascular bundles formed inside the stele constitute vascular tissue system.
  • Xylem, phloem and cambium are the major parts of the vascular bundle. Vascular bundle may be of following types :

(i) Radial: The xylem and phloem strands alternate with each other separated by parenchymatous cells. such kinds of vascular bundles are called radial and found mainly in roots.
(ii) Conjoint: A vascular bundle having both xylem and phloem together, is called conjoint. Normally the xylem and phloem occur on the same radius. They occur in stems. Such vascular bundles are of two types :
(a) Collateral: A vascular bundle in which the phloem lies towards outerside and xylem towards inner side, is called collateral, e.g., Sunflower.
Collateral bundle having a cambium between xylem and phloem is said to be of the open type, e.g., Dicot stem.
Collateral bundle lacking a cambium between xylem and phloem is said to be of the closed type, e.g., Monocot stem.
(b) Bicollateral: A vascular bundle having the phloem strands on both outer and inner side of xylem, is called bicollateral. e.g., Cucurbita.
(iii) Concentric: A vascular bundle in which one tissue is completely surrounded by the other, is called concentric. The concentric bundles are of two types :
(a) Amphivasal (Leptocentric): The phloem lies in the centre and remains completely surrounded by xylem. e.g., Dracaena, Yucca.

Various types of vascular bundles : (a) radial (b) conjoint closed (c) conjoint open (b) Amphicribal (Hadrocentric): The xylem lies in the centre and remains completely surrounded by phloem. e.g., Ferns.

7. PRIMARY STRUCTURES IN PLANT

Difference between internal structure of root and stem

Description Root Stem
(i) Epidermis or Epiblema Epiblema or piliferous layer without cuticle Epidermis usually with cuticle.
(ii) Hair Unicellular Multicellular.
(iii) Chlorenchyma in cortex Absent Usually present in young stems but absent in old stem.
(iv) Endodermis Very distinct Poorly developed or absent.
(v) Vascular bundle Radial Conjoint, collateral or bicollateral concentric.
(vi) Xylem Exarch (protoxylem towards periphery) Endarch. (protoxylem towards centre)

Origin of Lateral roots: Lateral roots arise endogenously, i.e., from the cells inside the endodermis. They arise from pericycle cells.

8. INTERNAL STRUCTURE OF LEAF

  • Like the root and stem, the leaf consists of three tissue systems, the dermal system, consisting of the upper and lower epidermis, the ground tissue system, the main photosynthetic tissue, which consists of mesophyll, and the vascular system, comprising of veins of various degrees.
  • Common leaves are bifacial and are further of two types, dorsiventral and isobilateral. Unifacial leaves are cylindrical and occur in onion and garlic.
  • The upper as well as the lower surface of the leaf is covered by a uniseriate epidermis. However, in some plants (e.g., Nerium, Ficus, etc.) the epidermis is multiseriate.
  • All epidermal cells of a leaf are alike. The epidermal cells are compactly arranged and their outer walls are usually thickened. The epidermis is covered by a layer of cuticle the thickness of the, which varies considerably and the xerophytic species have a thicker cuticle.
  • In some xerophytic leaves, especially those of grasses, the epidermal cells situated in longitudinal furrows are large with thin flexible walls. These cells are said to be motor cells or bulliform cells, and they help in rolling of leaves in dry weather.
  • A characteristic feature of the leaf epidermis is the presence of numerous small openings, called stomata. They occur either on both sides of the leaf (leaf is said to be amphistomatic), confined to the lower surface of the leaf (leaf is known as hypostomatic) or to the upper surface as in floating leaves of aquatic plants (leaf is called epistomatic). A stoma consists of two highly specialized epidermal cells, known as guard cells, enclosing a space. In some plants (e.g., Nerium), stomata are present in sunken cavities, called stomatal crypts.
  • The bulk of the internal tissue of the leaf, enclosed by the upper and lower epidermis, forms mesophyll. It is composed of thin walled parenchymatous cells containing numerous chloroplasts. The mesophyll is differentiated into palisade and spongy parenchyma in dicot leaves.
  • The spongy parenchyma consists of irregular and loosely arranged cells, enclosing large intercellular spaces. These air spaces are connected with the substomatal chambers and maintain gaseous exchange with the outside through stomata.
  • The palisade parenchyma is composed of more or less cylindrical and elongated cells arranged compactly with their long axis perpendicular to the epidermis.

Vascular system

  • The mid rib in most dicotyledons consists of a single large collateral vascular bundle with an adaxial xylem and abaxial phloem.
    The cells surrounding the vascular bundles in the leaf are mostly morphologically distinct from the mesophyll cells. These cells constitute the bundle sheath. In dicotyledons the vascular bundles are surrounded by thin walled parenchymatous cells that extend in the direction parallel to the veins. In monocotyledons, the vascular bundles are completely or partially surrounded by one or two bundle sheaths, each consisting of a single layer of cells.

Difference between dicot and monocot leaf

Character Dicot leaf Monocot leaf
(i) Type of leaf Dorsiventral (bifacial) Isobilateral.
(ii) Stomata Usually more on lower epidermis. Equal on lower and upper epidermis (amphistomatic).
(iii) Mesophyll Made up of two types of tissues (a) Palisade parenchyma. (b) Spongy parenchyma with large intercellular spaces Only spongy parenchyma is present, which has very small intercellular spaces.
(iv)Bundle sheath Made up of parenchyma. Just above and below the vascular bundle some parenchymatous cells or collenchymatous cells are present (upto epidermis). Made of parenchyma, but just above and below the vascular bundles sclerenchymatous cells (upto epidermis) are found.
(v) Bulliform or motor cells Absent. Present on upper epidermis.
T.S. of leaf : (a) Dicot (b) Monocot

Kranz type anatomy occurs in both monocot and dicot leaves of some tropical and arid area plants.


Secondary Xylem Biology

Secondary Xylem Biology: Origins, Functions, and Applications provides readers with many lenses from which to understand the whole scope and breadth of secondary xylem. The book builds on a basic comprehension of xylem structure and development before delving into other important issues such as fungal and bacterial degradation and biofuel conversion.

Chapters are written by recognized experts who have in-depth knowledge of their specific areas of expertise. It is a single information source containing high quality content, information, and knowledge related to the understanding of biology in woody plants and their applications.


SBI3U

Course Title: Biology, Grade 11, University Preparation (SBI3U)
Course Name: Biology
Course Code: SBI3U
Grade: 11
Course Type: University Preparation
Credit Value: 1.0
Prerequisite: SNC2D, Science, Grade 10, Academic
Curriculum Policy Document: Science, The Ontario Curriculum, Grades 11 and 12, 2008 (Revised)
Course Developer: Virtual High School
Department: Science
Department Head: Timothy Irvine, B.Sc.(Hon), B.Ed., OCT
Development Date: 1995
Most Recent Revision Date: 2012
Tuition Fee (CAD): $579

Fast track courses offer an accelerated assessment turnaround time which allows students the opportunity to move through the course at a faster pace.

Course Title: Biology, Grade 11, University Preparation (SBI3U)
Course Name: Biology
Course Code: SBI3U
Grade: 11
Course Type: University Preparation
Credit Value: 1.0
Prerequisite: SNC2D, Science, Grade 10, Academic
Curriculum Policy Document: Science, The Ontario Curriculum, Grades 11 and 12, 2008 (Revised)
Course Developer: Virtual High School
Department: Science
Department Head: Timothy Irvine, B.Sc.(Hon), B.Ed., OCT
Development Date: 1995
Most Recent Revision Date: 2012
Tuition Fee (CAD): $729

Repeat (or upgrade) courses are intended for students who wish to improve their grades in courses they have already successfully completed.

Course Title: Biology, Grade 11, University Preparation (SBI3U)
Course Name: Biology
Course Code: SBI3U
Grade: 11
Course Type: University Preparation
Credit Value: 1.0
Prerequisite: SBI3U, Biology, Grade 11, University Preparation
Curriculum Policy Document: Science, The Ontario Curriculum, Grades 11 and 12, 2008 (Revised)
Course Developer: Virtual High School
Department: Science
Department Head: Timothy Irvine, B.Sc.(Hon), B.Ed., OCT
Development Date: 1995
Most Recent Revision Date: 2012
Tuition Fee (CAD): $579

This course furthers students&rsquo understanding of the processes that occur in biological systems. Students will study theory and conduct investigations in the areas of biodiversity evolution genetic processes the structure and function of animals and the anatomy, growth, and function of plants. The course focuses on the theoretical aspects of the topics under study, and helps students refine skills related to scientific investigation.

Diversity of Living Things

In this unit, students will demonstrate an understanding that all living things can be classified through the principles of taxonomy and phylogeny. They will use sampling and classification techniques to investigate the principles of scientific classification. Students will analyse the effects of human activity on the diversity of living organisms in ecosystems.

In this unit, students will demonstrate an understanding of the theory of evolution and the evidence that supports it. They will examine the mechanisms by which it occurs, including thorough consideration of natural selection and punctuated equilibrium, and evaluate the logic that has drawn scientists to their conclusions. Students will also analyse the economic and environmental implications of artificial selection technology, and evaluate the impact of environmental changes on natural selection and species at risk.

Genetic Processes

In this unit, students will evaluate recent advances in our knowledge of genetic processes and demonstrate an understanding that genetic and genomic research can have both social and environmental implications. They will investigate how variability and diversity of living organisms results from the distribution of genetic material during the process of meiosis. Students will also analyse data to solve basic genetic problems.

Animals: Structure and Function

In this unit, students will demonstrate an understanding of how groups of organs with specific structures and functions work together as systems, which interact with other systems in the body. They will investigate by means of computer simulation and independent experimentation, the functional responses and relationships between major organ systems. Students will also be asked to consider how the development and uses of technology to maintain health are related to the changing needs of society.

Plants: Anatomy, Growth and Function

In this unit, students will demonstrate an understanding that plants have specialised structures with distinct functions that enable them to respond and adapt to their environment. They will investigate the structures and functions of plant tissues and factors affecting growth. Students will consider the importance of the plant variety to the survival and sustainability of ecosystems.

This is a proctored exam worth 30% of your final grade.

Resources required by the student:

Note: This course is entirely online and does not require or rely on any textbook.

  • Online lab access to third party software
  • A scanner, smart phone camera, or similar device to upload handwritten or hand-drawn work

Resources provided by Virtual High School:

Overall Curriculum Expectations

A. Scientific Investigation Skills and Career Exploration
A1 demonstrate scientific investigation skills (related to both inquiry and research) in the four areas of skills (initiating and planning, performing and recording, analysing and interpreting, and communicating)
A2 identify and describe careers related to the fields of science under study, and describe the contributions of scientists, including Canadians, to those fields.
B. Diversity of Living Things
B1 analyse the effects of various human activities on the diversity of living things
B2 investigate, through laboratory and/or field activities or through simulations, the principles of scientific classification, using appropriate sampling and classification techniques
B3 demonstrate an understanding of the diversity of living organisms in terms of the principles of taxonomy and phylogeny.
C. Evolution
C1 analyse the economic and environmental advantages and disadvantages of an artificial selection technology, and evaluate the impact of environmental changes on natural selection and endangered species
C2 investigate evolutionary processes, and analyse scientific evidence that supports the theory of evolution
C3 demonstrate an understanding of the theory of evolution, the evidence that supports it, and some of the mechanisms by which it occurs.
D. Genetic Processes
D1 evaluate the importance of some recent contributions to our knowledge of genetic processes, and analyse social and ethical implications of genetic and genomic research
D2 investigate genetic processes, including those that occur during meiosis, and analyse data to solve basic genetics problems involving monohybrid and dihybrid crosses
D3 demonstrate an understanding of concepts, processes, and technologies related to the transmission of hereditary characteristics.
E. Animals: Structure and Function
E1 analyse the relationships between changing societal needs, technological advances, and our understanding of internal systems of humans
E2 investigate, through laboratory inquiry or computer simulation, the functional responses of the respiratory and circulatory systems of animals, and the relationships between their respiratory, circulatory, and digestive systems
E3 demonstrate an understanding of animal anatomy and physiology, and describe disorders of the respiratory, circulatory, and digestive systems.
F. Plants: Anatomy, Growth and Function
F1 evaluate the importance of sustainable use of plants to Canadian society and other cultures
F2 investigate the structures and functions of plant tissues, and factors affecting plant growth
F3 demonstrate an understanding of the diversity of vascular plants, including their structures, internal transport systems, and their role in maintaining biodiversity.

Teaching & Learning Strategies:

As in a conventional classroom, instructors employ a range of strategies for teaching a course:

  • Well-presented, clear writing and helpful graphics and diagrams
  • Hands-on laboratory activities
  • Research assignments, with direct instruction and coaching

In addition, teachers and students have at their disposal a number of tools that are unique to electronic learning environments:

  • Electronic simulation activities
  • Discussion boards and email
  • Assessments with real-time feedback
  • Interactive activities that engage both the student and teacher in subject

Assessment, Evaluation and Reporting Strategies of Student Performance:

Our theory of assessment and evaluation follows the Ministry of Education's Growing Success document, and it is our firm belief that doing so is in the best interests of students. We seek to design assessment in such a way as to make it possible to gather and show evidence of learning in a variety of ways to gradually release responsibility to the students, and to give multiple and varied opportunities to reflect on learning and receive detailed feedback.

Growing Success articulates the vision the Ministry has for the purpose and structure of assessment and evaluation techniques. There are seven fundamental principles that ensure best practices and procedures of assessment and evaluation by Virtual High School teachers. VHS assessments and evaluations,

  • are fair, transparent, and equitable for all students
  • support all students, including those with special education needs, those who are learning the language of instruction (English or French), and those who are First Nation, Métis, or Inuit
  • are carefully planned to relate to the curriculum expectations and learning goals and, as much as possible, to the interests, learning styles and preferences, needs, and experiences of all students
  • are communicated clearly to students and parents at the beginning of the course and at other points throughout the school year or course
  • are ongoing, varied in nature, and administered over a period of time to provide multiple opportunities for students to demonstrate the full range of their learning
  • provide ongoing descriptive feedback that is clear, specific, meaningful, and timely to support improved learning and achievement
  • develop students&rsquo self-assessment skills to enable them to assess their own learning, set specific goals, and plan next steps for their learning.

For a full explanation, please refer to Growing Success.

The Final Grade:

The evaluation for this course is based on the student's achievement of curriculum expectations and the demonstrated skills required for effective learning. The final percentage grade represents the quality of the student's overall achievement of the expectations for the course and reflects the corresponding level of achievement as described in the achievement chart for the discipline. A credit is granted and recorded for this course if the student's grade is 50% or higher. The final grade will be determined as follows:

  • 70% of the grade will be based upon evaluations conducted throughout the course. This portion of the grade will reflect the student's most consistent level of achievement throughout the course, although special consideration will be given to more recent evidence of achievement.
  • 30% of the grade will be based on final evaluations administered at the end of the course. The final assessment may be a final exam, a final project, or a combination of both an exam and a project.

The Report Card:

Student achievement will be communicated formally to students via an official report card. Report cards are issued at the midterm point in the course, as well as upon completion of the course. Each report card will focus on two distinct, but related aspects of student achievement. First, the achievement of curriculum expectations is reported as a percentage grade. Additionally, the course median is reported as a percentage. The teacher will also provide written comments concerning the student's strengths, areas for improvement, and next steps. Second, the learning skills are reported as a letter grade, representing one of four levels of accomplishment. The report card also indicates whether an OSSD credit has been earned. Upon completion of a course, VHS will send a copy of the report card back to the student's home school (if in Ontario) where the course will be added to the ongoing list of courses on the student's Ontario Student Transcript. The report card will also be sent to the student's home address.

Program Planning Considerations:

Teachers who are planning a program in this subject will make an effort to take into account considerations for program planning that align with the Ontario Ministry of Education policy and initiatives in a number of important areas.

Planning Programs for Students with Special Education Needs

Virtual High School is committed to ensuring that all students, especially those with special education needs, are provided with the learning opportunities and supports they require to gain the knowledge, skills, and confidence needed to succeed in a rapidly changing society. The context of special education and the provision of special education programs and services for exceptional students in Ontario are constantly evolving. Provisions included in the Canadian Charter of Rights and Freedoms and the Ontario Human Rights Code have driven some of these changes. Others have resulted from the evolution and sharing of best practices related to the teaching and assessment of students with special educational needs.

Virtual High School pays particular attention to the following beliefs: (1) all students can succeed, (2) each student has his or her own unique patterns of learning, (3) successful instructional practices are founded in evidence-based research, tempered by experience, (4) an open and accessible learning environment with differentiated instruction are effective and interconnected means of meeting the learning or productivity needs of any group of students, (5) classroom teachers are the key educators for a student's literacy and numeracy development, (6) classroom teachers need the support of the larger community to create a learning environment that supports students with special education needs, and finally, (7) fairness is not sameness.

The provision of special education programs and services for students at Virtual High School rests within a legal framework The Education Act and the regulations related to it set out the legal responsibilities pertaining to special education. They provide comprehensive procedures for the identification of exceptional pupils, for the placement of those pupils in educational settings where the special education programs and services appropriate to their needs can be delivered, and for the review of the identification of exceptional pupils and their placement.

If the student requires either accommodations, then Virtual High School will take into account these needs of exceptional students as they are set out in the students' existing Individual Education Plan. The online courses offer a vast array of opportunities for students with special educations needs to acquire the knowledge and skills required for our evolving society. Students who use alternative techniques for communication may find a venue to use these special skills in these courses. There are a number of technical and learning aids that can assist in meeting the needs of exceptional students as set out in their Individual Education Plan. In the process of taking their online course, students may use a personal amplification system, tele-typewriter (via Bell relay service), an oral or a sign-language interpreter, a scribe, specialized computer programs, time extensions, ability to change font size, oral readers, etc.

Accommodations (instructional, environmental or assessment) allow the student with special education needs access to the curriculum without changes to the course curriculum expectations. VHS will develop a Personal Education Plan (PEP) based on the student’s existing Individual Education Plan.

Program Considerations for English Language Learners

This Virtual High School online course provide a number of strategies to address the needs of ESL/ELD students. This online course must be flexible in order to accommodate the needs of students who require instruction in English as a second language or English literacy development. The Virtual High School teacher considers it to be his or her responsibility to help students develop their ability to use the English language properly. Appropriate accommodations affecting the teaching, learning, and evaluation strategies in this course may be made in order to help students gain proficiency in English, since students taking English as a second language at the secondary level have limited time in which to develop this proficiency. Virtual High School determines the student's level of proficiency in the English Language upon registration. This information is communicated to the teacher of the course following the registration and the teacher then invokes a number of strategies and resources to support the student in the course. On a larger scale, well written content will aid ESL students in mastering not only the content of this course, but as well, the English language and all of its idiosyncrasies. Virtual High School has created course content to enrich the student's learning experience. Many occupations in Canada require employees with capabilities in the English language. Enabling students to learn English language skills will contribute to their success in the larger world.

Environmental Education

Helping students become environmentally responsible is a role assumed by Virtual High School. The first goal is to promote learning about environmental issues and solutions. The second goal is to engage students in practicing and promoting environmental stewardship in their community. The third goal stresses the importance of the education system providing leadership by implementing and promoting responsible environmental practices so that all stakeholders become dedicated to living more sustainably. Environmental education teaches students about how the planet's physical and biological systems work, and how we can create a more sustainable future. Good curriculum design following the resource document - The Ontario Curriculum, Grades 9-12: Environmental Education, Scope and Sequence of Expectations, 2011, will assist Virtual High School staff to weave environmental education in and out of the online course content. This ensures that the student will have opportunities to acquire the knowledge, skills, perspectives and practices needed to become an environmentally literate citizen. The online course should provide opportunities for each student to address environmental issues in their home, in their local community, or even at the global level.

Healthy Relationships

Every student is entitled to learn in a safe, caring environment, free from violence and harassment. Students learn and achieve better in such environments. The safe and supportive social environment at Virtual High School is founded on healthy relationships between all people. Healthy relationships are based on respect, caring, empathy, trust, and dignity, and thrive in an environment in which diversity is honoured and accepted. Healthy relationships do not tolerate abusive, controlling, violent, bullying/harassing, or other inappropriate behaviours. To experience themselves as valued and connected members of an inclusive social environment, students need to be involved in healthy relationships with their peers, teachers, and other members of the Virtual High School community.

The most effective way to enable all students to learn about healthy and respectful relationships is through the school curriculum. Virtual High School teachers can promote this learning in a variety of ways. For example, they can help students develop and practise the skills they need for building healthy relationships by giving them opportunities to apply critical-thinking and problem solving strategies and to address issues through group discussions, role play, case study analysis, and other means. Virtual High School can also have a positive influence on students by modelling the behaviours, values, and skills that are needed to develop and sustain healthy relationships, and by taking advantage of “teachable moments” to address immediate relationship issues that may arise among students.

At Virtual High School, all staff strive to create a climate of cooperation, collaboration, respect, and open-mindedness. These attitudes and attributes enable our students to develop an awareness of the complexity of a range of issues. Moreover, in examining issues from multiple perspectives, students develop not only an understanding of various positions on these issues but also a respect for different points of view. Virtual High School students will hopefully develop empathy as they analyse events and issues from the perspectives of people all over the world. These attitudes and attributes provide a foundation on which students can develop their own identity, explore interconnectedness with others, and form and maintain healthy relationships.

Equity and Inclusive Education

The Virtual High School equity and inclusive education strategy focuses on respecting diversity, promoting inclusive education, and identifying and eliminating discriminatory biases, systemic barriers, and power dynamics that limit the ability of students to learn, grow, and contribute to society. Antidiscrimination education continues to be an important and integral component of this strategy.

In an environment based on the principles of inclusive education, all students, parents, caregivers, and other members of the school community - regardless of ancestry, culture, ethnicity, sex, physical or intellectual ability, race, religion, gender identity, sexual orientation, socio-economic status, or other similar factors - are welcomed, included, treated fairly, and respected. Diversity is valued, and all members of the Virtual High School community feel safe, comfortable, and accepted. Every student is supported and inspired to succeed in a culture of high expectations for learning. In an inclusive education system, all students see themselves reflected in the curriculum, their physical surroundings, and the broader environment, so that they can feel engaged in and empowered by their learning experiences.

Virtual High School can give students a variety of opportunities to learn about diversity and diverse perspectives. By drawing attention to the contributions of women, the perspectives of various ethno-cultural, religious, and racial communities, and the beliefs and practices of First Nations, Mêtis, and Inuit peoples, teachers enable Virtual High School students from a wide range of backgrounds to see themselves reflected in the curriculum. It is essential that learning activities and materials used to support the curriculum reflect the multicultural nature of society that is Canada. In addition, Virtual High School differentiates the instruction and assessment strategies to take into account the background and experiences, as well as the interests, aptitudes, and learning needs, of all students.

Financial Literacy Education

Financial literacy may be defined as having the knowledge and skills needed to make responsible economic and financial decisions with competence and confidence. Since making financial decisions has become an increasingly complex task in the modern world, students need to have knowledge in various areas and a wide range of skills in order to make informed decisions about financial matters. Students need to be aware of risks that accompany various financial choices. They need to develop an understanding of world economic forces as well as ways in which they themselves can respond to those influences and make informed choices. Virtual High School considers it essential that financial literacy be considered an important attribute of a well-educated population. In addition to acquiring knowledge in such specific areas as saving, spending, borrowing, and investing, students need to develop skills in problem solving, inquiry, decision making, critical thinking, and critical literacy related to financial and other issues. The goal is to help students acquire the knowledge and skills that will enable them to understand and respond to complex issues regarding their own personal finances and the finances of their families, as well as to develop an understanding of local and global effects of world economic forces and the social, environmental, and ethical implications of their own choices as consumers. The Ministry of Education and Virtual High School are working to embed financial literacy expectations and opportunities in all courses as appropriate, as part of the ongoing curriculum review process.

Literacy, Mathematical Literacy, and Inquiry Skills

Literacy is defined as the ability to use language and images in rich and varied forms to read, write, listen, view, represent, and think critically about ideas. It involves the capacity to access, manage, and evaluate information to think imaginatively and analytically and to communicate thoughts and ideas effectively. Literacy includes critical thinking and reasoning to solve problems and make decisions related to issues of fairness, equity, and social justice. Literacy connects individuals and communities and is an essential tool for personal growth and active participation in a cohesive, democratic society. Literacy involves a range of critical-thinking skills and is essential for learning across the curriculum. Literacy instruction takes different forms of emphasis in different subjects, but in all subjects, literacy needs to be explicitly taught. Literacy, mathematical literacy, and inquiry/research skills are critical to students' success in all subjects of the curriculum and in all areas of their lives.

Many of the activities and tasks that students undertake in the Virtual High School courses involve the literacy skills relating to oral, written, and visual communication. For example, they develop literacy skills by reading, interpreting, and analysing various texts. In addition, they develop the skills needed to construct, extract information from, and analyse various types information presented in a variety of media forms. In all Virtual High School courses, students are required to use appropriate and correct terminology, including that related to the concepts of disciplinary thinking, and are encouraged to use language with care and precision in order to communicate effectively.

Inquiry and research are at the heart of learning in all subject areas at Virtual High School. Students are encouraged to develop their ability to ask questions and to explore a variety of possible answers to those questions. As they advance through the grades, they acquire the skills to locate relevant information from a variety of print and electronic sources. The questioning they practiced in the early grades becomes more sophisticated as they learn that all sources of information have a particular point of view and that the recipient of the information has a responsibility to evaluate it, determine its validity and relevance, and use it in appropriate ways. The ability to locate, question, and validate information allows a student to become an independent, lifelong learner.

Critical Thinking and Critical Literacy

Critical thinking is the process of thinking about ideas or situations in order to understand them fully, identify their implications, make a judgement, and/or guide decision making. Critical thinking includes skills such as questioning, predicting, analysing, synthesizing, examining opinions, identifying values and issues, detecting bias, and distinguishing between alternatives. Students who are taught these skills become critical thinkers who can move beyond superficial conclusions to a deeper understanding of the issues they are examining. They are able to engage in an inquiry process in which they explore complex and multifaceted issues, and questions for which there may be no clear-cut answers.

Students use critical-thinking skills in Virtual High School courses when they assess, analyse, and/or evaluate the impact of something and when they form an opinion about something and support that opinion with a rationale. In order to think critically, students need to examine the opinions and values of others, detect bias, look for implied meaning, and use the information gathered to form a personal opinion or stance, or a personal plan of action with regard to making a difference. Students approach critical thinking in various ways. Some students find it helpful to discuss their thinking, asking questions and exploring ideas. Other students, including many First Nations, Mêtis, and Inuit students, may take time to observe a situation or consider a text carefully before commenting they may prefer not to ask questions or express their thoughts orally while they are thinking.

The development of these critical-thinking skills is supported in every course at Virtual High School. As students work to achieve the curriculum expectations in their particular course, students frequently need to identify the possible implications of choices. As they gather information from a variety of sources, they need to be able to interpret what they are listening to, reading, or viewing to look for instances of bias and to determine why a source might express a particular bias.

The Role of the School Library

The school library program in many schools can help build and transform students' knowledge in order to support lifelong learning in our information- and knowledge-based society. The school library program of these schools supports student success across the curriculum by encouraging students to read widely, teaching them to examine and read many forms of text for understanding and enjoyment, and helping them improve their research skills and effectively use information gathered through research. Virtual High School teachers assist students in accessing a variety of online resources and collections (e.g., professional articles, image galleries, videos, databases). Teachers at Virtual High School will also guide students through the concept of ownership of work and the importance of copyright in all forms of media.

The Role of Information and Communications Technology

Information literacy is the ability to access, select, gather, critically evaluate, and create information. Communication literacy refers to the ability to communicate information and to use the information obtained to solve problems and make decisions. Information and communications technologies are utilized by all Virtual High School students when the situation is appropriate within their online course. As a result, students will develop transferable skills through their experience with word processing, internet research, presentation software, and telecommunication tools, as would be expected in any other course or any business environment. Although the Internet is a powerful learning tool, there are potential risks attached to its use. All students must be made aware of issues related to Internet privacy, safety, and responsible use, as well as of the potential for abuse of this technology, particularly when it is used to promote hatred.

The Ontario Skills Passport: Making Learning Relevant and Building Skills

The Ontario Skills Passport (OSP) is a free, bilingual, web-based resource that provides teachers and students with clear descriptions of the "Essential Skills" and work habits important in work, learning, and life. Virtual High School can engage students by using OSP tools and resources to show how what they learn in class can be applied in the workplace and in everyday life. For further information on the Ontario Skills Passport, including the Essential Skills and work habits, visit http://www.skills.edu.gov.on.ca.

Education and Career/Life Planning

As online students progress through online courses, teachers are available to help the student prepare for employment in a number of diverse areas. With the help of teachers, students will learn to set and achieve goals and will gain experience in making meaningful decisions concerning career choices. The skills, knowledge and creativity that students acquire through this online course are essential for a wide range of careers. Throughout their secondary school education, students will learn about the educational and career opportunities that are available to them explore and evaluate a variety of those opportunities relate what they learn in their courses to potential careers in a variety of fields and learn to make appropriate educational and career choices. The framework of the program is a four-step inquiry process based on four questions linked to four areas of learning: (1) knowing yourself - Who am I? (2) exploring opportunities - What are my opportunities? (3) making decisions and setting goals - Who do I want to become? and (4) achieving goals and making transitions - What is my plan for achieving my goals?

Cooperative Education and Other Forms of Experiential Learning

By applying the skills they have developed, students will readily connect their classroom learning to real-life activities in the world in which they live. Cooperative education and other workplace experiences will broaden their knowledge of employment opportunities in a wide range of fields. In addition, students will increase their understanding of workplace practices and the nature of the employer-employee relationship. Virtual High School will try to help students link to Ministry programs to ensure that students have information concerning programs and opportunities.

Planning Program Pathways and Programs Leading to a Specialist High Skills Major

Virtual High School courses are well suited for inclusion in Specialist High Skills Majors (SHSMs) or in programs designed to provide pathways to particular apprenticeship, college, university, or workplace destinations. In some SHSM programs, courses at Virtual High School can be bundled with other courses to provide the academic knowledge and skills important to particular economic sectors and required for success in the workplace and postsecondary education, including apprenticeship training.

Health and Safety

In order to provide a suitable learning environment for the Virtual High School staff and students, it is critical that classroom practice and the learning environment complies with relevant federal, provincial, and municipal health and safety legislation and by-laws, including, but not limited to, the Workplace Safety and Insurance Act, the Workplace Hazardous Materials Information System (WHMIS), the Food and Drug Act, the Health Protection and Promotion Act, the Ontario Building Code, and the Occupational Health and Safety Act (OHSA). The OHSA requires all schools to provide a safe and productive learning and work environment for both students and employees.

Ethics

Virtual High School courses provide varied opportunities for students to learn about ethical issues and to explore the role of ethics in both public and personal decision making. During the inquiry process, students may need to make ethical judgements when evaluating evidence and positions on various issues, and when drawing their own conclusions about issues, developments, and events. Teachers may need to help students in determining appropriate factors to consider when making such judgements. In addition, it is crucial that Virtual High School teachers provide support and supervision to students throughout the inquiry process, ensuring that students engaged in an inquiry are aware of potential ethical concerns and address them in acceptable ways. Teachers at Virtual High School will ensure that they thoroughly address the issue of plagiarism with students. In a digital world in which there is easy access to abundant information, it is very easy to copy the words of others and present them as one's own. Students need to be reminded, even at the secondary level, of the ethical issues surrounding plagiarism, and the consequences of plagiarism should be clearly discussed before students engage in an inquiry. It is important to discuss not only dishonest plagiarism but also more negligent plagiarism instances. Students often struggle to find a balance between writing in their own voice and acknowledging the work of others in the field. Merely telling students not to plagiarize, and admonishing those who do, is not enough. The skill of writing in one's own voice, while appropriately acknowledging the work of others, must be explicitly taught to all Virtual High School courses. Using accepted forms of documentation to acknowledge sources is a specific expectation within the inquiry and skill development strand for each course.


Effect of High Add Power, Medium Add Power, or Single-Vision Contact Lenses on Myopia Progression in Children: The BLINK Randomized Clinical Trial

Importance: Slowing myopia progression could decrease the risk of sight-threatening complications.

Objective: To determine whether soft multifocal contact lenses slow myopia progression in children, and whether high add power (+2.50 D) slows myopia progression more than medium (+1.50 D) add power lenses.

Design, setting, and participants: A double-masked randomized clinical trial that took place at 2 optometry schools located in Columbus, Ohio, and Houston, Texas. A total of 294 consecutive eligible children aged 7 to 11 years with -0.75 D to -5.00 D of spherical component myopia and less than 1.00 D astigmatism were enrolled between September 22, 2014, and June 20, 2016. Follow-up was completed June 24, 2019.

Interventions: Participants were randomly assigned to wear high add power (n = 98), medium add power (n = 98), or single-vision (n = 98) contact lenses.

Main outcomes and measures: The primary outcome was the 3-year change in cycloplegic spherical equivalent autorefraction, as measured by the mean of 10 autorefraction readings. There were 11 secondary end points, 4 of which were analyzed for this study, including 3-year eye growth.

Results: Among 294 randomized participants, 292 (99%) were included in the analyses (mean [SD] age, 10.3 [1.2] years 177 [60.2%] were female mean [SD] spherical equivalent refractive error, -2.39 [1.00] D). Adjusted 3-year myopia progression was -0.60 D for high add power, -0.89 D for medium add power, and -1.05 D for single-vision contact lenses. The difference in progression was 0.46 D (95% CI, 0.29-0.63) for high add power vs single vision, 0.30 D (95% CI, 0.13-0.47) for high add vs medium add power, and 0.16 D (95% CI, -0.01 to 0.33) for medium add power vs single vision. Of the 4 secondary end points, there were no statistically significant differences between the groups for 3 of the end points. Adjusted mean eye growth was 0.42 mm for high add power, 0.58 mm for medium add power, and 0.66 mm for single vision. The difference in eye growth was -0.23 mm (95% CI, -0.30 to -0.17) for high add power vs single vision, -0.16 mm (95% CI, -0.23 to -0.09) for high add vs medium add power, and -0.07 mm (95% CI, -0.14 to -0.01) for medium add power vs single vision.

Conclusions and relevance: Among children with myopia, treatment with high add power multifocal contact lenses significantly reduced the rate of myopia progression over 3 years compared with medium add power multifocal and single-vision contact lenses. However, further research is needed to understand the clinical importance of the observed differences.

Trial registration: ClinicalTrials.gov Identifier: NCT02255474.

Conflict of interest statement

Conflict of Interest Disclosures: Dr Walline reported receiving nonfinancial support from Bausch + Lomb and grants from the National Eye Institute (NEI) during the conduct of the study and personal fees from SightGlass outside the submitted work. Dr Mutti reported receiving grants from NEI and nonfinancial support from Bausch + Lomb during the conduct of the study and personal fees from Welch Allyn outside the submitted work. Dr Jones-Jordan reported receiving nonfinancial support from Bausch + Lomb and grants from the National Institutes of Health (NIH)/NEI during the conduct of the study. Dr Sinnott reported receiving grants from NIH during the conduct of the study. Dr Giannoni reported receiving grants from NIH/NEI and nonfinancial support from Bausch + Lomb during the conduct of the study. Dr Bickle reported receiving study materials from Bausch + Lomb during the conduct of the study. Dr Schulle reported receiving nonfinancial support from Bausch + Lomb and grants from NIH/NEI during the conduct of the study. Dr Nixon reported receiving grants from NIH and nonfinancial support from Bausch + Lomb during the conduct of the study. Dr Pierce reported receiving grants from NIH during the conduct of the study. Dr Berntsen reported receiving grants from NIH/NEI and nonfinancial support from Bausch + Lomb during the conduct of the study and personal fees from Visioneering Technologies, Inc and Contact Lens Spectrum outside the submitted work. No other disclosures were reported.


Secondary Sources: Fed Sensitivity, Job Growth, Slave Trade

--Fed Sensitivity: The Economist charts the asset classes most responsive to Fed action. "America's jobs market has improved, and markets are now reckoning the Fed will start to taper its bond purchases in coming months. According to EPFR Global, a data provider, this has put pressure on bond prices and investors have fled riskier assets bond fund outflows reached a record in the week ending June 5th. Researchers at Barclays Capital have looked at which assets are most sensitive to the Fed's balance sheet, by dividing the change in the asset prices over periods of QE, by the change in the size of the Fed's balance sheet. At present, markets are adjusting to the Fed's balance sheet merely expanding more slowly than expected. At some point they will have to adjust to its outright shrinkage. Since emerging-market equities and European and American high-yield bonds showed the greatest sensitivity to Fed balance sheet expansion, they could be expected to also fall most when it shrinks. Judged by how an asset deviated from its historical value during QE, Turkish equities and "defensive" stocks (those that do not move with the business cycle, like food) are most vulnerable."

--Job Growth:John Taylor notes that job growth is barely keeping pace with inflation. "Last Friday's employment report revealed little sign of a lift off of the employment to population ratio. It's the same as it was in May of last year—58.6 percent. Employment grew by about 1 percent (or 1.6 million) over the past year. But the working age population also grew at about 1 percent (or 2.4 million), so the ratio of employment to population stayed the same. For the policy book I edited with Lee Ohanian and Ian Wright last year we put the employment to population ratio on the cover. Here's the sad extrapolation."

--Slave Trade:James Fenske and Namrata Kala examine how the slave trade continues to effect the economics of Africa. "The slave trade continues to shape modern Africa. This column analyzes environmental shocks to the supply side of the trans-Atlantic slave trade and their long-term effects. During warm periods, African ports exported fewer slaves because lower agricultural productivity raised slavers' costs. These temperature fluctuations had long-run impacts, and ports that experienced a warmer period during the decades when the slave trade was most active appear more developed today."

Compiled by Phil Izzo

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