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12.2: Internal Leaf Structure - Biology

12.2: Internal Leaf Structure - Biology


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Learning Objectives

  • Describe the microscope internal structure of leaves, including the epidermis, mesophyll, and vascular bundles.
  • Compare the adaptations of mesophytic, hydrophytic, and xerophytic leaves.
  • Identify the unique features of pine and corn leaves.
  • Compare the structures of sun and shade leaves.

Tissue Organization in Leaves

All three tissue types are represented in leaves. The epidermis represents the dermal tissue, the mesophyll that fills the leaf is ground tissue, and the vascular bundles that form the leaf veins represent vascular tissue (Figure (PageIndex{1})). These three tissues will be discussed using a eudicot leaf that is adapted to a moderate amount of water (mesophytic leaf). Variations in leaf structure are discussed later on this page.

Epidermis

The outermost layer of the leaf is the epidermis; it is present on both sides of the leaf and is called the upper and lower epidermis, respectively. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis helps in the regulation of gas exchange. It contains stomata (singular = stoma; Figure (PageIndex{2})), openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing, and the guard cells are sometimes flanked by subsidiary cells. Guard cells are the only epidermal cells to contain chloroplasts. In most cases, the lower epidermis contains more stomata than the upper epidermis because the bottom of the leaf is cooler and less prone to water loss.

The epidermis is usually one cell layer thick; however, in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration. A protective layer called the cuticle covers surface of the epidermal cells (Figure (PageIndex{3})). The cuticle is rich in lignin (which lends some rigidity) and waxes (which function in waterproofing). The cuticle reduces the rate of water loss from the leaf surface. Other leaves may have small hairs (trichomes) on the leaf surface. Trichomes help to deter herbivory by restricting insect movements, or by storing toxic or bad-tasting compounds; they can also reduce water loss by blocking air flow across the leaf surface (Figure (PageIndex{4})). For this reason, trichomes (like stomata) are frequently denser on the lower side of the leaf.

Mesophyll

Below the epidermis are layers of cells known as the mesophyll, or “middle leaf.” Mesophyll cells contain many chloroplasts and specialize in photosynthesis. The mesophyll of most leaves typically contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma (Figure (PageIndex{5})). The palisade parenchyma (also called the palisade mesophyll) has column-shaped and may be present in one, two, or three layers. The palisade cells specialize in capturing incoming sunlight (including slanted sun rays), rotating chloroplasts to the top of the leaf and then allowing them to regenerate by cycling them toward the leaf's center. They also decrease the intensity of sunlight for the spongy mesophyll. Although palisade cells may appear tightly packed in a cross section because there are many rows of cells behind those in the foreground, there is actually ample space (intercellular air spaces) between them. Below the palisade parenchyma are seemingly loosely arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The intercellular air spaces found between mesophyll cells facilitate gaseous exchange.

Vascular Bundles (Veins)

Like the stem, the leaf contains vascular bundles composed of xylem and phloem (Figure (PageIndex{6-7})). When a typical stem vascular bundle (which has xylem internal to the phloem) enters the leaf, xylem usually faces upwards, whereas phloem faces downwards. The conducting cells of the xylem (tracheids and vessel elements) transport water and minerals to the leaves. The sieve-tube elements of the phloem transports the photosynthetic products from the leaf to the other parts of the plant. The phloem is typically supported by a cluster of fibers (sclerenchyma) that increase structural support for the veins. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.

Leaf Adaptations

The broad, flat shape of most leaves increases surface area relative to volume, which helps it capture sunlight; however this also provides more opportunity for water loss. The anatomy of a leaf has everything to do with achieving the balance between photosynthesis and water loss in the environment in which the plant grows. Plants that grow in moist areas can grow large, flat leaves to absorb sunlight like solar panels because sunlight is likely more limiting than water. Plants in dry areas must prevent water loss and adapt a variety of leaf shapes and orientations to accomplish the duel tasks of water retention and sunlight absorption. In general, leaves adapted to dry environments are small and thick with a much lower surface area-to-volume ratio.

In regards to water, there are three main types of plants: mesophytes, hydrophytes, and xerophytes. Mesophytes are typical plants which adapt to moderate amounts of water ("meso" means middle, and "phyte" means plant). Many familiar plants are mesophytes, such as lilac, Ranunculus (buttercup), roses, etc. Hydrophytes grow in water ("hydro" refers to water). Their leaf blades are frequently highly dissected (deeply lobed) to access gases dissolved in water, and their petioles and stems have air canals to supply underwater organs with gases. Hygrophytes (not discussed further) live in constantly wet environment, their leaves adapted to rapidly release water through the stomata. They sometimes even excrete of water drops through the leaf margins (guttation). Xerophytes are adapted to the scarce water ("xero" refers to dryness). Xerophytes are found in deserts and Mediterranean climates (such as in much of California), where summers are hot and dry. The leaves of mesophytes are called mesophytic, hydrophyte leaves are called hydrophytic, and so on. The structure of mesophytic leaves was already described (Figure (PageIndex{1})). Adaptaions in hydrophytic and xerophytic leaves and discussed below in more detail.

Hydrophytic Leaves

The structure of a hydrophytic leaf differs from a mesophytic leaf due to selective pressures in the environment -- water is plentiful, so the plant is more concerned with staying afloat and preventing herbivory. Hydrophytic leaves have a thin epidermal layer and the absence of stomata in the lower epidermis (Figure (PageIndex{8})). In the spongy mesophyll, there are large pockets where air can be trapped, helping the leaf float. This type of parenchyma tissue, specialized for trapping gases, is called aerenchyma. Sharp, branched sclereids (astrosclereids) traverse the mesophyll of a hydrophytic leaf. These provide the leaf structural support, as well as prevention of herbivory. Vascular tissue is somewhat reduced in hydrophytic leaves.

Xerophytic Leaves

Xerophytic leaves (Figure (PageIndex{9})) have thick cuticles to limit water loss, especially on the upper epidermis (Figure (PageIndex{10})). Both the upper and lower epidermis consists of several layers (multiple epidermis). Sometimes the additional layers are called the hypodermis ("hypo" meaning under; "dermis" meaning skin). Depressions in the lower epidermis creates a pockets that are lined with trichomes, and the stomata are located at the base of these pockets (called stomatal crypts; figure (PageIndex{10})). The trichomes help capture evaporating moisture and maintain a relatively humid environment around the stomata. These stomatal crypts are located only on the underside of the leaves, where they experience less sun exposure and therefore less water loss. The upper epidermis is free from stomata.

Pine Leaves

Pines evolved during a period in Earth’s history when conditions were becoming increasingly dry, and pine needles have many adaptations to deal with these conditions. Many of these adaptations are similar the xerophytic leaves of some angiosperms (described above) because pines themselves are xerophytes.

The epidermis of the leaf seems to be more than one cell layer thick (figure (PageIndex{11})). These subsequent layers of epidermis-like tissue under the single, outer layer of true epidermis are called the hypodermis , which offers a thicker barrier and helps prevent water loss. The epidermis itself is coated on the outside by a thick layer of wax called the cuticle. Because waxes are hydrophobic, this also helps prevent water loss through the epidermis. The stomata are typically sunken, occurring within the hypodermis instead of the epidermis. Sunken stomata create a pocket of air that is protected from the airflow across the leaf and can aid in maintaining a higher moisture content (figure (PageIndex{12})).

Within the mesophyll, there are several canals that appear as large, open circles in the cross section of the leaf. These are resin canals. The cells lining them secrete resin (the sticky stuff that coniferous trees exude, often called pitch), which contains compounds that are toxic to insects and bacteria. When pines evolved, not only was the Earth becoming drier, but insects were evolving and proliferating. These resin canals are not features that help the plant survive dry conditions, but they do help prevent herbivory. In addition to prevention of herbivory, resin can aid in closing wounds and preventing infection at wound sites.

There are two bundles of vascular tissue embedded within a region of cells called transfusion tissue, which functions in transporting materials to and from the mesophyll cells. The transfusion tissue and vascular bundles are surrounded by a distinct layer of cells called the endodermis. This is similar to the tissue of the same name in the root, but the cells are not impregnated with the water-repelling compound suberin.

Finally, the overall shape of the leaf allows for as little water loss as possible by decreasing the relative surface area, taking a rounder shape as opposed to a flatter one. This low surface area-to-volume ratio is characteristic of xerophytes.

Corn Leaves

The model organism for monocots in botany is usually corn (Zea mays). In corn, there are approximately the same number of stomata on both the upper and lower epidermis. The mesophyll is not divided into two distinct types. The vascular bundles all face the same directly (appearing circular in cross section) because they run parallel to each other.

Corn is not necessarily a xerophyte, but it is adapted to deal with high temperatures. One of these adaptations, C4 type photosynthesis is discussed in Photorespiration and Photosynthetic Pathways and results in a cell arrangement called Kranz anatomy. The vascular bundles are surrounded by obviously inflated parenchyma cells that form a structure called a bundle sheath, and these are packed with chloroplasts (Figure (PageIndex{13})). (Bundle sheaths surround vascular bundles of other types of leaves as well, but the bundle sheath cells are much smaller). Mesophyll cells encircle the bundle sheath cells. In C4 photosynthesis, carbon dioxide is first gathered by the mesophyll cells and temporarily stored as a four-carbon sugar. This four-carbon sugar is transferred to the bundle sheath cells, where it is broken down to release carbon dioxide. It is in the bundle sheath cells where a process called the Calvin cycle, and glucose is ultimately produced. C4 photosynthesis concentrates carbon dioxide inside the bundle sheath cells, reducing the need to frequently open stomata for gas exchange. This helps conserve water.

When moisture is plentiful, the corn leaves are fully expanded and able to maximize photosynthesis. When moisture is limited, the leaves roll inward, limiting both moisture loss and photosynthetic capacity. This is accomplished by the presence of bulliform cells in the upper epidermis (Figure (PageIndex{14})). These clusters of enlarged cells are swollen with water when there is abundant water available. As the water content in the plant decreases, these cells shrivel, causing the upper epidermis to curl or fold inward at these points. This adaptation to sun exposure can be found in many other grasses, as well (corn is a member of the Poaceae, the grass family).

Sun and Shade Leaves

The light intensity experienced by a developing leaf influences its structure. Leaves that develop when consistently exposed to direct sunlight (sun leaves) thus differ from leaves exposed to low light intensities (shade leaves) in several ways (Figure (PageIndex{15})). Relative to shade leaves, sun leaves are smaller and thicker. This reduces surface area relative to volume, conserving water, which would otherwise be easily lost under bright sunlight and resultantly warmer temperatures. In contrast, the broad, thin shape of shade leaves helps capture sufficient light when light intensity is low. The thicker cuticle of sun leaves also limits water loss. They have more palisade parenchyma and more vascular tissue. Sun leaves can maintain a high photosynthetic rate at high light intensities, but shade leaves cannot.


Leaf structure and Adaptations for Photosynthesis: Grade 9 Understanding for IGCSE Biology 2.21

The leaf is the organ in a plant specially adapted for photosynthesis. You need to understand the structure of the tissues in a leaf together with their functions.

Upper Epidermis: this is the tissue on the upper surface of the leaf. It produces a waxy layer, called the cuticle, which is not made of cells but is a waterproof barrier to prevent excessive evaporation through the hot upper surface of the leaf. The upper epidermis cells have no chloroplasts so light passes through them easily.

Palisade Mesophyll: this tissue is where 80% of the photosynthesis takes place in the leaf. The palisade cells have many chloroplasts in their cytoplasm and the box-like shape and arrangement of these cells ensures they are packed tightly together.

Spongy Mesophyll: this tissue contains large air spaces which are linked to the atmosphere outside the leaf through microscopic pores called stomata on the lower surface. Spongy mesophyll cells also contain chloroplasts and photosynthesis occurs here too. The air spaces reduce the distance carbon dioxide has to diffuse to get into the mesophyll cells and the fact that these cells have fairly thin cell walls which are coated with a film of water together means that gas exchange between air space and mesophyll is speeded up.

Lower Epidermis is the most dull tissue in the leaf. The only interesting thing about it is that it contains specialised cells called guard cells which enclose a pore called a stoma. Carbon dioxide can diffuse into the leaf through the stomata when they are open (usually at day time) and water evaporates out of the stomata in a process called transpiration.

Adaptations of a Leaf for Photosynthesis

  • Large Surface Area – to maximise light harvesting
  • Thin – to reduce distance for carbon dioxide to diffuse through the leaf and to ensure light penetrates into the middle of the leaf
  • Air Spaces – to reduce distance for carbon dioxide to diffuse and to increase the surface area of the gas exchange surface inside the leaf
  • Stomata – pores to allow carbon dioxide to diffuse into the leaf and water to evaporate out (transpiration)
  • Presence of Veins – veins contain xylem tissue (carries water and minerals to the leaf from the roots) and phloem (transports sugars and amino acids away from the leaf)
  • Chloroplasts – mesophyll cells and guard cells contain many chloroplasts. These organelles contain the light harvesting pigment chlorophyll and are where all the reactions of photosynthesis occur

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Internal Tree Leaf Structures

The leaf blade is composed of tissue layers, each having an important part to play in a functioning leaf. Find these structures on the attached diagram of cellular leaf tissues.

Epidermis – The leaf's outer layer and protective "skin" surrounding leaf tissues.

Cuticle – A waxy protective coating on the leaf epidermis that prevents water loss on leaves, green stems, and fruits.

Leaf hairs – Coverings on a leaf's epidermis that may or may not exist with every tree species.

Palisade layer – A tightly packed layer of long tube-like parenchyma tissues filled with chloroplasts for photosynthesis.

Chloroplasts – Sub-cellular, photosynthetic structures in leaves and other green tissues. Chloroplasts contain chlorophyll, a green plant pigment that captures the energy in light and begins the transformation of that energy into sugars.

Vascular bundle – Xylem and phloem tissues, commonly known as leaf veins.

Spongy mesophyll – Layer of parenchyma tissues loosely arranged to facilitate movement of oxygen, carbon dioxide, and water vapor. It also may contain some chloroplasts.

Stomata – Natural openings in leaves and herbaceous stems that allow for gas exchange (water vapor, carbon dioxide and oxygen).

Guard cells – Specialized kidney-shaped cells that open and close the stomata.


Leaf Structure Observations

When viewing the surface of the leaf under the stereo microscope, students will be able to clearly see hair-like structures (trichome) on the leaf surface that serve a number of functions ranging from trapping insects to trapping water/moisture. Students will also observe the intricate leaf veins (vascular bundles) running across the surface of the leaf.

With some leaves (such as the maple leaf), it's possible to isolate the vascular bundles (vein structures) for viewing under the microscope.

  • Simmer the leaf for about an hour and a half
  • Once the leaf starts feeling slimy, remove from the pot and place on a plate/Petri dish
  • Add a small amount of water and gently remove the soft part using a small brush from both sides of the leaf
  • Place the leaf vein (vascular bundles) between two hard surfaces (such as a book) to prevent from twisting
  • View the leaf vein under the microscope (stereo microscope or under low power on compound microscope)

Plant Organs

Like animals, plants contain cells with organelles in which specific metabolic activities take place. Unlike animals, however, plants use energy from sunlight to form sugars during photosynthesis. In addition, plant cells have cell walls, plastids, and a large central vacuole: structures that are not found in animal cells. Each of these cellular structures plays a specific role in plant structure and function.

In plants, just as in animals, similar cells working together form a tissue. When different types of tissues work together to perform a unique function, they form an organ organs working together form organ systems. Vascular plants have two distinct organ systems: a shoot system, and a root system. The shoot system consists of two portions: the vegetative (non-reproductive) parts of the plant, such as the leaves and the stems, and the reproductive parts of the plant, which include flowers and fruits. The shoot system generally grows above ground, where it absorbs the light needed for photosynthesis. The root system, which supports the plants and absorbs water and minerals, is usually underground. Figure 6 shows the organ systems of a typical plant.

Figure 6. The shoot system of a plant consists of leaves, stems, flowers, and fruits. The root system anchors the plant while absorbing water and minerals from the soil.


Function of the Leaf

As one of the most important constituents of plants, leaves have several essential functions:

Photosynthesis

The primary function of the leaf is the conversion of carbon dioxide, water, and UV light into sugar (e.g., glucose) via photosynthesis (shown below). The simple sugars formed via photosynthesis are later processed into various macromolecules (e.g., cellulose) required for the formation of the plant cell wall and other structures. Therefore, the leaf must be highly specialized to combine the carbon dioxide, water, and UV light for this process. Carbon dioxide is diffused from the atmosphere through specialized pores, termed stomata, in the outer layer of the leaf. Water is directed to the leaves via the plant’s vascular conducting system, termed the xylem. Leaves are orientated to ensure maximal exposure to sunlight, and are typically thin and flat in shape to allow sunlight to penetrate the leaf to reach the chloroplasts, which are specialized organelles that perform photosynthesis. Once sugar is formed from photosynthesis, the leaves function to transport it down the plant via specialized structures called the phloem, which run in parallel to the xylem. The sugar is typically transported to the roots and shoots of the plant, to support growth.

Transpiration

Transpiration refers to the movement of water through the plant, and subsequent evaporation via the leaves. When the stomata open to accommodate the diffusion of carbon dioxide into the plant for photosynthesis, water flows out. This process also serves to cool the plant via evaporation of the water from the leaf, as well as regulate the plant’s osmotic pressure.

Guttation

Guttation refers to the excretion of xylem from the edges of leaves and other vascular plants due to increased levels of water in the soil at night, when the stomata are closed. The pressure caused at the roots results in the leakage of water from the xylem out of specialized water glands at the edges of leaves.

Storage

Leaves are a primary site of water and energy storage since they provide the site of photosynthesis. Succulents are particularly adept at water storage, as evidenced by the thick leaves. Due to the high levels of nutrients and water, many animal species ingest the leaves of plants as a source of food.

Defense

In general, the types of leaf can be divided into six major types, although there are also plants with highly specialized leaves:

Conifer Leaf

Conifer leaves are needle-shaped or in the form of scales. Conifer leaves are typically heavily waxed and highly adapted to colder climates, arranged to dispel snow and resist freezing temperatures. Some examples include Douglas firs and spruce trees. The images below illustrate this type of leaf.

Microphyll Leaf

Microphyll leaves are characterized by a single vein that is unbranched. Although this type of leaf is abundant in the fossil record, few plants exhibit this type of leaf today. Some examples include horsetails and clubmosses. The image below illustrates this type of leaf.

Megaphyll Leaf

Megaphyll leaves are characterized by multiple veins that can be highly branched. Megaphyll leaves are broad and flat, and generally comprise the foliage of most plant species. The image below illustrates this type of leaf.

Angiosperm Leaf

Angiosperm leaves are those found on flowering plants. These leaves are characterized by stipules, a lamina, and a petiole. The illustration below shows an example of an angiosperm leaves.

Fronds

Fronds are large, divided leaves characteristic of ferns and palms. The blades can be singular or divided into branches. The image below presents an example of a frond.

Sheath Leaf

Sheath leaves are typical of grass species and monocots. Thus, the leaves are long and narrow, with a sheathing surrounding the stem at the base. Moreover, the vein structure is striated and each node contains only one leaf. The image below presents an example of a sheath leaf.

1. The primary function of a leaf is:
A. Water evaporation for cooling
B. Photosynthesis
C. Provide shade to the shoot and root structures of the plant
D. Transpiration

2. Which of the following statements is TRUE regarding guttation:
A. It typically occurs at night.
B. It occurs when the stomata are closed.
C. It results from increased water pressure in the soil.
D. All of the above


Types of Leaf Forms

Leaves may be categorized as simple or compound, depending on how their blade (or lamina) is divided.

Learning Objectives

Differentiate among the types of leaf forms

Key Takeaways

Key Points

  • In a simple leaf, the blade is completely undivided leaves may also be formed of lobes where the gaps between lobes do not reach to the main vein.
  • In a compound leaf, the leaf blade is divided, forming leaflets that are attached to the middle vein, but have their own stalks.
  • The leaflets of palmately-compound leaves radiate outwards from the end of the petiole.
  • Pinnately-compound leaves have their leaflets arranged along the middle vein.
  • Bipinnately-compound (double-compound) leaves have their leaflets arranged along a secondary vein, which is one of several veins branching off the middle vein.

Key Terms

  • simple leaf: a leaf with an undivided blade
  • compound leaf: a leaf where the blade is divided, forming leaflets
  • palmately compound leaf: leaf that has its leaflets radiating outwards from the end of the petiole
  • pinnately compound leaf: a leaf where the leaflets are arranged along the middle vein

Leaf Form

There are two basic forms of leaves that can be described considering the way the blade (or lamina) is divided. Leaves may be simple or compound.

Simple and compound leaves: Leaves may be simple or compound. In simple leaves, the lamina is continuous. (a) The banana plant (Musa sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. (b) In palmately compound leaves, such as those of the horse chestnut (Aesculus hippocastanum), the leaflets branch from the petiole. (c) In pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory (Carya floridana). (d) The honey locust has double compound leaves, in which leaflets branch from the veins.

In a simple leaf, such as the banana leaf, the blade is completely undivided. The leaf shape may also be formed of lobes where the gaps between lobes do not reach to the main vein. An example of this type is the maple leaf.

In a compound leaf, the leaf blade is completely divided, forming leaflets, as in the locust tree. Compound leaves are a characteristic of some families of higher plants. Each leaflet is attached to the rachis (middle vein), but may have its own stalk. A palmately compound leaf has its leaflets radiating outwards from the end of the petiole, like fingers off the palm of a hand. Examples of plants with palmately compound leaves include poison ivy, the buckeye tree, or the familiar house plant Schefflera sp. (commonly called “umbrella plant”). Pinnately compound leaves take their name from their feather-like appearance the leaflets are arranged along the middle vein, as in rose leaves or the leaves of hickory, pecan, ash, or walnut trees. In a pinnately compound leaf, the middle vein is called the midrib. Bipinnately compound (or double compound) leaves are twice divided the leaflets are arranged along a secondary vein, which is one of several veins branching off the middle vein. Each leaflet is called a “pinnule”. The pinnules on one secondary vein are called “pinna”. The silk tree (Albizia) is an example of a plant with bipinnate leaves.


Leaf Tissues

Leaf tissues are composed of layers of plant cells. Different plant cell types form three main tissues found in leaves. These tissues include a mesophyll tissue layer that is sandwiched between two layers of epidermis. Leaf vascular tissue is located within the mesophyll layer.

The outer leaf layer is known as the epidermis. The epidermis secretes a waxy coating called the cuticle that helps the plant retain water. The epidermis in plant leaves also contains special cells called guard cells that regulate gas exchange between the plant and the environment. Guard cells control the size of pores called stomata (singular stoma) in the epidermis. Opening and closing the stomata allows plants to release or retain gases including water vapor, oxygen, and carbon dioxide as needed.

The middle mesophyll leaf layer is composed of a palisade mesophyll region and a spongy mesophyll region. Palisade mesophyll contains columnar cells with spaces between the cells. Most plant chloroplasts are found in palisade mesophyll. Chloroplasts are organelles that contain chlorophyll, a green pigment that absorbs energy from sunlight for photosynthesis. Spongy mesophyll is located below palisade mesophyll and is composed of irregularly shaped cells. Leaf vascular tissue is found in the spongy mesophyll.

Vascular Tissue

Leaf veins are composed of vascular tissue. Vascular tissue consists of tube-shaped structures called xylem and phloem that provide pathways for water and nutrients to flow throughout the leaves and plant.


A lymphocyte is a white blood cell that contains a large nucleus (Figure 12.10). Most lymphocytes are associated with the adaptive immune response, but infected cells are identified and destroyed by natural killer cells, the only lymphocytes of the innate immune system. A natural killer (NK) cell is a lymphocyte that can kill cells infected with viruses (or cancerous cells). NK cells identify intracellular infections, especially from viruses, by the altered expression of major histocompatibility class (MHC) I molecules on the surface of infected cells. MHC class I molecules are proteins on the surfaces of all nucleated cells that provide a sample of the cell’s internal environment at any given time. Unhealthy cells, whether infected or cancerous, display an altered MHC class I complement on their cell surfaces.

Figure 12.10. Micrograph shows a round cell with a large nucleus occupying more than half of the cell.

After the NK cell detects an infected or tumor cell, it induces programmed cell death, or apoptosis. Phagocytic cells then come along and digest the cell debris left behind. NK cells are constantly patrolling the body and are an effective mechanism for controlling potential infections and preventing cancer progression. The various types of immune cells are shown in Figure 12.11.

Figure 12.11 Cells involved in the innate immune response include mast cells, natural killer cells, and white blood cells, such as monocytes, macrophages and neutrophils.


Contents

Some studies of plant anatomy use a systems approach, organized on the basis of the plant's activities, such as nutrient transport, flowering, pollination, embryogenesis or seed development. [4] Others are more classically [5] divided into the following structural categories:

About 300 BC Theophrastus wrote a number of plant treatises, only two of which survive, Enquiry into Plants (Περὶ φυτῶν ἱστορία), and On the Causes of Plants (Περὶ φυτῶν αἰτιῶν). He developed concepts of plant morphology and classification, which did not withstand the scientific scrutiny of the Renaissance.

A Swiss physician and botanist, Gaspard Bauhin, introduced binomial nomenclature into plant taxonomy. He published Pinax theatri botanici in 1596, which was the first to use this convention for naming of species. His criteria for classification included natural relationships, or 'affinities', which in many cases were structural.

It was in the late 1600s that plant anatomy became refined into a modern science. Italian doctor and microscopist, Marcello Malpighi, was one of the two founders of plant anatomy. In 1671 he published his Anatomia Plantarum, the first major advance in plant physiogamy since Aristotle. The other founder was the British doctor Nehemiah Grew. He published An Idea of a Philosophical History of Plants in 1672 and The Anatomy of Plants in 1682. Grew is credited with the recognition of plant cells, although he called them 'vesicles' and 'bladders'. He correctly identified and described the sexual organs of plants (flowers) and their parts. [6]

In the eighteenth century, Carl Linnaeus established taxonomy based on structure, and his early work was with plant anatomy. While the exact structural level which is to be considered to be scientifically valid for comparison and differentiation has changed with the growth of knowledge, the basic principles were established by Linnaeus. He published his master work, Species Plantarum in 1753.

In 1802, French botanist Charles-François Brisseau de Mirbel, published Traité d'anatomie et de physiologie végétale (Treatise on Plant Anatomy and Physiology) establishing the beginnings of the science of plant cytology.

In 1812, Johann Jacob Paul Moldenhawer published Beyträge zur Anatomie der Pflanzen, describing microscopic studies of plant tissues.

In 1813 a Swiss botanist, Augustin Pyrame de Candolle, published Théorie élémentaire de la botanique, in which he argued that plant anatomy, not physiology, ought to be the sole basis for plant classification. Using a scientific basis, he established structural criteria for defining and separating plant genera.

In 1830, Franz Meyen published Phytotomie, the first comprehensive review of plant anatomy.

In 1838 German botanist Matthias Jakob Schleiden, published Contributions to Phytogenesis, stating, "the lower plants all consist of one cell, while the higher plants are composed of (many) individual cells" thus confirming and continuing Mirbel's work.

A German-Polish botanist, Eduard Strasburger, described the mitotic process in plant cells and further demonstrated that new cell nuclei can only arise from the division of other pre-existing nuclei. His Studien über Protoplasma was published in 1876.

Gottlieb Haberlandt, a German botanist, studied plant physiology and classified plant tissue based upon function. On this basis, in 1884 he published Physiologische Pflanzenanatomie (Physiological Plant Anatomy) in which he described twelve types of tissue systems (absorptive, mechanical, photosynthetic, etc.).

British paleobotanists Dunkinfield Henry Scott and William Crawford Williamson described the structures of fossilized plants at the end of the nineteenth century. Scott's Studies in Fossil Botany was published in 1900.

Following Charles Darwin's Origin of Species a Canadian botanist, Edward Charles Jeffrey, who was studying the comparative anatomy and phylogeny of different vascular plant groups, applied the theory to plants using the form and structure of plants to establish a number of evolutionary lines. He published his The Anatomy of Woody Plants in 1917.

The growth of comparative plant anatomy was spearheaded by British botanist Agnes Arber. She published Water Plants: A Study of Aquatic Angiosperms in 1920, Monocotyledons: A Morphological Study in 1925, and The Gramineae: A Study of Cereal, Bamboo and Grass in 1934. [7]

Following World War II, Katherine Esau published, Plant Anatomy (1953), which became the definitive textbook on plant structure in North American universities and elsewhere, it was still in print as of 2006. [8] She followed up with her Anatomy of seed plants in 1960.


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