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How do plants reproduce?
Is it really due to the birds and the bees? Not always. Even though it is spotted, this plant is known as the kangaroo fern, not the cheetah fern. And all those spots are spores. So what's a spore? Each spore can grow into a new individual without the need for fertilization.
Life Cycle of Plants
All plants have a characteristic life cycle that includes alternation of generations. Plants alternate between haploid and diploid generations. Alternation of generations allows for both asexual and sexual reproduction. Beginning with the diploid sporophyte, spores form from meiosis. Asexual reproduction with spores produces haploid individuals called gametophytes, which produce haploid gametes by mitosis. Sexual reproduction with gametes and fertilization produces the diploid sporophyte. A typical plant’s life cycle is diagrammed in Figure below.
Life Cycle of Plants. This diagram shows the general life cycle of a plant.
Early plants reproduced mainly with spores and spent most of their life cycle as haploid gametophytes. Spores require little energy and matter to produce, and they grow into new individuals without the need for fertilization. In contrast, most modern plants reproduce with gametes using pollen and seeds, and they spend most of their life cycle as diploid sporophytes. Many modern plants can also reproduce asexually using roots, stems, or leaves. This is called vegetative reproduction. One way this can occur is shown in Figure below.
Strawberry plants have horizontal stems called stolons that run over the ground surface. If they take root, they form new plants.
- All plants have a characteristic life cycle that includes alternation of generations.
- Asexual reproduction with spores produces a haploid gametophyte generation.
- Sexual reproduction with gametes and fertilization produces a diploid sporophyte generation.
- Define alternation of generations.
- What type of reproduction occurs in an alternation of generations life cycle?
- Draw a diagram of a typical plant life cycle that illustrates the concept of alternation of generations.
Life Cycle of Selaginella (With Diagram) | Plants
Selaginella is a very large genus of about 700 tropical to temperate species growing almost everywhere on this Earth. Although so large, the genus shows a uniformity lacking in the genus Lycopodium. The genus is more abundant in the tropical rain forests in weak light and is also common in the warmer plains of India where Lycopodium does not survive. Some species are also xerophytic.
Most of them grow perennially by vegetative propagation while some are annual.
Some of the species are rather small (a few centimstres) resembling mosses while others may grow even to a length of 20 metres. 58 species are recorded by Alston (1945) from India which include Selaginella rupestris, S. pentagona, S. megaphylla, S. proniflora, S. ciliaris, S. delicatula, S. chiysocaulos, S. pallidissima, S. sanguinolenta, etc. S. kraussiana from Africa is commonly grown in greenhouses as an ornamental plant.
Selaginella shows considerable variation in the symmetry of the plant but most species are prostrate and creeping, a few are even climbing. A few xerophytic species (e.g., S. lepidophylla and S. pilifera) typically roll up the leaves and become ball-like but open up into normal green plants when put in water even when dead. These are often sold inside bottles as curiosities.
Selaginella shows great uniformity even cytologically. Most species show the basic number n = 9. A few species are polyploids. However, the genus Selaginella has been divided into two subgenera according to the sporophytic form. In the subgenus Homoeophyllum (Selaginella rupestris, etc.), the plant body has a tendency to be upright with radially arranged, more or less uniform (isophyllous) leaves.
In the sub­genus Heterophyllum the plant body is more or less dorsiventral and prostrate with a distinct heterophylly (anisophyily) of the leaves. There are only about 50 species, in the subgenus Homoeophyllum while all the rest are in the subgenus Heterophyllum. As the latter is the commoner and more typical form this is being described in what follows. The most typical species is Selaginella kraussiana.
Figure 553 shows diagrammatically the life cycle of Selaginella.
The common Selaginella (viz., S. kraussiana— Fig. 546A & B) has a horizontal creeping stem branching along one plane giving rise to a more or less dorsiventral plant body. The branching is dichotomous but often appears to be monopodial because of stronger growth of the main stem. Here and there rhizophores grow vertically down from the forking’s of the dichotomies.
Adventitious roots develop where the rhizophores meet the soil. Plants are often ‘floating’ being kept propped up by the rhizophores.
In the Heterophyllum subgenus of Selaginella the leaves are of two types (dimorphic). There are four rows of leaves—two rows of smaller leaves on the upper side and two rows of larger leaves on the lower side. Actually, the leaves are arranged in an artiso-phyllous opposite order (Fig. 546C) though in some species the arrangement may look spiral.
At each node there is one small leaf on the upper side and one large leaf on the lower side. Flattening (dorsiventrality) of the stem may disturb this arrangement.
The leaves (Fig. 546C) are microphyllous with a single median vein and lanceolate to ovate in shape becoming narrower at stem attachment. In some species the smaller leaves become filiform. Each leaf bears on its upper surface near its base a small, membranous, tongue-like outgrowth called the ligule. The ligule shrinks and becomes inconspicuous in mature leaves.
The original embryonic root is short-living. All other roots are adventitious from the tips of the rhizophores.
The apical growth of most species is by a single pyramidal cell with three cutting faces. But, in a few species a group of meristem cells has been observed.
The anatomy of the mature stem is very distinct. A t.s. of S. kraussiana (Fig. 547A) shows a single layer of epidermal piosenchymatous fibres without any stomatal opening. Below it is the cortex of angular parenchymatous cells usually without intercellular spaces. The outer cortex cells are thickened in some, specially, xerophytic species.
The vascular cylinder in Selaginella species varies from protostelic to siphonostelic and monostelic to polystelic (with 2 to 16 separate steles). But, on the whole the stelar arrangement is simpler than in Lycopodium. S. kraussiana is distelic. The course of the steles is shown in Fig. 547B.
Each stele is surrounded by an endodermis which is a normal compact ring is very young stems but each cell becomes greatly elongated in the axis radial to the stele and also laterally separated from one another by big air gaps in the mature stem. Thus, the endodermal cells in the mature stem are the trabeculae of long radiating cells bridging the wide space between the steles and the inner face of the cortex.
In spite of this radial elongation, the endodermal cells still retain the transverse girdles of casparian strips. Inside each endodermis is one layer of peri-cycle enclosing a simple protostele.
In the stele there is an exarch xylem at the centre, the metaxylem showing stalariform tracheides and a protoxylem lies towards the circumference. The xylem is surrounded by one or two layers of parenchyma which again is surrounded by a single layer of sieve tubes with sieves on lateral walls forming the phloem outside which is the peri-cycle.
In some other species of Selaginella some complexities are noted in the stele. While secondary growth is not normal, in S. selagirvlloides a few secondary xylems do develop.
In certain species (e.g., S. rupestris, S. densa, S. arizonica, S. rupicola) the raetaxylem is almost wholly formed of true vessels formed by the dissolution of end walls with only a few trachcides intermixed. These are usually species showing spiral arrangement of vegetative leaves.
The adventitious roots (all roots in a mature plant are adventitious) are delicate, sparingly branched structures. A t.s. of a root shows a very simple arrangement with a small monarch stele.
A t.s. of the rhizophore which shows a monarch arrangement is also as simple as that of a root. This fact, combined with its positively geotropic nature, has led some to believe that it is a root. But, its exogenous origin, lack of a cap and the experimental production of leafy shoot from the apices of decapitated rhizophores seem to prove that it is actually only a modified stem. Bower (1908) and Goebel (1905), however, concluded that it is an organ sui generis which is neither a root nor a stem.
A t.s. of the leaf (Fig. 548A) shows a simple organisation. There is a simple vas­cular bundle at the centre. The epidermis is one layer thick and the stomata are usually present on the lower epi­dermis only. The mesophyll is uniformly formed either of all spongy or all pali­sade-like elongated cells full of air spaces. Meso­phyll cells have single cup- shaped or many chloroplastids according to species.
In all cases there are several spindle-shaped, pyrenoid-like bodies at the centre of each chloroplast. The bodies may be trans- through ligule at base of leaf, formed into rudimentary starch grains. The only other Embryophyte with similar chloroplasts is the Anthocerotopsida.
The ligule (Fig. 548B) is differentiated rather early. It matures and withers away also early. It has a cup-shaped sheath at base. The sheath cells in S. kraussiana have casparian strips as in an endodermis. Immediately above the sheath is the hemi­spherical Glossop-odium formed of large cells full of a greatly vacuolated protoplasm.
The structure of the ligule has suggested that it is a water-absorbing or exuding structure serving for the protection of the growing leaf or the sporangium.
When the plants become mature the branches terminate in the sporangiferous spike or strobili. The sporophylls are all alike resembling the larger vegetative leaves but the sporangia are of two types—microsporangia and megasporangia (or macrosporangia) so that the sporophylls also are called microsporophylls and megasporophylls.
The sporophylls are spirally arranged on the stem roughly forming four vertical rows. In most species microsporophylls and megasporophylls are borne on the same strobilus but in a few cases they may be in different strobili. The position of the micro- and megasporophylls vary in the different species.
In some the microsporophylls are on top and the mega­sporophylls bellow, in others the microsporophylls are on one side of the strobili while the megasporophylls are on the other vertical half. In S. kraussiana (Fig. 549A) the megasporophylls occupy a part of the base on one half.
In the development of the sporangium, each sporangium develops from a group of initials (it has been disputed that in some cases there is a single initial) either on the stem immediately on the sporophyll or at the axil of the sporophyll (Fig. 549B & G).
When mature, the sporangium is always located at the axil of the sporophyll below the ligule and is reniform or obovoid in appearance with a stalk. The initial cells divide into the upper jacket initials and the lower archesporial cells.
The former develops a jacket two cells in thickness and the latter differentiates into an outer tapetum and an inner group of spore mother cells. In the microsporangium (Fig. 549D) most of the spore mother cells are functional and give rise to the microspore tetrads on reduction division. In the mega-sporangium only one spore mother cell remains functional.
On reduc­tion division this gives rise to a tetrad of megaspores arranged tetrahedrally. The mega- spores increase greatly in size with the mega-sporangium so that a megaspore is much larger than a microspore and a mega-sporangium is larger than a microsporangium.
Enlargement of the megaspores cause the mega-sporangium (Fig. 549E) to have a four-lobed appearance, each lobe enclosing a megaspore. This pressure and the drying up of the sporangial wall cause a splitting of the mega-sporangium into two valves along an un-thickened vertical line on the wall. In some cases some of the four megaspores may even fail to enlarge giving rise to mega-sporangia with 1, 2 or 3 megaspores.
The Male and the Female Gametophytes:
The microspores are very small and with a 2-layered wall (exine and intine). The exine is usually ornamented with spines. Its shape is more or less tetrahedral with one side rounded. This soon germinates to form the male gametophyte. A small lens- shaped prothallial cell is cut off on one side and the other large cell becomes the aniheridial Initial (Fig. 550A).
The antheridial initial develops an antheridium which shows four central primary somatogenous cells surrounded by a jacket of 8 cells (Fig. 550B). The microscope may be shed from the plant during any of these stages. After shedding, the primary spermatogenous cells undergo repeated divisions forming up to 256 sperm cells.
Meanwhile, the exine cracks and the jacket cells with the prothallial cell disintegrate so that the bi-ciliated sperms (Fig. 550D) swim freely inside the mature gametophyte (Fig. 550C).
The megaspore also shows a 2-layered wall with a very thick and sculptured exine and a thin inline. A triradiate ridge is prominent on the top of the tetrahedral structure (Fig. 551 A).
The stage at which the megaspore is shed is extremely variable. It may be shed from the sporangium before any trace of cellular organisation, shortly after the first archegonia are formed or it may be retained within the sporangium until after fertilisation and a considerable development of the embryo. Development of the female gametophyte begins with a conspicuous enlargement of the megaspore.
The mega- spore nucleus divides repeatedly without immediate formation of cell walls (free cell formation). A conspicuous central vacuole with many free nuclei around it in a gradually thickening peripheral cytoplasm is organised (Fig. 551B).
Very often, the outer thickened part of the mega-sporangial wall grows much faster than the thinner inner wall leaving a gap between the two as seen in Figure 551B. In the next stage, a cellular tissue 2 or 3 cells in thickness is formed by wall formation at the pyramidal end of the female gametophyte.
This tissue is separated from the rest of the gameto­phyte by a conspicuous diaphragm below which the free nuclei remain round the central vacuole (Fig. 551C). Eventually the central vacuole is obliterated by the in­creasing cytoplasm but this lower part remains multinucleate until the embryo begins to develop.
The apical tissue now becomes exposed by the cracking of the spore wall above it along the triradiate fissure. The exposed tissue may become green, and, on falling to ground, rhizoids may develop on it in patches (Fig. 551D).
However, this photosynthetic tissue and rhizoids are of minor importance as far as the nutri­tion of the gametophytes is concerned as the reserved food in the lower part is of much greater importance for the nutrition of the gametophyte and the growing embryo. All superficial cells on the apical tissue are potential archegonial initials.
An archegonial initial divides periclinally to form a primary cover cell and a central cell (Fig. 551E). The central cell again divides periclinally to form a primary canal cell and a primary ventral cell.
The primary ventral cell forms a ventral canal cell and an egg by another periclinal division. The cover cell forms a 2-tiered neck which protrudes out and spreads apart to open the inner passage (Fig. 551F). The primary canal cell does not divide so that there is a single canal cell. While the archegonia are being formed, the free nuclei below form walls and organise into large cells (Fig. 551G).
This tissue below is nutritive to the archegonia and the future embryo above so that this is the endosperm of Selaginella which is a gametophytic tissue as distinguished from the triploid endosperm .of the Angiosperms.
Fertilisation may take place while the female gametophyte is still within the unshed mega-sporangium on the plant or after it is shed. Apogamy and parthenogenesis are known in several species. Embryos are known to arise out of archegonial initials, eggs in unopened archegonia and also out of sporophytic cells in species not forming microspores.
The New Sporophyte:
The first division of the zygote is usually transverse (Fig. 552A). The upper cell gives rise to the one or more celled suspensor and the lower cell to the embryo. The em­bryo initial divides vertically (Fig. 552B). The two resultant cells again divide vertically to form four cells one of which cuts off the stem initial from its base (Fig. 552C).
Gradually, the embryo organises into sectors on one side of which is the stem between two cotyledons, on the other side is the foot (for sucking food material from the endosperm) occupying a major part of the base and the rhiziphore initial on the top of it (Fig. 552D).
The new sporophyte eventually pierces through the surrounding gametophyte and establishes itself as an independent plant after growing adventitious roots from the rhizophore. It is to be noted that, unlike most Pteridophytes, it shows two coty­ledons and even a hypocotyl.
Life Cycle of Lycopodium (With Diagram) | Plants
Lycopodium is a large genus with about 180 species having world-wide distribution in tropical to temperate regions. They are herbaceous, terrestrial plants or erect to pendent epiphytes. The stems are densely covered with microphylls and are protostelic. The genus is homosporous with the sporangia on the adaxial faces of the sporophylls.
This large genus Lycopodium shows a great diversity in forms.
Because of this, it is customary to distinguish within this genus two subgenera: Urostachya and Rhopalostachya.
Urostachya shows erect (e.g., L. selago) or pendent (e.g., L. phlegmaria) plants which are never creeping. They may be un-branched or dichotomously branched. The adventitious roots come out only through the base of the stem and are not to be found along the surface of the stem. Even if any root develops on the upper stem, it traverses through the cortex and emer­ges only at the base.
The sporophylls are green and usually of the same size as the vegetative leaves. In L. selago (Fig. 537) there are sterile (vegetative leaves) and fertile (sporophylls) patches on the stem. In most species of this subgenus there are no organised strobili. In some species (e.g., L. phlegmaria), however, the sporophylls, though green, are shorter and loca­lised at the tips forming distinct dichotomously branched strobili.
In Rhopalostachya, on the other hand, the stem is prostrate with adventi­tious roots developing on the under surface of the prostrate stem. The branching, though dichotomous at first, becomes monopodial later. The sporophylls are smaller, paler in colour, usually with dentate margins although the vegeta­tive leaves are smooth-margined and form dis­tinct strobili.
The chromosome numbers of the Urostachya are much higher (2n=up to 528 as against up to 68 in the Rhopalostachya).
Because of this variation, some modern Pteridologists are not satisfied by merely breaking the genus into two subgenera but they suggest several genera—even families.
Pichi-Sermolli (1959) supports breaking up into four genera: Huperzia, Lycopodium, Lepidotis and Diphasium— the first one. of Urostachya and the other three out of Rhopalostachya. The four genera are also supported by Love and Love (1958) from the cytological point of view.
Lycopodium clavatum (Fig. 538A) is a temperate to sub- tropical, terrestrial species very common on the Indian hills, specially the Himalayas. The sporophyte has a weak, prostrate stem trailing along the surface and rooted down with adventitious roots growing anywhere on the lower surface.
The branching is dichotomous becom­ing monopodial by the strong development of aerial branches here and there. The stem is closely covered spirally by small, simple (microphyllous), sessile, lanceolate leaves (Fig. 538B) with mildly serrate margins and single median veins.
Lycopodium plants may grow almost perennially by the dying out of older parts and the growth of the branches. Gemma-like reproductive buds are also known in several species.
The stem of Lycopodium is protostelic without any cambium. It grows at the tip by several apical cells. A. t.s. of the stem of L. clavatum (Fig. 539A), which may be cy­lindrical or somewhat fluted, shows an epidermis of one layer of thick-walled cells broken here and there by stomata.
Below it there is a thick cortex traversed by leaf trace bundles. The outer layers of the cortex are sclerenchymatous while the cortex below is parenchymatous. In mature specimens the innermost region of the cortex also has thickened cell walls.
The cortex is bounded on the inside by a layer of endodermis with the usual thickened radial walls (casparian strips). Internal to the endodermis is a pericycle of one or more layers of parenchymatous cells. The- stele in this case is protostelic of the plectostele type. The xylem elements are arranged in more or less parallel plates with the phloem between these patches.
The xylem is formed only of tracheides and is exarch with the protoxylem (smaller spiral and annular tracheides) at the ends of the plates and the metaxylem (large scalariform tracheides) forming the general mass. The phloem shows sieve cells and parenchyma. The sieve cells are elongated with tapering ends and with sieve plates on the lateral walls. Leaf traces do not cause leaf gaps in the stele.
In other species of Lycopodium the stelar structure shows great variation. In L. serratum (Fig. 539a) the stele is actinostelic (star-shaped xylem). In L. annotimum (Fig. 539b) the stelar structure is broken up. The breaking up is the maximum in L. cernuum (Fig. 539c) where the stele is a meshwork of innumerable xylem patches with surrounding phloem tissue.
This type is termed as a mixed protostele. These types of stelar structure may be considered as showing the course of stelar evolution —the actinostele being the most primitive and the mixed protostele the most modern. The plectostele is placed before the mixed protostele. The stelar structures are so charac­teristic that Lycopodium species may be identified from anatomy alone.
The anatomy of the leaf is very simple with a surrounding one-layer epidermis broken by stomata, a uniform parenchymatous mesophyll with numerous air spaces and chloroplastids, and a median concentric vascular bundle.
The slender roots show simpler structures with usually monarch steles.
The reproductive shoots arise as erect branches from the horizontal stem late in the season (Fig. 538A). The lower part of the reproductive shoot is comparatively sparsely leaved and the tip branches dichotomously into two or more spike-like strobili (sporangiferous spikes) compactly covered by sporophylls (Fig. 540A).
The sporophylls are of one type only (homosporous). The sporophylls (Fig. 540B) are differentiated from the vegetative leaves by the wider bases and more serrations in the margins.
The sporangia are comparatively large, reniform of subglobose, orange to light-yellow in colour and with short stalks when mature. These develop on the adaxial (ventral) face of the sporophyll a little above the axil. In other species the sporangium is known to develop from the axil or even from the stem just above the axil.
The sporangium is a massive structure developing from a group of initial cells (Fig. 540C). This is known as the eusporangiate mode of sporangium development. In the nearly mature sporangium (Fig. 540D & E) there is a stalk, a. jacket (2 or more layers thick), a mas­sive sporogenous tissue and a nutritive tapetum formed partly by the inner layer of the jacket and partly by some outer sporogenous cells.
The sporogenous cells soon be­come spore mother cells (Fig. 540E) which become rounded, separate from one another and undergo reduction division to form the spore tetrads. The mature sporangium splits along a vertical line of weakness (stomium) and the spores are released.
The spore is tetrahedral with the usual intine and a sculptured exine showing a triradiate ridge (Fig. 541 A). A few chloroplastids are usually present.
The spore of L. clavatum remains viable for a long time and may germinate only after a year or more. The exine splits at the triradiate fissures and a tissue, developing very slowly (taking another year or more), comes out forming a top-shaped (Fig. 541B), underground, non-green, tuberous gametophyte or prothallus.
With age this game­tophyte loses its shape and becomes much convoluted (Fig. 541G). The gametophyte ceases to grow if it does not become infected by a fungus at an early stage of develop­ment.
A vertical t.s. of the mature gametophyte (Fig. 541B) shows an outer epidermis with some rhizoids an outer cortex filled by mycorrhizal fungi an inner cortex with an outer parenchymatous and an inner palisade zone and a central parenchymatous storage tissue where the outermost cells are elongated. The top of this top-shaped gametophyte is lobed and the antheridia, the archegonia and the growing embryos are located on these lobes. This type of gametophyte is noted in many creeping species.
A second type of gametophyte is represented by L. cernuum (Fig. 541D). In these the spore germinates without passing through any resting stage. The gametophyte is usually smaller, annual, partly aerial and partly underground. The lobed crown with antheridia and archegonia becomes green.
A still third type of gametophyte is found among the epiphytic species like L. phlegmaria (Fig. 541E). The prothalli are saprophytic, growing on trunks below a coating of humus. Here a central, small, tuberous body develops a number of colourless, slender, cylindrical arms on whose surfaces the antheridia and the archegonia develop.
L. selago shows gametophytes of both the first and the second types according as it grows on the surface or below the soil level.
All gametophytes are monoecious. In the development of an antheridium, an epidermal cell divides transversely forming an upper jacket initial and a lower primary antheridial initial (Fig. 542A). The jacket initial ultimately forms a jacket layer one cell in thickness with a triangular cell at the top centre.
The lower cell forms a mass of tissue which ultimately become very small cubical sperm mother cells (Fig. 542B). Each sperm mother cell gives rise to a biflagellate (rarely triflagellate) sperm (Fig. 542C) resembling rather the Bryophytes. The antheridia are almost wholly sunken in the gametophytic tissue. The sperms are liberated by the breaking down of the triangular cell at top.
The archegonium (Fig. 542D) also develops similarly from a superficial archegonial initial cell. The first division gives rise to an upper primary cover cell and a lower central cell. The central cell divides to form a lower primary ventral cell and an upper primary canal cell.
The primary canal cell divides transversely to form usually four (1 to 3 in L. cernuum, 7 in L. selago, up to 16 in L. complanatum) canal cells while the ventral cell forms the egg often after cutting off a ventral canal cell. The primary cover cell develops the neck 3 to 4 cells high. In the mature archegonium the neck portion protrudes out while the venter remains sunken.
Fertilisation takes place in the usual way. The neck canal cells and the ventral canal cell (if any) disintegrate and come out exuding citric acid and citrates which probably chemically attract the sperms one of which fertilises the egg developing the zygote.
The zygote divides transversely to form an upper suspensor and a lower embryonic cell (Fig. 543A). The embryonic cell divides into eight cells in two tiers of which the upper 4 near the suspensor gives rise to the foot (for absorption of food material from the gametophyte) and the lower 4 to the stem on one side and a cotyledonary leaf (Fig. 543 B & C) on the other side.
As the embryo grows it rises erect above the gametophyte, the first root developing from the point where the cotyledon and the foot joins. The first leaves are scaly. The new sporophyte soon gets established as an independent plant (Fig. 543D).
In L. cernuum, the 8-celled embryo develops a massive globose structure called the protocorm which becomes green and develops rhizoids below. The structure deve­lops a few erect, conical outgrowths functioning as leaves (Fig. 543E). The protocorm remains in this condition for some time and then the apical meristem bursts into a normal shoot.
A mycorrhizal fungus grows inside this structure. This intermediate protocorm structure has been considered to be of some evolutionary significance by some (Protocorm Theory) while others consider it as a mere passing phase. Treub con­sidered this as an undifferentiated primitive stage of sporophyte which was present in all Pteridophytes but has been lost in most of them.
Phylloglossum (described below) shows a permanent protocorm while this occurs occasionally in Ophioglossum. Goebel, Bower, etc., however, consider the protocorm merely as an occasional adaptation to meet the strain of sporophytic development under special conditions. The Protocorm Theory is now only of historical importance.
Figure 544 shows diagrammatically the life cycle of Lycopodium.
This is a peculiar plant represented by a single species Phylloglossum drummondii found only in Australia, Tasmania and New Zealand. It is a small plant with a fleshy, tuberous, perennial stem and a few stiff, awl-shaped leaves (Fig. 545) reminding the protocorm stage of Lycopodium cernuum. A compact sporangiferous spike is developed on a stalk at the tip of the stem.