In this article we will discuss about the life cycle of lycopodium, explained with the help of suitable diagrams.
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.