Lycopodium, commonly known as ‘club-moss’ and ‘ground pine’, grows chiefly in tropical and subtropical forests but some species are also distributed in arctic and temperate regions.
The common Indian species are L. hamilvonii, L. phlegmaria, L. cernuum, L. selago and L. clavatum.
The Sporophyte of Lycopodium:
In habit, all species vary widely, but all have slender, weak-stemmed, comparatively small, herbaceous or shrubby sporophytes. Many species are somewhat prostrate with stems creeping above or below the surface of the soil. Other terrestrial species have upright or semi-erect stems, which later on become more or less horizontal. Some species may also be epiphytic with pendent bodies, while still others are twining to some extent.
In the simplest case, the sporophyte has a simple stem covered with numerous, moss-like leaves, each bearing a single large sporangium on its upper side. In this sense the whole sporophyte is a strobilus (e.g. L. selago. L. pitheyoides, etc,).
In more complex types, the sporophytes have much-branched stems whose lower leaves are sterile and act as foliage leaves, and this gradual sterilization process can be traced to more than one form where the sporophyte is distinctly differentiated into a vegetative region bearing foliage leaves, and a reproductive region bearing the sporophylls.
In such cases, the sporophylls are quite different in form from the foliage leaves and are localized and compacted to form a distinct strobilus (e.g., L. clavatum). These strobili are often separated from the vegetative body and are borne on slender stalks with rudimentary leaves.
Branching of the stem is characteristically dichotomous. The two branches of a forking may be equal or unequal. The leaves are very numerous, simple and small (usually 2-10 mm. long, sometimes up to 25-30 mm.), arranged in close spirals, whorls or opposite pairs, or the arrangement may be somewhat irregular.
The primary root is short-lived but many adventitious roots arise, singly or in acropetal clusters, from the under-side of the older parts of the stem and the branching is strikingly dichotomous. Root hairs are abundant and persistent in some terrestrial species and lateral roots do not develop.
A cross-section of the stem shows two distinct regions, the cortex and the central cylinder of the stele. The cortex is bounded externally by an epidermis, one cell in thickness and with stomata. The cortex varies in thickness as well as in structure. In some species it is soft and parenchymatous throughout, in others the outer or the inner portion of the cortex undergoes sclerification, while in still others the entire cortex becomes sclerified.
The cortex is limited internally by an endodermis with characteristic radially thickened walls. Lying within the endodermis is the pericycle 3-6 cells in thickness. Internal to the pericycle is the central core of vascular cylinder which is a protostele with xylem exarch. In the simplest case, the xylem appears as a star-like mass with a variable number of rays.
In between the rays lies the phloem being separated from the xylem by narrow strip of parenchyma. In more advanced types of sporophytes, numerous furrows appear in the xylem cylinder so that the xylem breaks up isolated strands forming plate-like lobes or mesh-like mass, with included phloem bands.
Reproduction in Lycopodium:
Lycopodium reproduces both by vegetative and sexual methods.
There are several means by which vegetative reproduction takes place:
(1) Tips of lateral branches become flattened and with wing-like leaves, known as bulbils or gemmae annually fall to the ground, take roots and form new plants.
(2) The rhizome progressively grows at the apex and its older part dies, the branches become separated and form new plants.
(3) In some species during winter the entire plant dies, but the apical portion behaves as a resting bud.
(4) In epiphytic species, portions of plant body may give rise to new plants.
(5) Roots and detached leaves of bulbils can also give rise to new individuals.
Lycopodium is homosporous and the spores are produced within large, short-stalked, reniform to somewhat sub-globose sporangia, borne singly either in the axils or on the stem, a little above the sporophylls.
The sporophylls are variable in form, size and colour in different species, sometimes resembling foliage leaves. These sporophylls usually become localized and aggregated to form cones or strobili, at the apex of the main stem or on lateral branches. The strobili are either stalked or sessile. The sporangium is eusporangiate in development.
Within the jacket of the sporangium the sporogenous tissue is surrounded by a special nutritive layer known as tapetum. The cells of the sporogenous tissue ultimately cease to divide to form spore mother cells, each of which by reduction division gives rise to a spore tetrad.
With reduction division and formation of spores, the gametophytic or haploid generation begins. Each spore shows a weak tri-radiate ridge and its wall is either smooth or shows honeycomb or net-like thickenings.
The Gametophyte of Lycopodium:
When the spores are mature, a narrow transverse strip of cells (stomium) is gradually differentiated at the apex of the sporangium, which ruptures transversely and liberates the spores. Each spore, under favourable conditions, germinates and produces the gametophytic plant. There are two main types of gametophytes.
In tropical species (L. cernuum) usually the spores after liberation germinate quickly and form short-lived gametophytes, on the surface of the ground, which are very small, green (excepting the basal portion), somewhat cylindrical to ovoid bodies with lobed apices.
In other cases, especially in creeping and epiphytic species (L. clavatum), the spores after a shorter or longer period of rest (3-8 years) germinate and form non-green, subterranean, somewhat tuberous or carrot- shaped, much larger gametophytes, sometimes much convoluted, and these grow to maturity very slowly, taking several years (6-15 years) and nourishing the young sporophytes.
In other species, transitional forms occur and these gametophytes are partly subterranean with a green, lobed, aerial portion (crown) bearing the sex organs. Usually, both types of gametophytes are associated with an endophytic fungus forming a mycorrhiza, which is a prominent feature of the gametophyte.
The gametophyte of Lycopodium is monoecious (homothallic) and numerous sex organs, antheridia and archegonia, are borne either on the crown, or in between its lobes, or on the central cushion in flattened types of the gametophytes.
The antheridia vary in size, shape and number of spermatozoids and either project slightly or remain wholly embedded within the gametophytic tissue. There are many spermatozoid mother cells within the single-layered antheridial wall and each gives rise to a biflagellate (rarely three) spermatozoid resembling the spermatozoid of Bryophyta.
The archegonia are either short or long and embedded in the tissue of the gametophytes with their necks protruding upwards. At maturity, each archegonium contains an egg cell, a ventral canal cell and 6, sometimes 10-13, canal cells (but only one neck canal cell in shorter archegonium).
When the archegonium attains maturity its neck canal cells and ventral canal cell disintegrate forming a passage open to the ovum for the spermatozoids to fertilize. The walls of the antheridium breaks up, the spermatozoids are set free, these are washed to the archegonium and one of them, finding its way through the neck, ultimately fertilizes the ovum.
The fertilized ovum soon covers itself with a wall and forms the oospore. With fertilization and formation of oospore, the sporophytic or diploid generation begins.
The New Sporophyte of Lycopodium:
The oospore, by repeated divisions, gives rise to an embryo consisting of a suspensor cell, an absorbing organ called the foot, the stem, a leaf and a root belated in development. From this embryo the young sporophyte gradually develops and this may be supported and nourished by the gametophyte for several years. In some cases several young sporophytes may be borne simultaneously on the same gametophyte.
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