Most species of Selaginella inhabit damp forests of tropical climates and are distributed all over the world; some species also grow in the temperate regions. These are found in the hills and are abundantly cultivated, in private gardens. Some are xerophytic and grow on rocky cliffs or dry sandy soil. They are mostly perennial, a few are small, delicate annuals.
The Sporophyte of Selaginella:
In general appearance, they are usually long, slender, much- branched, dorsiventral, creeping stems. All forms branch freely, chiefly in one plane, and the branching is in most cases dichotomous or pseudo-monopodial.
The stems are thickly clothed with numerous small, more or less ovate leaves, usually of two distinct kinds- some large and some small, arranged usually in four longitudinal rows, two of which spring from the lower and two from the upper part of the stem.
The leaves of the lower surface are much larger than the upper ones, one small leaf and one large leaf arise at each node. At the base of the ventral surface of the leaf, there is a membranous ligule which is characteristic of the Selaginella.
The roots are mostly adventitious, because the first root dies early. The branching of the roots is dichotomous in alternate planes. At each ramification of the stem, a root-like organ, the rhizophore, is developed, which on reaching the soil produces the roots there.
Three views have been put forward regarding the morphological nature of the rhizophore:
(a) Root Nature:
Rhizophores resemble the roots being positively geotropic and leafless, in having anatomical structure-like root and species with polystelic stem shows monostelic rhizophore, but they differ from the roots in having no root-caps and root-hairs and being exogenous in origin.
Rhizophores resemble stems in being exogenous in origin, lack of root-caps and root-hairs, position definite in relation to stem, i.e., at basal dichotomy, and when decapitated develop sometimes leafy shoots.
(c) Organs Suigeneris:
A cross-section of the stem shows two regions clearly differentiated into the cortex and the stele or steles. The cortex is many- layered consisting of either entirely thin-walled parenchyma or the outermost part of it being thick-walled and highly lignified (sclerified parenchyma), with or without intercellular spaces and is bounded on the outside by a single-layered epidermis, consisting of thick walled cells and with a cuticle.
The number of meristele ranges from one to three or more, each of which is surrounded by an air-space, which is bridged by radially elongated cells with prominent casparian strips, the trabeculae, which represent the endodermis. Surrounding each meristele there is a single layer of pericycle consisting of conspicuous parenchyma cells. The vascular bundles are hadrocentric; the xylem is exarch and diarch.
A transverse section of the rhizophore shows an epidermis and a well-developed cortex enclosing the stele. The endodermis is not clearly marked out. There is a solitary protoxylem and the phloem entirely encircles the xylem.
Selaginella is heterosporous, because the asexual reproductive units, the spores, are of two kinds; the smaller ones are microspores and the larger ones are megaspores, which are produced in different kinds of sporangia. The sporangia are mostly reniform or ovoid, sometimes flattened and shortly stalked. The two kinds of sporangia differ greatly in size, the mega-sporangia being much larger than the microsporangia.
The sporophylls bearing mega-sporangia are called mega-sporophylls and those bearing micro- sporangia are called micro-sporophylls. The sporophylls, which are nearly of equal size, are generally collected into more or less distinct, four-angled cone or sporangiferous spike or strobilus. These strobili are terminally situated at the apices of the branches.
Each strobilus usually consists of both types of sporophylls, but in some species only one type of sporophylls may occur. The order of arrangement of the two types of sporophylls is variable in different species. Each mega-sporophyll bears in its axil a mega-sporangium within which occurs only one functional megaspore mother cell and it gives rise to four megaspores due to reduction division.
On the contrary, each microsporophyll bears in its axil a micro-sporangium containing many microspore mother cells, each of which produces four microspores due to reduction division, so that many microspores occur in each micro-sporangium. Both types of spores are tetrahedral and the wall shows a tri-radiate ridge and ornamentations. With reduction division and formation of spores, the gametophytic or haploid generation begins.
The Gametophyte of Selaginella:
The microspores, when still included within the micro-sporangium, begin to germinate but are ultimately set free by transverse rupture of the sporangium wall. The result of germination of each microspore is a male prothallus (prothallial cell) which is extremely reduced to a single cell.
It bears the so-called rudimentary antheridium (the primary spermatogenous cells—four in number) being surrounded by a jacket of sterile cells (jacket cells) and the whole remain included within the spore wall.
From the primary spermatogenous cells about 128 to 256 spermatozoid mother cells are produced. Biflagellate spermatozoids are developed from the spermatozoid mother cells of the so-called antheridium and these ultimately float freely in the cavity of the spore-wall. The spore wall bursts and liberates the spermatozoids in the surrounding film water.
Similarly megaspores germinate before they are set free from the mega-sporangium. On germination, the spore wall does not burst immediately and there follow within it free nuclear divisions forming a large number of nuclei, which are distributed in the general mass of cytoplasm surrounding a large central vacuole.
As the nuclei increase in number, the cytoplasmic layer becomes thicker and the vacuole becomes smaller and smaller until it is completely filled up with cytoplasm. Wall formation about the nuclei follows from the periphery near the apical region (towards the tri-radiate ridge) forming a tissue which gradually extends inwards.
In some cases, after forming a tissue consisting of 3-10 layers of cells from the periphery, the wall formation stops temporarily and the inner wall of the lowermost layer of cells becomes thickened to form so-called diaphragm, which separates the peripheral tissue from the non-cellular portion.
The megaspores, at about this stage, are liberated from the mega-sporangium and the wall of each megaspore ultimately bursts along the tri-radiate ridge exposing the archegonia and part of the female gametophyte.
The female gametophyte then becomes green and also develops rhizoids, which come out through the tri-radiate fissure. The female gametophyte, thus ultimately becomes independent of the sporophyte but not free from the megaspore, being still enclosed within spore wall.
Within each archegonium the ovum or oosphere develops. When the archegonium attains maturity, the neck cells and ventral canal cells disorganize. The biflagellate spermatozoids, discharged from the antheridium of the neighbouring a rudimentary male prothallus, swim towards the archegonium in dew or rain water, and ultimately one of them fertilizes the ovum. The fertilized ovum, on secreting a wall around itself, becomes an oospore. With fertilization and formation of oospore the sporophytic or diploid, generation begins.
The New Sporophyte of Selaginella:
The oospore gradually gives rise to an embryo possessing a stem, two cotyledons, a foot, a root, and a suspensor, and from this embryo the Selaginella plant is derived.