In this article we will discuss about the process of reproduction in selaginella with the help of diagram.
Selaginella is heterosporous. Two types of sporangia (larger ones megasporangia and smaller ones microsporangia) containing larger and smaller spores are borne on sporophylls which form cones terminally either on the stem or branches.
The cones are inconspicuous due to small size and the sporophylls are similar to vegetative leaves (Fig. 8.3A, B). In some forms, vegetative leaves are produced above the cone (Fig. 8.3C) due to continued meristematic activity.
Each sporophyll is ligulate and according to its sporangium is called megasporophyll or microsporophyll. The sporophylls are spirally arranged but due to condensation of axis they appear opposite and are four-ranked.
Normally, mega and microsporophylls are borne on the same cone, the former at the base and the latter at the distal end (Fig. 8.3D) or the two types form opposite rows (Fig. 8.3E, F). In some species the cones bear either megasporangia or microsporangia but both occur on the same plant. In S. selaginoides basal sporangia are nonfunctional.
Sporangia which are reniform to ovoid, with a short stalk, are borne on the adaxial face between ligule and base of the sporophyll. However, at maturity the sporangia are almost axillary in position. Megasporangia are much larger than microsporangia but in some species they are of the same size. Microsporangia are slightly elongate. Growth of the strobilus is apical and in a longi-section sporangia in various stages of development can be seen (Fig. 8.3 D-F).
In S. sulcata only one megaspore is formed, whereas in S. rupestris there are usually two megaspores. In some species there are more than one functional sporocytes so that up to twelve or rarely more megaspores result. Heterospory, therefore, is due to degeneration of sporocytes and increase in the size of survivors. Although both types of sporangia occur on the same plant, but in no instance one sporangium produces two types of spores.
The developing sporangia have two layered wall, all cells of which except in the apical strip are thickened, and are separated from the sporogenous tissue by a conspicuous cell layer, the tapetum. In a mature sporangium, only the outermost wall layer persists and rest decompose before dehiscence. Spore dispersal takes place due to apical dehiscence of wall, brought out by hygroscopic changes.
It remains to be seen whether mechanism is different in mega- and micro-sporangia. Further, is it passive or active or forced. There is either dispersal of spores (spore ejection) or the entire sporangium is ejected from the strobilus. In species from xeric environment there is passive dehiscence of sporangia and dispersal of spores.
In S. rupestris, though the megasporangia dehisce but the megaspores are not shed. The development of female gametophyte and fertilization takes place in situ and the young sporophytes can be seen growing on the parent sporophyte (Fig. 8.3G). However in S. selaginoides there is an active discharge of megaspores described as “compression and slingshot ejection”. This is the first record of active megaspore ejection in any heterosporous form and is suggested to be a primitive feature in this genus.
The spores (micro- and megaspores) are tetrahedral with a prominent triradiate mark and characteristic ornamentation. Heterospory introduces a new mode of gametophyte development into the life cycle. The gametophytes are formed within the spore wall, i.e., are endosporic. Nuclear divisions begin in spores before dispersal and when shed the gametophytes are in various stages of development.
At the time of liberation, the male gametophyte normally consists of 13 cells; one small prothallial cell, eight jacket cells and four androgonial cells. The microspore divides unequally to produce bigger antheridial cell and a smaller prothallial cell. The antheridial cell divides to produce four cells which intersect at the centre of spore.
Each of these cells cuts off one primary androgonial or spermatogenous cell towards inside and one jacket cell towards outside. The androgonial cells undergo further divisions to produce 128 or 256 biflagellate antherozoids which are liberated when the spore wall cracks open along the triradiate mark. The sperms in Selaginella are the smallest among vascular plants.
The stage at which the megagametophyte is shed is variable. It may be liberated as a single cell or after the differentiation of archegonia. Rarely it is retained in the sporangium. Development of megagametophyte is initiated by free-nuclear divisions in megaspore and with the appearance of a large vacuole these nuclei get dispersed to the periphery.
This is followed by cell wall formation and a cushion-shaped tissue differentiates beneath the triradiate mark and ultimately the vacuole is obliterated by the formation of cells. In some species the lowermost cells of cushion become thickened and form a diaphragm separating it from the rest of the gametophyte.
Archegonial initials differentiate on the surface of cushion (Fig. 8.4A). The archegonia are embedded in the gametophyte at the venter region and their two-celled necks project at the gametophytic surface. The axial row comprises one neck canal cell, one venter canal cell and an egg.
On the cushion, under three arms of triradiate mark, appear mounds of tissue, which are covered with rhizoids, and their growth cracks open the spore wall (Fig. 8.4B). The mounds are prominent in some species (S. galeotti) and inconspicuous in others. The rhizoids help entangle the spermatozoids and facilitate fertilization, but in some species the female gametophytes are without rhizoids.
The first division of fertilized egg is transverse forming a suspensor cell above and an embryonic cell below. The contributions of these cells to embryo, its position and organization are variable in different species. In some species the embryo develops in the cushion region whereas in S. kraussiana venter of the archegonium extends carrying the embryo deeper in nutritive tissue.
Otherwise normally elongating suspensor serves this purpose (Fig. 8.4A). In S. martensii, which represents typical embryogeny, outer cell forms the suspensor, and the lower embryonic cell forms embryo with shoot apex, root apex, and swollen foot. Contrary to this, in S. denticulata all parts of the embryo are derived from outer cell. In between these two extremes are S. kraussiana and S. galeotti. In the former the outer cell forms the vestigial suspensor and also the foot, and in the latter suspensor, foot, and root are derived from the outer cell.