In this article we will discuss about the origin of seed in selaginella.
An evolutionary sequence of origin of seed from homosporous form should involve:
(a) Production of two kinds of spores,
(b) Reduction in the number of functional megaspores,
(c) Final reduction to a single megaspore in the sporangium,
(d) Retention of megaspore within the megasporangium,
(e) Elaboration of the megasporangium apex,
(f) Formation of integuments,
(g) Formation of a new structure micropyle for microspore reception and to provide an opening for seed germination.
To an extent some of these changes can be seen in surviving heterosporous pteridophytes and to varying extents in extinct pteridophytes.
The megasporangium of Stauropteris burntislandica a fossil, presents several exceptional features, the pertinent few out of which are as follows:
(a) Basal half or more of the sporangium is sterile, possibly serving as food reservoir,
(b) A slender vascular strand running through the sterile portion. This is the most extensive vascularization of any fern or fern-like sporangium,
(c) Few specimens show a tapering apex with a minute opening.
It is a specialized megasporangium but not a seed. Accordingly, a seed can be conceived to have evolved from it, involving the three steps (Fig. 8.6) as below:
(a) Reduction in the number of megaspores to one and increase in size of the survivor,
(b) Sinking of megaspore towards the basal part of megasporangium.
(c) Displacement of the vascular strand and its division into two branches which developed around the megaspore.
From this, with only a slight modification of the sporangium wall and apex, the vascularized nucellus of the medullosan-type seed can be derived. With some more extensive modifications or specialization of tissue in the peripheral wall of megasporangium, a lyginopterid seed can be derived.