Botany and Plant Science

In the stem of Selaginella, the endodermal cells are elongated and form bridge-like structures called trabeculae that connect the stele to the cortex.

In Selaginella, the development of the male gametophyte is endosporic, meaning it occurs entirely within the microspore wall.

The megaspore of Selaginella germinates to produce the female prothallus, which bears the archegonia.

The 13-celled male gametophyte of Selaginella consists of 1 prothallial cell and a 12-celled antheridium.

In Selaginella, the megasporophyll bears the megasporangium, which is functionally analogous to the carpel in angiosperms that encloses the ovule. This comparison highlights the evolutionary trend toward seed-bearing structures.

The male gametophyte of Selaginella is highly reduced. It consists of a single prothallial cell and an antheridium.

The stem of Selaginella typically contains a protostele, which is the simplest type of stele consisting of a solid core of xylem surrounded by phloem.

The ligule is a small, scale-like outgrowth found on the adaxial surface of the leaves of Selaginella, which is a characteristic feature of the order Selaginellales.

Selaginella can reproduce vegetatively through various methods including fragmentation, tubers, and resting buds, which allow it to propagate effectively.

The male gametes (antherozoids) of Selaginella are biflagellate, meaning they have two flagella to aid in swimming toward the archegonia.

In S. bryopteris and S. lepidophylla, megaspores are retained inside the megasporangium (not megasporogonium, which appears to be a typographical error in the question) for germination. This retention of megaspores is a step toward seed habit and occurs in specific Selaginella species. The other species pairs listed are known to release megaspores externally.

The folding of lamina in bud condition is called as ptyxis. Arrangement of leaves on the stem branches is called as phyllotaxy. Vernation is the phenomenon of arrangement of leaves in the bud. 

Selaginella is a tracheophyte that exhibits heterophylly (different leaf sizes), lacks companion cells (typical of pteridophytes), and has vessels in its xylem, which is an advanced trait.