Botany and Plant Science

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.

Lycopsida is a class of Pteridophyta, not Bryophyta. Bryophyta includes classes like Hepaticopsida, Anthocerotopsida, and Bryopsida.

In the life cycle of Dryopteris, the asexual sporophyte is the dominant individual. This is perennial and evergreen and differentiates into rhizome (stem), roots and leaves ( fronds). When the fronds mature these become fertile and are called as sporophylls since they contain sori. In each sorus stalked sporangia is present arising from the placenta, the swollen outgrowth on the divisions of the pinna or leaflets.

Strobilus is called the fertile branch, because it bears the reproductive structures such as microsporophylls and megasporophylls.

In Dryopteris, the neck consists of a wall or jacket layer of 4 vertical rows of sterile neck cells. The neck cells enclose a central neck canal. They contains a single, long neck canal cells with two nuclei.

Selaginella and Salvinia are considered the first plants to exhibit a significant step toward the evolution of seed habit because embryo develops in female gametophyte which is retained on parent sporophyte. It occurs because Selaginella and Salvinia both are heterosporous, i.e., these plants produce two kinds of spores, macro (large) and micro (small) spores. In the heterosporous species, the female gametophyte remains on the parent sporophytes for variable periods. The development of the zygotes into young embryos takes place within the female gametophytes. This event is a precursor to the seed habit (tendency towards seed formation) considered an important step in evolution. Hence, heterospory leads to seed habits in plants.

The presence of Ligule - a small tongue shaped appendage on the adaxial side of the leaf near the leaf base is a distinctive feature of the Selaginella. This consists of elongated secretory cells the glossopodium which tends to keep the sporangium wet. Another distinctive feature is the cones or strobilus on which the sporophylls are present.