In Selaginella the stelar systeim h do~ shows profounder modifications.
The leaves of the true mosses and those of the club-mosses (Lycopodium, Selaginella) being somewhat alike in general appearance and in ontogeny, might be, and indeed have been, regarded as homologous on that ground.
The single genus of this order (Selaginella) contains between three and four hundred species.
Selaginella is heterosporous, the megasporangia being often found towards the base of the cone.
Thus the position of the root in Selaginella is different from what obtains in the other Vascular Cryptogams. A point of interest in this heterosporous genus is that the formation of the prothallus may commence before the megaspore is liberated from the sporangium.
The cones, which in some instances at least were heterosporous, presented a general resemblance to those of Lycopodium and Selaginella, a single sporangium being situated on the upper surface of each sporophyll.
Each leaf bears a ligule resembling that of Selaginella in structure and position.
The spores, which are set free by the rotting of the sporangial wall, germinate much as in the case of Selaginella, though the similarity may be a case of independent resemblance.
The primary structure of the stem was thus of a simple Lycopodiaceous type, resembling on a larger scale what we find in the upright stem of Selaginella spinosa.
Some analogy among recent Lycopods is afforded by the stem of Isoetes, and by the base of the stem in Selaginella spinosa; in the fossils the process was of a more normal type, but some of its details need further investigation.
The most interesting point in the structure of the leaf-base is the presence of a ligule, like that of Isoetes or Selaginella, which was seated in a deep pit, opening on the upper surface of the cushion, just above the insertion of the lamina.
In the cone attributed to the Lower Carboniferous Lepidodendron Veltheimianum) the arrangement was that usual in Selaginella, the microsporangia occurring above and the megasporangia below in the same strobilus (diagram, fig.
Another case of a " seed-bearing " Lycopod has lately been discovered by Miss Benson in Miadesmia membranacea, a slender Selaginella-like plant from the Lower Coal Measures of Lancashire.
In Selaginellites Suissei there was a definite strobilus bearing both microand megasporangia; in each of the latter from 16 to 24 megaspores were contained; in Selaginellites primaevus, however, the number of megaspores was only 4, and the resemblance to a recent Selaginella was thus complete.
The antiquity of the Selaginella type indicates that this group had no direct connexion with the Lepidodendreae, but sprang from a distinct and equally ancient herbaceous stock.
The first formed portion of the stern in all species of Selaginella which have been investigated possesses an exarch haplostele.