Selaginella is heterosporous, the megasporangia being often found towards the base of the cone.
The development of the microand megasporangia is the same up to the stage of isolation of the spore mother-cells.
They consist either of microsporangia or megasporangia, which are arranged in basipetal succession on the receptacle.
Each sorus includes both microsporangia, with numerous spores, and megasporangia, each of which contains a single megaspore with a complicated wall.
The abortion of certain spores, which is known to have taken place both in the homosporcus C. Binneyana and in the megasporangia of C. Casheana, may throw some light on the origin of the heterosporous condition.
It is very doubtful whether any homosporous Lepidostrobi existed, but there is reason to believe that here, as in the closely allied Lepidocarpon, microsporangia and megasporangia were in some cases borne on different strobili.
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.
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.