The development of the microsporangia and the contained spores (pollen -grains) P (P g is closely comparable with that of the microsporangia in Gymnosperms or heterosporous ferns.
The spores produced in each sporangium vary from very many to a single one in the case of some heterosporous forms. These latter bear spores of two kinds, microspores and megaspores, in separate sporangia.
Some Calamites were heterosporous, sporangia with microspores and megaspores being found in the same cone.
As general characteristics of the Lycopodiales, the simple form of the leaves, which are generally of small size, and the situation of the sporangia on the upper surface of the sporophylls, which are often associated in cones, close to their insertion on the axis, may be mentioned; there are both homosporous and heterosporous forms, the prothalli exhibiting corresponding differences.
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.
In some of the heterosporous forms (Lepidocarpon, Miadesmia) the sporangia were sometimes surrounded by an integument; and since only a single megaspore attained maturity, the structure of the megasporangium suggests a comparison with an ovule.
Hydropterideae.-Two very distinct orders of heterosporous Filicales, the Salviniaceae and the Marsiliaceae, are included in this group. The difficulty of determining their exact relationship to the other orders of Ferns is increased by the more or less completely aquatic habit of the plants and the modifications and reductions in structure associated with this.
In the absence of direct evidence from Palaeobotany, and bearing in mind the modifications associated with adaptation to an aquatic life in other plants, the recognition of any more definite affinity for these heterosporous ferns than that indicated above appears to be inadvisable.
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.
That the cones were heterosporous there can be no doubt, though little is known as yet of the microsporangia.
The discovery of Sigillariostrobus, which was the fructification of Subsigillariae as well as of the ribbed species, has finally determined the question of the affinities of the genus, once keenly discussed; Sigillaria is now clearly proved to have been a genus of heterosporous Lycopods, with the closest affinities to Lepidodendron.
Selaginellites elongatus, another heterosporous species, is remarkable for having no differentiated strobilus, a condition not known in the recent genus.
We cannot as yet say whether these bodies represent a somewhat unusual type of fern sporangium or whether they are microsporangia: if the latter supposition is correct the plant must have been heterosporous; but we are still without evidence on this point.