The synangia consist of a stout wall composed of thick-walled cells, succeeded by a layer of more delicate and smaller elements; and internal to the wall occur two rows of sporangial loculi containing microspores.
The male sporophylls are similar in form to the vegetative leaves, but smaller; sunk in their parenchyma are numerous tubular loculi, containing large pollen-grains, which are pluricellular like those of Cordaites; the female fructification had not yet been identified with certainty.
The careful study of the development of the synangium of Tmesipteris, which consists of two loculi, and of Psilotum, which consists of three, has shown that their structure can be explained as originating by the septation of a single sporangium resembling that of Lycopodium.
Sometimes the two cavities are restricted to the two ends of the spore, the polaribilocular type and the two loculi may be united From Beitrcge zur Wissenschaftlichen Botanik.
These organs may be raised on a short stalk, their cavity subdivided into loculi, and provided in some cases with hooks.