Green organism is developed; this is the prothallus (gametophyte, sexual generation; fig.
Those of Ophioglossum are cylindrical, while the dorsiventral prothallus of Botrychium bears the sexual organs on the upper surface.
The single large megaspore contained in each megasporangium produces a small prothallus, which bears one or a few archegonia; these are exposed on the surface of the prothallus at the summit of the germinated megaspore (fig.
The prothallus developed from the spore is green and in most cases dorsiventral, bearing the archegonia and antheridia on the under surface.
A, Prothallus viewed from the lower surface; ar, archegonia; an, antheridia; rh, rhizoids (much enlarged).
The microspores on germination produce a small, greatly reduced male prothallus bearing one or two antheridia which give rise to a number of spirally coiled, multiciliate spermatozoids.
There is some reason to believe that the prothallus of Psilotum resmbles some Lycopodium prothalli, but conclusive evidence is wanting; that of Tmesipteris is unknown.
Microspore spherical or oval, with or without a bladder-like extension of the exine, containing a prothallus of two or more cells, one of which produces two non-motile or motile male cells.
In the former the prothallus produces one or more fern-plants vegetatively, the projection which develops into the sporophyte in many cases occupying the position of an archegonium.
The genus Ephedra, with its prothallus and archegonia, which are similar to those of other Gymnosperms, may be safely regarded as the most primitive of the Gnetales.