Heterospory is only known in the Hydropterideae.
In several cases heterospory, unknown among recent Equisetaceae, has been demonstrated in their Palaeozoic representatives.
In most other respects the two genera agree; there is evidence for the occurrence of heterospory in some strobili referred to Palaeostachya.
The spores of this genus are curiously winged, and intermediate in size between the microspores and megaspores of Lepidostrobus; the question of homospory or heterospory is not yet decided.
Each sporangium had, on one side only, a longitudinal or slightly oblique annulus, several cells in width; the numerous spores were all of the same size; certain differences among them, which have been interpreted as indicating heterospory, have now proved to depend merely on the state of preservation.