The development of the microsporangia and the contained spores (pollen-grains) is closely comparable with that of the microsporangia in Gymnosperms or heterosporous ferns.    From Wordnik.com.  [Encyclopaedia Britannica, 11th Edition, Volume 2, Part 1, Slice 1] Reference

When only the heterosporous lycophyte.    From Wordnik.com.  [PLoS ONE Alerts: New Articles] Reference