308 PLANT MORPHOLOGY 



primitive The Paleozoic ancestors of the spermatophytes must have 

 developed heterospory, ]>iit the only known heterosporous ferns are the 

 two families of Hydropteridales, both of which are modern and highly 

 speciali/ed. Yet, aside from heterospory, there are many resemblances 

 between the eusporangiate ferns and the Cycadofilicales of the Paleozoic, 

 which are the most primitive group of gymnosperms. 



