CYCADOFILICALES 29 



separated taxonomically. There is not the sHghtest doubt that these 

 resemblances have been attained independently. None of them have 

 been transmitted from one to another. 



But we could not regard the fern-seed-fern resemblances in this 

 way. In the cases just mentioned the resemblances are superficial 

 and not at all misleading; while in the fern and seed-fern leaves the 

 structures, even to margins and venation, are so identical that the 

 leaves cannot be assigned to either group unless they are associated 

 with other structures. 



The most decisive evidence of a fern ancestry is the seed itself. 

 What is a seed and how does it differ from a spore? We cannot im- 

 agine a seed, phylogenetically, originating as a seed ; or even a heter- 

 osporous fern originating in the heterosporous condition. It is a fun- 

 damental of comparative morphology that homospory preceded het- 

 erospory. The Bryophytes never got beyond the homosporous stage, 

 and those ferns and lycopods which are regarded as primitive are 

 homosporous. In the advance from homospory the ferns and even 

 the seed plants show an ontogeny which indicates their phylogeny. 

 In all of the living heterosporous ferns and lycopods, the early de- 

 velopment of the sporangium is that of a typical homosporous fern. 

 In Azolla, the two kinds of sporangia develop alike, even producing 

 the same number of spores. In the microsporangia, all the spores de- 

 velop; while in the megasporangia all the spores, but one, abort, giv- 

 ing up their nourishment to the successful spore, which becomes the 

 large megaspore. In the various heterosporous Pteridophytes the 

 differentiation takes place in different ways, but one feature is found 

 in all — the disorganization of some of the spores to feed others, 

 which, with the abundant nutrition, become large. 



Just how far must the evolution of the megaspore advance before 

 we call it a seed? To place an absolute limit which would satisfy 

 one's ideas about logic is impossible; but, arbitrarily, we make the 

 retention of the megaspore within the sporangium, so that it is never 

 shed, the feature which distinguishes a megaspore from a seed. If 

 the megaspore is shed from the sporangium, it is only a megaspore; 

 if it is permanently retained, the megaspore has reached the dignity 

 of a seed. By this character, all botanists separate the Pteridophytes 

 from the Spermatophytes. 



