CYCADOFILICALES 57 



of pteridosperms." Theoretically, there remains a stage of progress 

 entirely lacking in our records, and that is the heterosporous fernlike 

 forms that preceded the evolution of seeds. The gap between the 

 homosporous Primofilices (or their unknown ancestors) and the seed- 

 bearing Cycadofilicales is an enormous one, including the evolution 

 of both heterospory and the seed. 



Proceeding on purely hypothetical grounds, the most natural 

 inference is that homosporous sporangia began to differentiate into 

 the heterosporous condition; that the microsporangia continued 

 with relatively little change throughout the cycadophyte phylum; 

 and that the megasporangia developed with comparative rapidity 

 into ovules. Miss Benson (41) has suggested that the ovule is a 

 transformed sorus, explaining the peculiar integument of Lagenostoma 

 by regarding the tissue-filled chambers as the sterilized peripheral 

 sporangia of a radiate sorus or synangium. Oliver (85) infers from 

 the free integument lobes of Physostoma that the integument of the 

 Lagenostoma group originated as a multiple structure, the nature of 

 the units (free lobes in Physostoma, coalescent chambers in Lagenos- 

 toma) being an open question. His suggestion is that they have 

 arisen as new formations, without any phylogenetic connection, being 

 related to the encasement of reproductive structures that so frequently 

 precedes or follows fertilization. This view would mean that the 

 origin of the integument was contemporaneous with the origin of the 

 seed itself; and that the cupule (as in Lagenostoma) is a second such 

 encasement. However true either of these suggestions may be with 

 reference to Lagenostoma, the relation of the seeds to the fronds and 

 a comparison of the position of seeds with that of microsporangiate 

 sori compel the belief that ovules often may have arisen by the trans- 

 formation of sori, a transformation probably introduced by restricting 

 a sorus to a single sporangium. If, as seems possible, a sorus of 

 free sporangia is a more primitive condition of ferns than a synangium, 

 it may be that free sporangia not grouped in a sorus represent a still 

 more primitive condition, and that neither synangium nor sorus holds 

 any relation to the origin of the ovule. After all, the important 

 and obvious fact is that an ovule is a transformed sporangium, and 

 that the phylogenetic stages in this transformation are absolutely 

 lacking. 



