30 HEREDITY AND EVOLUTION IN PLANTS 



ennial, often living on after the sporophyte has died. In 

 general the sporophyte possesses less sterile tissue in pro- 

 portion to fertile tissue than is the case with the lepto- 

 sporangiate forms. These characters mark the group as 

 more primitive than the leptosporangiate ferns, and they 

 are much less numerous, only about 100 species being 

 known from the entire world, while of the leptosporangiate 

 ferns between 3,000 and 4,000 species have been described. 

 Recent studies of the vascular anatomy of the Ophio- 

 glossaceae have disclosed features in common with the 

 Osmundaceae and Polypodiaceae. The fact that the vas- 

 cular bundles of the fertile spike originate in the same 

 manner as those extending into the pairs of pinnae of 

 the sterile segment points to the conclusion that the fertile 

 spike represents, or is homologous with, two fused pinnae 

 at the base of a fern leaf. From this and other evidence 

 the Ophioglossaceae, while "simpler" in structural fea- 

 tures, have been regarded as not having had a strobilar 

 origin (by progressive sterilization 1 ) from the liverworts, 

 and as not standing in the ancestral line of the modern lep- 

 tosporangiate ferns, but as having themselves been derived 

 at a very early period from a primitive fern stock closely 

 related to the Osmundaceae. On the other hand, Camp- 

 bell 2 has adduced evidence for the derivation of the fertile 

 spike of Ophioglossum from a sporogonium like that of the 

 liverwort, Anthoceros. This and other evidence indicates 

 that the Ophioglossaceae, and the eusporangiate ferns 

 as a group, are the oldest fern stock, and this conclusion 

 is supported by the geological record, for the oldest known 

 fossil ferns are eusporangiate. Further investigation is 

 necessary before the question can be definitely settled. 



J Cf. pp. 379, 432, and 574 infra. 



2 Campbell, D. H., Amer. Nat. 41 : 139-159. 1907. 



