CORDAITALES l8i 



or the two groups had a common origin from a still more ancient group 

 of fernlike plants. The historical evidence favors the latter alterna- 

 tive, for the two groups are distinct as far back as the record goes; 

 but it must be remembered that this is negative evidence. On the 

 other hand, although both groups are certainly extremely ancient, 

 their similarities suggest so close a relationship, confirmed by the 

 existence of series of anastomosing forms, that the other alternative 

 is worthy of consideration. 



On the whole, the structure of the seed is the most striking charac- 

 ter common to Cordaitales and Cycadofilicales. It is no longer pos- 

 sible to distinguish their seeds. The free nucellus and integument 

 that characterized certain seeds of both groups, the distribution of 

 the two sets of vascular strands, the large pollen chamber, the absence 

 of any visible embryo, are all characters that may be regarded as 

 primitive and as indicating a close relationship of the two groups. 

 As an alternative, it may be assumed that these resemblances do not 

 indicate a phylogenetic connection of the groups so much as they do 

 an early stage of seeds in general, and such seeds may have originated 

 independently many times from the original fern stock. This claim 

 can be met only by comparing the other structures of the two groups, 

 to discover whether there is any indication of more progress in one 

 than in the other, progress that would indicate that they are not at 

 the same level of primitiveness. 



The organization of strobili represents a distinct advance beyond 

 the condition of Cycadofilicales, and it is the same kind of modification 

 as that suggested in the case of the Bennettitales (p. 87). It consists 

 in the complete differentiation of sporophylls and foliage leaves, the 

 consequent reduction in size of the sporophylls, and their aggregation 

 into a strobilus by shortening the fertile axis. The association of 

 bracts and sporophylls in the strobilus would be natural, some of 

 the bracts (chiefly the outer ones) doubtless representing reduced foliage 

 leaves, and some of them (notably the inner ones) being sterile sporo- 

 phylls. A notable complication in the ovulate strobilus of Cordaitales 

 is the fact that the primary axis of the strobilus bears only bracts, 

 the ovules appearing on short secondary axes (a compound strobilus) . 



It is in the vegetative structures that there appears a distinct gra- 

 dation from Cycadofilicales toward Cordaitales. It has been seen 



