228 THE ANATOMY OF WOODY- PLANTS 



sarcotesta and sclerotesta, but are lacking in the nucellus or 

 megasporangium. In the seed now under consideration a fibro- 

 vascular envelope surrounds the megaspore and ends upwardly 

 in the pollen chamber. It is clear in the present instance that 

 the tracheary tissues invade the megasporangium precisely as 

 they do the microsporangium of Ginkgo and seed plants higher 

 in the evolutionary scale. The function of the tracheary tissue 

 which ends under the pollen chamber doubtless was that of 

 supplying water to facilitate the germination of the microspores, 

 and the fertilizing movements of the antherozoids originated 

 from these. It seems clear that we have in the type of seed 

 'figured in connection with the present paragraph a counterpart 

 to microsporangia with the endokinetic mode of dehiscence. A 

 number of ancient seeds of the anatomical organization indicated 

 here have been investigated, but unfortunately they have not been 

 connected with absolute certainty with any definite vegetative 

 types. 



The seed of the living Ginkgo throws no light upon the question 

 of the affinities of the second type of Paleozoic seeds, for, although 

 tracheary tissues are abundant in the base of the seed, they do not 

 penetrate into the megasporangium proper. It is likely that 

 degeneracy of the fibrovascular structures has here obscured the 

 real situation, since from the organization of the microsporangium 

 in the genus we should expect to discover tracheary elements in the 

 walls of the nucellus or megasporangium. 



In the seeds of the Cycadales we have realized the general 

 features of organization depicted in Fig. 167, which is, in fact, 

 modeled from the young seed of Cycas revoluta. The organization 

 of the microsporangium in the cycads does not support the hypoth- 

 esis that nucellar tracheids were once present in the seed of the 

 group and have disappeared in its modern representatives. The 

 conservative tendencies of microsporangial structures as contrasted 

 to those of the megasporangium or seed which is considerably 

 more rapidly influenced by the course of evolution constitute a 

 valuable situation from the standpoint of comparative morphology. 



The general anatomical features of the seed in the higher gymno- 

 sperms may next be considered. In Fig. 169 is illustrated the more 



