﻿26 Curtis : The Evolution of 



layer of non-propagative cells. In the higher members of this 

 group a more considerable sterilization of tissue is apparent in the 

 formation of elaters and a rudimentary seta. This differentiation 



J 



Anthoce 



is manifest in the differentiation of the sterile cells. The central 

 cell mass is no longer propagative and supports a 



mere 



shaped zone of archesporial tissue which is associated with assimi- 

 latory cells; the foot has developed root-like processes for the 

 absorption of nutriment from the gametophyte, and among some 

 forms it is especially noteworthy that certain parts of the arche- 

 sporial tissue becomes sterile, separating the sporogenous tissue 

 into irregular chambers. These features are worthy of especial 

 consideration, for in them are seen the rudiments of the segmen- 

 tations and differentiations of the higher plants. It will be seen 

 that the entire sporogenous tissue has been advanced to near the 

 surface of the sporogonium as a more advantageous position for 

 the distribution of spores and an assimilatory tissue has been 

 evolved to meet the demands made upon the plant by its ever- 

 increasing spore production, as it advances in the biological series, 

 and finally organs for the absorption of food, both liquid and 

 gaseous, have been added. Thus we have a rudimentary sporo- 

 phytic plant, still parasitic to be sure, but possessing three distinct 

 functions, progagation, assimilation and absorption, and from 

 some such type there is indication of the evolution of the higher 

 plants. 



The Musci are remotely connected with this ancestral form and 

 show few departures, save in the gradual sterilization of the upper 

 portion of the archesporium, but in passing to the Pteridophyta, 

 we are confronted by a greater gap than appears between any of 

 the other subkingdoms. So extensive has been the obliteration 

 of the progenitors of this division in the evolutionary series, that 

 not only is the ancestry in doubt, but the alliances of the three 

 surviving classes themselves must remain an uncertainty. How- 

 ever, the lines along which the evolution took place, as well as the 

 origin of new features, are generally manifest. The significant 

 features of the Pteridophyta are the finally complete separation of 

 the two generations, the separation of the sporogenous tissue into 



