﻿Assimilating Tissue in Sporophytes. 27 



isolated portions (sporangia) and the development of appendages, 

 lateral organs, from the sporophyte. This has been made possible 

 through the differentiation of tissue that accompanied the perpetu- 

 ation of the physiological distribution of labor noted above. The 

 sporophyte has become a self-supporting plant, save in its em- 

 bryological condition, through the perfection of its absorbtive 

 parts and also through the formation of appendages to which are 

 confined almost entirely the assimilatory function. These ap- 

 pendages, the leaves, doubtless came about in some of the classes 

 through their ceasing to function as organs of propagation, but 

 retaining the function of assimilation, and in other cases they may 

 quite as reasonably be supposed to represent outgrowths of the 

 assimilatory tissue of the sporogonial prototype. This increase of 

 the vegetative system of the sporophyte, which renders the ap- 

 proach to the Pteridophyta so striking, has become a necessity 

 since now the food absorbed is no longer organized, and more 

 especially in order to mature the spore, the production of which 

 reaches its culmination in this group. The stages in this evolu- 

 tion of the extant Pteridophyta have been hopelessly lost, but each 

 of the subclasses presents features that suggest an anthocerotal 

 progenitor. This relationship appears very manifest among some 

 of the simpler of the eusporangiate Filicinae. In this primitive 

 group appear several strong ancestral characters, and among the 

 more significant, as appertaining to this subject, may be men- 

 tioned the structure of the so-called fertile leaf. Here is found a 

 superficial archesporium, made septate by the sterilization of its 

 tissue, and associated are assimilating cells and stomata — prac- 

 tically the duplicate of the sporogonium of Notothylas and An- 

 thoceros. The assimilatory portion cannot longer for lack of room 

 be confined to the propagative organ, and is developed from it, 

 forming an appendage, the leaf, just as the root has taken on 

 new characters. Through forms like Botrychium, Danaca and 

 Angiopteris is presented a series of changes which are suggestive 

 of the changes which led the way to the gradual separation of the 

 sporogenous tissue into separate portions, sporangia, and their 

 distribution over the surface of the leaf. Thus in Anthoceros the 

 sporogenous tissue is imperfectly separated forming a synangium, 

 in Botrycluum this septation of the propagative cells is carried 



