THE EVOLUTION OF THE SPOROPHYTE 403 



of the Eiccia group with those of Marchantia, Porella, and 

 Anthoceros). Thus tissue which originally developed spores, 

 or had spore-forming possibilities, became set apart for vegeta- 

 tive functions alone. In this manner the foot and stalk became 

 established in Marchantia and Porella, and the heavy walls of 

 the spore case in Anthoceros. This principle has been called the 

 "sterilization of potential sporogenous tissue," but a simpler 

 expression would be " the assumption of vegetative functions 

 by .tissues with spore-forming possibilities." 



Second. Portions of the chlorophyll-bearing regions of the 

 sporophytes developed stomata as in Anthoceros and certain 

 mosses, and this was the first appearance of the elaborate mech- 

 anism for chlorophyll work (photosynthesis), which later became 

 so highly developed in the leaves of ferns and seed plants. 



The pteridophytes carried the advance much farther, through 

 the third, fourth, fifth, and sixth great steps in the development 

 of the sporophyte. 



Th,ird. The sporophyte became independent of the gameto- 

 phyte by developing roots, and to these added stems and fronds. 



Fourth. This condition was associated with the differentia- 

 tion of a vascular tissue that made it possible for the sporophyte 

 to grow to a considerable height above the ground, (1) by ena- 

 bling it to maintain a connection with a water supply through the 

 roots, and (2) by providing it with a strong framework through- 

 out the stem and leaves. In their strengthening functions the 

 fibro-vascular bundles were greatly assisted by the development 

 of rigid tissues (schlerenchyma). In other respects, also, the 

 entire tissue structure, or histology, of the sporophyte became 

 much more complicated. 



Fifth. Fronds were differentiated into spore leaves, or spo- 

 rophylls, and vegetative, or foliage leaves. The spore leaves 

 became grouped into cones, and by heterospory were differen- 

 tiated into microsporophylls and megasporophylls. 



Sixth. The embryo sporophyte of heterosporous pteridophytes, 

 through the shortening of the gametophytic phases, came to use 



