THE EVOLUTION OF THE SPOROPHYTE 403 



of the Riccia 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 Marcliantia 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 ex- 

 pression 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 in Anthoceros and in some of 

 the mosses, and this was the beginning of the elaborate mechan- 

 ism for chlorophyll work (photosynthesis), which is developed to 

 such a high degree 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. 



Third. 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 

 nbro-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 



