THE EVOLUTION OF PLANTS 1 29 



gradually increases, as we pass to more highly organized 

 forms, indicating a progressive sterilization of the fertile 

 tissue during evolutionary development. A survey of 

 the sporophytic phases of the Uverworts, mosses, and 

 ferns will show how these sporophytes gradually in- 

 crease in complexity and importance, from the simple 

 condition in the liverwort Riccia, with almost no sterile 

 tissue, through the sporogonium of the higher liverworts 

 and mosses, to the leafy sporophyte of the ferns (Fig. 65). 

 The final step in the development of the sporophyte was 

 the differentiation of plants bearing only large spores 

 (megasporophytes), and those bearing only small spores 

 (microsporophytes), represented in the Angiosperms re- 

 spectively by the pistillate and staminate plants. The 

 progressive sterilization accompanied a change in the 

 habitat of the plants from water to dry land.^ 



On the other hand, a careful student of fossil plants 

 has recently been led to state that, "it is beginning to 

 appear more probable that the Higher Cryptogams (ferns 

 and fern alhes) are a more ancient and primitive group 

 than the Bryophytes, which would seem to owe thei[ 

 origin to reduction from some higher type."^ In view of 

 this diversity of opinion, we learn at once that great cau- 

 tion must be used in interpreting the evidence — that we 

 must not "jump at conclusions." 



103. Resixlts of the Method of Comparative Anatomy. 

 By their study of comparative anatomy and morphol- 

 ogy, botanists have been led to propose the following 



'"The fern, as we normally see it, is an organism with, so to speak, 

 one foot in the water, the other on the land." Bower, F. O., The origin 

 of a land flora, p. 82. 



2 Scott, D. H. The Evolution of Plants, p. 18. 



