290 EVOLUTION OF PLANTS 



from the fertilized egg-cell, is very simple in structure, 

 and is devoted almost exclusively to spore-production, 

 having no power of independent growth, but living as 

 a parasite upon the tissues of the gametophyte. Within 

 the Hepaticse, however, are forms in which the sporo- 

 phyte becomes much more important, and in the genus 

 Anthoceros, especially, it reaches a large size and 

 becomes almost independent of the gametophyte owing 

 to the development of several layers of green tissue 

 communicating with the atmosphere by means of sto- 

 mata, exactly as in the higher plants. Here, too, only 

 a small part of the tissue is devoted to spore-formation, 

 and the growth of the sporophyte does not cease as soon 

 as the first spores are ripe. No root, however, is de- 

 veloped, and the sporophyte remains dependent upon 

 the gametophyte for its supply of water and for such 

 food elements as it cannot obtain from the air. The 

 duration of its growth is therefore determined by 

 that of the gametophyte. 



The gametophyte in the Bryophytes may reach a very 

 considerable size, and is sometimes quite complicated 

 in its structure, but this does not necessarily corre- 

 spond to the development of the sporophyte, which 

 reaches its highest expression in forms with a very 

 simple gametophyte. 



It is in the Pteridophytes, or ferns, that the sporo- 

 phyte first becomes entirely self-supporting. Here the 

 embryo-sporophyte closely resembles that of the mosses, 

 but soon develops the special organs, stem, root, and 

 leaf, which distinguish the fern-sporophyte and render 

 it independent of the gametophyte, which now withers 

 away as soon as the young sporophyte is established. 



