io8 TEXTBOOK OP BOTANY 



most of the work of food-making and food-storage was trans- 

 ferred from the sexual to the asexual generation. This 

 transfer of work could not take place in a plant like the moss, 

 whose asexual generation has no direct connection with the 

 soil and so must receive water and other substances from 

 the sexual generation. Before the asexual generation can 

 become independent, it must find a way of reaching the soil ; 

 and this some ancestor of the fern did by the development of 

 a root. Nothing at all like a root was ever developed, so 

 far as we kno\\', by a sexual plant ; and the possession by the 

 asexual generation of this new organ, which can draw upon 

 the resources of the soil and can firmly anchor the plant, is 

 one reason why the asexual generation of the fern (and of 

 seed plants as well) has come to be so large and complex. 

 The sexual generation of the fern, on the contrary, haidng 

 little need of food-making and food-storing tissues, has be- 

 come very small. The history of the life of the fern may be 

 represented by a diagram (Fig. 59) which is like that used for 

 the moss (Fig. 48) except for the relative length of the lines 

 representing the sexual and asexual generations. 



134. Other Fems. — A comparatively small' number of 

 species of fems live in temperate regions, where they do not 

 as a rule grow in large numbers. But in tropical and sub- 

 tropical countries we find many more species, and in such 

 countries fems often form conspicuous features of the land- 

 scape. Most of the fems bear spore sacs in groups on the 

 under surface of the leaf, and not in a line near the margin 

 as those of the bracken fern are borne. Among the common 

 species in the United States and Canada are the lady fem, 

 the rock fem, and the maidenhair fem. Others of interest, 

 but not so common, are the royal fem, our largest species ; 

 the ostrich fem, the cinnamon fem, and the walking fem. 

 Among those of the tropics are the various species of tree 

 fems already mentioned. Some small tropical fems live on 

 the trunks of trees high above the ground. 



