io8 TEXTBOOK OF 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 know, 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, having 

 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 Ferns. A comparatively small number of 

 species of ferns 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 ferns often form conspicuous features of the land- 

 scape. Most of the ferns 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 fern, 

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

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

 the ostrich fern, the cinnamon fern, and the walking fern. 

 Among those of the tropics are the various species of tree 

 ferns already mentioned. Some small tropical ferns live on 

 the trunks of trees high above the ground. 



