The Ferns and Their AlHes 



291 



can be transmitted from one part to another only slowly by 

 diffusion from cell to cell. They have no water-conducting 



Fig. 173. The life historj' of a fern. The prothallus (.-1) produces egg cells and sperms 

 in organs on the lower surface. One of the sperms set free from B unites with an egg 

 cell (shown in C), and produces a sexual spore. This germinates and produces the leafy 

 fern plant {D), which in turn produces asexual spores (G) on the lower side of the leaves. 

 By the bursting of the walls of the sporangium {n), the spores are set free. They 

 then germinate on the soil (in some species on rocks or trees) and produce a new genera- 

 tion of prothalli like the one shown in A . The prothallus is here shown about 4 times 

 its natural size. 



or food-conducting tissues, and for this reason they are all of 

 small size. 



The development of a conductive system is, therefore, an 

 additional step in the complete adjustment of plants to a land 

 en\'ironment. With a conductive system, the water may be 

 carried rapidly from one part of a plant to another. Con- 

 sequently stems and leaves may be raised far above the ground 

 level and yet receive suflFicient water from the roots to replace 

 that lost through transpiration. Likewise an adequate supply 

 of food may be transferred to the roots, which makes it pos- 

 sible for them to live in the soil, where they are unable to 



