The Ferns and Their Allies 



291 



can be transmitted from one part to another only slowly by 

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



H 



FIG. 173. The life history of a fern. The prothallus (A) 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 in sporangia (F and G) on the 

 lower side of the leaves. By the bursting of the walls of the sporangium (H), the 

 spores are set free. They then germinate on the soil (in some species on rocks or 

 trees) and produce a new generation of prothalli like the one shown in A . The pro- 

 thallus 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 

 environment. 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 sufficient 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 



