THE CONDITIONS OF PLANT LIFE 29 



mechanism of these devices is usually dependent upon 

 moisture. Of these may be cited the ring of thickened 

 cells which surrounds the spore-case in most ferns, and 

 the strongly hygroscopic elaters of most liverworts, 

 and the curious structures forming the " peristome " 

 about the opening of the spore-bearing capsule of the 

 common mosses. 



The fruits and seeds of the flowering plants offer 

 numberless examples of specialized structures, evidently 

 adaptations to special environment. In the lower mem- 

 bers of the group, such as the pond-weeds and similar 

 simple aquatic types, the fruits are very simple and 

 the seeds are set free through its decay, falling to the 

 bottom of the water, where they remain until conditions 

 are suitable for germination. A similar condition of 

 things prevails among a good many land plants, e.g. 

 some of the grasses, but in very many of the higher 

 types special contrivances have been evolved by means 

 of which the distribution of the seeds is facilitated. 

 The violent opening of many seed vessels ; the wings 

 and floats developed by many seeds and fruits ; the 

 hooks, prickles, etc., found in many fruits and seeds, 

 by means of which they adhere to animals, and are 

 thus transported, are a few of these many devices, and 

 the presence of edible parts in fruits and seeds is 

 largely to be placed in the same category. 



Summary 



All plants agree in requiring for their existence 

 certain food substances which must be absorbed in the 

 form of solutions or gases. Of these food substances 



