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 
