SEED DISTRIBUTION 



147 



by their leaves. But even perennials generally depend upon 

 the growth of seeds to continue the species. A forest of white 

 pines, for example, when all its trees have died of old age or 

 been killed by plant or insect enemies, by destructive winds 

 or forest fires, can only be renewed by the growth of young 

 pines from the seed. And in the case of annual plants all the 



FIG. 136. Dandelion fruits 



a, akene ; 6, beak of pappus; br, bracts; p, pappus (representing the limb of the 

 calyx) ; r, common receptacle for all the fruits. Twice natural size 



individuals in existence at any one time will have died in a 

 year, or little more, from that date, to be replaced by a new 

 crop sprung from seeds. 



It is important to notice how well suited most seeds are to 

 withstand conditions that would kill ordinary plants. Seeds 

 are not injured by the lowest natural temperatures, and they 

 resist considerably higher temperatures than those found in 

 most climates. Lack of moisture does not usually harm them. 

 And while some kinds of seeds remain capable of sprouting 

 only for a few days, most kinds will remain good for a year 

 and many for several years. The seed is a matured ovule 



