238 



PRACTICAL BOTANY 



home of small crustaceous lichens. As these grow, die, and 

 decay, and are replaced by others of their kind, the living and 

 decaying bodies tend to disorganize the rock. Weathering 

 processes also assist in crumbling the rock, and after a time 

 there is soil enough to permit the growth of other lichens and 

 mosses and finally of larger plants. These pioneer plants are 



eventually driven 

 from the rock by 

 others that can 

 live in the meager 

 soil that is pro- 

 duced by the li- 

 chens and mosses. 

 Certain kinds of 

 crustaceous lichens 

 are looked upon 

 as the forerunners 

 of higher vegeta- 

 tion in rocky re- 

 gions which are 

 too bare to per- 

 mit other forms 

 of vegetation to 

 live. They are 

 almost universally 

 distributed over 

 the earth. The 

 time required for the production of soil sufficient for the growth 

 of other plants depends largely upon the nature of the rock 

 and upon the climate. Upon some lava beds it is said^ that 

 after almost two hundred years I'rom their formation crusta- 

 ceous lichens in places are still the only plants to be found. 

 Lichens as food for herbivorous animals are of considerable 

 importance in regions where other foods are scanty or where 

 for parts of the year cold and snow render otlier vegetation 

 1 Warming, CEcology of Plants, chap. xvii. 



Fig. 1! 



A cup lichen (Cladonia) 



This lichen often appears on moist ground, and at times 



forms the cup-like reproductive hodies, even sometimes 



having some of these form upon other cups. Two and 



one-half times natural size 



