ECOLOGY 



99 



species of Spermatophytes living to-day have all appeared on 

 the earth in comparatively recent times. 



221. Plant remains, therefore, in the form of coal, constitute 

 a very considerable part of the earth's crust and have largely 

 determined the modern conditions of human activity. But 



Fig. 55. — Sarracenia purpurea, in bloom. In this species the leaves are shaped 



like a pitcher. X1/3. 



vegetation is at the present time an important agency in both 

 constructively and destructively modifying the physiographic 

 features of the earth. 



222. All forms of vegetation assist in breaking up rocks and 

 dissolving minerals and thus contributing greatly to the general 

 process of weathering or decay of rocks. Vegetation is also 



