466 RELATION TO ENVIRONMENT. 



ing of the ground, which "heaves" some plants from the soil. 

 Leaves and other plant remains mulch the soil and check evapora- 

 tion of water. The influence of the chemical condition of the 

 soil is very marked in alkaline areas where the concentration 

 of salt in the soil permits a very limited range of species. So 

 the physical and mechanical conditions of the soil influence 

 plants because the moisture content of the ground is so closely 

 dependent on its physical condition. Rocky and gravelly soil, 

 other things being equal, is dry. Clay is more retentive of 

 moisture than sand, and moisture also varies according to the 

 per cent of humus mixed with it, the humus increasing the per- 

 centage of moisture retained. 



902. Climatic factors. These factors are operative over very 

 wide areas. There are two climatic factors: rainfall or atmos- 

 pheric moisture, and temperature. A very low annual rainfall 

 in warm or tropical countries causes a desert; an abundance of 

 rain permits the growth of forests; extreme cold prevents the 

 growth of forests and gives us the low vegetation of arctic and 

 alpine regions. 



903. Biotic factors. These are animals which act favorably 

 in pollination, seed distribution, or unfavorably in destroying or 

 injuring plants, and man himself is one of the great agencies 

 in checking the growth of some plants while favoring the growth 

 of others. Plants also react on themselves in a multitude of 

 ways for good or evil. Some are parasites on others; some in 

 symbiosis (see p. 85) aid in providing food; shade plants are 

 protected by those which overtop them; mushrooms and other 

 fungi disintegrate dead plants to make humus and finally plant 

 food; certain bacteria by nitrification prepare nitrates for the 

 higher plants (see p. 83). 



II. Vegetation Types and Structures. 



904. Responsive type of vegetation. In studying vegetation 

 in relation to environment we are more concerned with the 

 form of the plants which fits them to exist under the local con- 



