221 



substances. It is characterized by its great adhesiveness 

 when wet and by an extraordinary power of expansion and 

 contraction under the influence of moisture and heat respect- 

 ively. In the dry season it is traversed by great fissures, 

 often many feet in depth. As a rule it is very fertile, especially 

 for herbaceous plants and field crops, one of its names being 

 due to its suitability for the cultivation of cotton. The Babul 

 (Acacia arabicd) is frequently found on this class of soil, as 

 also is Eutea frondosa. 



Aspect, slope and the character of the underlying strata Aspect, 

 also obviously affect the loss of water from the soil through Slope and 

 evaporation and drainage. S^t?" 18 



Finally, there is the important factor of the presence, or Effect of 

 absence, of vegetation on the soil. A covering of forest vegeta- Vegetation, 

 tion protects the soil from insolation and the effect of winds and 

 thus diminishes evaporation ; it leads to the formation of humus 

 which absorbs and retains water well ; the force of rain is broken 

 by the leaves, twigs, and branches, from which it falls gently, 

 and percolates into the soil, whereas on an area bare of vegeta- 

 tion much of the rain water runs off and is lost, especially on 

 steep slopes. It must also not be forgotten that the rain water 

 contains appreciable quantities of useful nitrogenous compounds 

 and of mineral salts which are essential plant food-materials, so 

 that, by intercepting the rain water alone, humus may con- 

 siderably increase the fertility of a soil. 



The character of the soil appears to be the principal factor Soil the 

 determining the distribution of the Sal tree, this species Principal 

 requiring a loose well-drained soil, containing a con- j-j^?*. 

 siderable proportion of humus. It cannot thrive on the mining the 

 heavy soil which usually overlies trap rocks and, in Central Distribution 

 India, the extension of Sal westwards is abruptly checked of Salt 

 by these rocks, Teak forest commencing where the Sal forest ends. 



200. The effect of temperature on (3) TEMPER- 



the distribution of plants has been recognised from very early ATURE - 

 days, and it was found that, if the surface of the globe was Vegetation 

 divided into zones of the same mean annual temperature, each due to Tem- 

 of these zones was characterized by possessing particular P erature - 

 species of plants, which did not thrive far beyond the limits 

 of their particular zone. These zones are given in consecutive 

 order below, that with the highest mean temperature first, 

 and that with the lowest last. 



I. Tropical Zone. This is sometimes sub- divided into 

 the Equatorial, Tropical and Sub-Tropical Zones. 



