316 SOIL TYPES [chap. 



and texture of the soil, viz., into sands, loams, and clays, 

 with subdivisions dependent on the presence or absence 

 of calcium carbonate, and upon the situation, as causing 

 the accumulation or otherwise of humus. 



In attempting to review the vegetation appropriate 

 to different types of soil it will be found that two 

 distinct factors must be taken into account the 

 relations of the soil to water, and its chemical con- 

 stitution which factors often interact in a complex 

 fashion, different causes producing the same effect. A 

 plant, for example, may be found upon sand because 

 of its dryness, or, because of the absence of calcium 

 carbonate usually associated with sand ; another 

 plant, having adapted its structure to use very small 

 quantities of water, may equally well be found on a 

 dry sand, or on a clay which holds so much water 

 as to be injurious to the ordinary plant. 



Plants have adapted themselves to conditions of 

 dryness in very diverse ways , in some cases, as in 

 gorse or broom, the leaf surface is much restricted ; in 

 others, the thickness of the cuticle has been increased, 

 or the surface of the leaf is thickly clad with hairs ; in 

 other cases the leaves possess special tissues for storing 

 water. Such plants are known as " xerophytes," and 

 are found on soils which appear to differ very much 

 from one another, for a soil may contain plenty of 

 water and yet be physiologically dry, because of the 

 presence of some other constituent hindering the 

 absorption of water by the plant. 



The areas on which xerophytic plants are found 

 include not only the true desert areas, where great 

 heat and intense illumination occur during the larger 

 part of the year, but also the pervious sandy soils re- 

 taining very little moisture sand dunes, shingle flats, 

 and the like. Again, the plants of alkali soils and ol 



