THE WATER-RELATION 113 



(Calluna vulgaris) and Bog Moss (Sphagnum), thrive best under a 

 deficiency of lime, and regularly inhabit such soils. Other plants, 

 such as Dog's Mercury (Mercurialis perennis), are lime-loving. It is 

 not so much that the calcium requirements of plants varies, but 

 rather that lime is the chief basic substance of the soil, and its presence 

 affects the properties of the soil very considerably, especially the 

 soil-reaction. Some plants prefer an acid soil, others a neutral or 

 slightly alkaline one. Plants are also liable to be affected by the 

 presence of non-essential salts such as sodium chloride. Most plants 

 are unable to make satisfactory growth in a soil, such as that of a 

 salt-marsh or other maritime area, in which sodium chloride is rela- 

 tively abundant, because of the harmful effect of the salt. The group 

 of plants known as Halophytes (Chapter XI.) are, however, able to 

 thrive in such places, and their development is actually improved by 

 the presence of the salt. Enough has been said to indicate the com- 

 plexity of the relationship between the plant and the soil, the number 

 of factors that come into play, and the degree to which soil factors 

 affect the distribution of plants. 



B.B. 



