88 BRITISH PLANTS 



wet soils, the greater part of the heat absorbed is used up 

 in evaporating the water, and very little remains to raise 

 the temperature of the mass. A soil constantly damp 

 is therefore cold. In a dry soil little water is present, 

 and most of the heat absorbed is utihzed in raising the 

 temperature of the particles. In a wet country like ours 

 a pure clay soil is not generally desirable, for, besides being 

 cold and wet, it is heavy and expensive to work. In the 

 South of France, however, where the climate is warm and 

 dry, such a soil is valuable because of its power of retaining 

 water during long intervals of drought. 



Aeration of the Soil.^ — -The presence of an adequate 

 amount of air in the ground is very important. The 

 underground parts of plants have the same need of 

 oxygen for respiration as the parts growing in the air. 

 The oxygen passes into the roots in solution in water, 

 or, if the roots are old and woody, through the lenticels, 

 or air-holes, upon their surface. A poorly aerated soil 

 is a soil deficient in oxygen, and therefore unfavourable 

 to plant-growth. Deficiency of oxygen leads to imperfect 

 respiration, and this entails a checking or lowering of all 

 the vital functions, root-absorption among them. Defici- 

 ency of air in the soil also means the absence of bacteria 

 and earthworms as well as the production of " souring " 

 acids from rotting material, which has a still greater 

 unfavourable effect upon absorption. 



The Chemical Properties of the Soil. — Generally speak- 

 ing, the chemical properties of the soil are far less im- 

 portant than its physical. In two cases only does the 

 chemical nature of the soil seeux to have a marked effect 

 on the fiora : 



1. Salts. — No other salt is so universally present in 

 soils as common salt (sodium chloride). Generally, the 

 quantity is small, but in some localities the amount is 

 considerable, especially along the margins of the sea, 

 and in the proximity of salt lakes and marshes. The 

 characteristic plants in these places are halophytes (Gr. 

 halos, salt). These are succulent (e.g., glasswort, Fig. 10), 

 a type usually associated with desert-conditions. In a 

 desert rain seldom falls, but when it does the plants 

 absorb as much of it as they can, and store it up in special 

 tissues for use during the long intervening periods of 

 drought. The seashore is a physiological desert. Though 



