336 ECOLOGY 



and diluent. It is obvious, therefore, that a suitable 

 water-supply is the most important edaphic or soil 

 factor. Water carries into the soil gases from the 

 air, one of which, carbon dioxide, gives to water 

 the power of dissolving part of the mineral matter 

 of the soil, e.g. potash and lime. On the physical 

 character of the soil depends the amount of water 

 available for plants. The rainfall of a district will 

 play an important part in determining the character 

 of the vegetation. 



We generally find that plants growing in a sour, wet, 

 cold soil have peculiarly modified shoots. The leaves are 

 reduced , and either up-rolled or back-rolled ; the cuticle is 

 thick ; the stomata are sunk in grooves or pits ; and similar 

 devices occur which serve to reduce transpiration. Plants 

 growing in a wet soil may possess structures characteristic 

 of those found in habitats liable to periods of drought. If 

 the soil, though it contains much water, is too acid, too 

 cold, or if the water is otherwise rendered difficult to absorb, 

 it is said to be ' physiologically dry '. 



CHAPTER XXVII 



PLANTS OF HEDGEROWS AND WALLS 



Uses and distribution of hedgerows. Hedgerows provide 

 endless material for the study of plants and offer numerous 

 problems for solution. They are, however, fences intro- 

 duced by man and not a natural feature of the vegetation 

 of a country. Hedges are useful in many ways. They 

 protect the crops and surface soil from the drying and tear- 



