38 PLANT GEOGRAPHY 



ranean produce more steppe than forest and a type of 

 vegetation adapted to economy of transpiration, whilst 

 the summer rains of east Australia produce a much 

 more luxuriant tree-growth. 



The minor distinctions of habitat, or situation, within 

 the great climatic regions are largely determined by the 

 presence of water in the soil. Thus in England Alders 

 and Willows grow in undrained land, Elms will flourish 

 on stiff clays, Oaks prefer a loam as being less liable to 

 stagnation, while Beech requires a warm, well-drained 

 soil. It is not surprising that no external influence has 

 resulted in such marked modifications of plant structure 

 and form as has the greater or less supply of moisture 

 m the air and in the soil, or other medium in which 

 plants grow. This, however, is not a question of the 

 amount of water present, but of the amount available to 

 the plant. Salt water and acid soil-water, such as that 

 in many stagnant bogs, cannot be utilised by the 

 majority of plants; nor will roots absorb water when 

 the soil-temperature falls below a certain point. 



Apart from aquatic plants, which we will consider 

 separately, the two most marked types of adaptation in 

 this respect are those of (i.) plants growing under con- 

 ditions in which an abundant and constant supply of 

 water necessitates increased facilities for its discharge, 

 which are termed hygrophytes ; and (ii.) those of plants 

 in which a scanty supply has led to the development of 

 means for increasing absorption and diminishing tran- 

 spiration, which plants are termed xerophytes. Plants 

 of intermediate character, not specially modified for 

 excessive moisture or drought, are termed mesophytes, 

 among which we may class those plants, like our de- 

 ciduous broad -leaved trees and shrubs, which develop 

 bud-scales and corky periderm xerophilous characters 

 in ihe cold and dry season of winter, but thin leaves, 

 like those of hygrophytes, in summer. These have been 

 termed tropophytes. Hygrophytes may have a poorly 

 developed root-system; long stems, often climbing; large 

 thin leaves with numerous stomata and, sometimes, with 

 long acuminate points or " drip-tips " and other water- 

 pores from which drops of water exude; and no prickles, 

 hairiness, or waxy bloom on the surface, or succulence in 

 texture. This type is abundantly represented in the 

 Amazon forests. 



