TYPES OF VEGETATION 17 



choice ;the same is true of the climatic divisions. Neither 

 of the divisions, however, is very satisfactory from an 

 ecological point of view. A proper ecological grouping 

 must be based on something more than cKmate. It 

 should take into consideration not one factor of the 

 environment, however important, but all the factors, 

 both of soU and climate, which influence vegetation, and 

 lead to the estabhshment of plant-communities in definite 

 habitats. 



There is no sharp ^ne dividing one type of vegetation 

 from another, any more than there is a sudden change 

 from one climate to another. Woodland, for example, 

 gradually merges into heath, moor, or grassland, while 

 the transition from the grassland to the desert is almost 

 imperceptible. 



In temperate regions like our own, some 25 to 30 inches 

 of rain are required annually for the maintenance of 

 permanent natural forest, and as we approach the Tropics 

 the amount increases. When this minimum amount is 

 not reached — i.e., when the soU is too dry for forest — 

 tree-growth becomes diminished, and gradually gives 

 way to grass. Park-land or savannah is grassland, 

 interrupted with trees and woods, the latter occurring 

 generally in the wetter parts, and frequently marking 

 the line of the watercom'ses. 



With the increasing dryness of the soil, stunted and 

 thorny bushes make their appearance, constituting in 

 some regions (AustraKa, South Africa) definite com- 

 munities of scrub or bush ; tussocks of hard, wiry grasses 

 imperfectly clothe the soil, and as the moisture further 

 diminishes, the vegetation gradually disappears, the bare 

 places increase, until in the " desert " only a few specially 

 equipped plants are able to eke out a precarious existence, 

 and break the uniformity of the bare earth. 



Again, when we ascend the mountains of Scotland or 

 Wales, the trees vanish from every exposed spot, and 

 the great shoulders of the uplands are covered with dreary 

 stretches of bog, moor, and heath. The rainfall is heavy, 

 but the soil is cold ; covering the rock-surfaces are vast 

 accumulations of peat, which has its own characteristic 

 vegetation, giving a definite stamp to the scenery. 



These illustrations give us some idea of the influence 

 of climate upon the vegetation. The most important 



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