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 climate. It 
should take into consideration not one factor of the 
environment, however important, but all the factors, 
both of soil and climate, which influence vegetation, and 
lead to the establishment of plant-communities in definite 
habitats. 
There is no sharp line 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—7.e., when the soil 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 watercourses. 
With the increasing dryness of the soil, stunted and 
thorny bushes make their appearance, constituting in 
some regions (Australia, 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 
2 
