CHAPTER XXII 
PLANT ASSOCIATIONS AND FORMATIONS 
In the course of this book we have made several attempts 
to classify plants ecologically. The simplest and most 
obvious was to divide the vegetation into types according 
to their appearance when beheld as masses of individuals 
in the landscape. This physiognomic grouping gave us 
the great types of vegetation which we know as wood- 
land, moorland, grassland, and desert (p. 16). We 
found that these types could only be associated with a 
definite kind of climate if subdivided according to climatic 
differences. This led us to a climatic grouping of plants 
(p. 16). Later on, in dealing with the relation between 
water and plants (Chapters II. to VI.), and then with the 
relation between water and the soil (Chapter IX.), and, 
last of all, between the soil and the plant (Chapter X.), we 
were forced to enlarge our conceptions of climate. Most 
of the climatic factors act indirectly upon the plant 
through the soil, and when we come to consider the details 
of the vegetation, the soil as the exponent of climate 
becomes more and more important (p. 84). 
We pointed out on p. 17 that a satisfactory and 
scientific ecological grouping of the vegetation can only 
be obtained if we take into consideration all the external 
factors of the environment influencing the vegetation. 
Some factors, of course, are more important than others, 
but almost any factor may, in some place or another, 
assume an importance which will leave a stamp on the 
vegetation growing there. Considered thus, the term 
habitat has a special meaning when used in ecology. If 
means the abode of a plant or community of plants as 
biologically conditioned by the factors of the environment. 
In defining the habitat of any plant, therefore, we must 
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