228 BRITISH PLANTS 
frequently explore the ground at different levels. This is 
shown well in a wood where bluebells, bracken, and 
grasses show a distinct stratification (Fig. 108). 
To make a complete study of an association, something — 
more than a mere list of plants should be made. The 
habitat, which determines the distribution of the plant, 
should be examined in detail, and the adaptation of the 
plants to their environment carefully studied. The 
physical and chemical nature of the soil and the under- 
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d REVISE RS AES 
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Fie. 108.—Srcrion or Som in A Woop, SHOWING STRATIFICATION OF 
a Prants. (AFTER WOODHEAD.) 
a, Holcus mollis ; b, Pteris aquilina ; c, Scilla nutans ; d, humus; e, light 
loam ; /, stiff clay. 
lying rock ; the rainfall, its total amount, and the number 
of wet days in the year; the humidity of the air; the 
temperature of both air and soil, recorded, if possible, at 
frequent intervals throughout the year; the amount of 
water in the soil, and, what is of very great importance, 
the character and amount of the substances in solution— 
are all factors of the habitat which require careful study 
in a detailed survey of the vegetation. As examples 
of what can be done in this respect the papers on 
