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- 



5* d 



FIG. 108. SECTION OF SOIL IN A WOOD, SHOWING STRATIFICATION OF 

 PLANTS. (AFTER WOODHEAD.) 



a, Holcus mollis ; 6, 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 



