8 BRITISH PLANTS. 
2. The Physical Effects of Air. 
Of these the most important in relation to plants are : 
(a) The effects of air in motion—i.e., wind (Chapter 
VIIT.), and— 
(6) Its rate of diffusion through perforated membranes, 
such as exist in plants in the form of surfaces pierced 
with small holes or stomata. 
C. THE SOIL. 
The earth, or edaphic factor, is the third great factor 
of the environment. The soil is the medium in which 
the great majority of plants are fixed, and from which 
they draw all the materials necessary for nutrition, 
excepting only the carbonic acid gas required for photo-. 
synthesis and oxygen for respiration. 
The soil is considered under the following heads : 
(a) The physical and chemical properties of the soil 
(Chapter IX.). 
(6) The relations of soi] to water (Chapter IX.). 
(c) The decaying organic matter in the soil—humus 
(Chapter X.). 
(d) The biology of the soil (Chapter X.). 
Of all the factors in the environment water is the most 
important. It leaves a stamp upon the vegetation that 
no other factor does. A large portion of Part I. is 
concerned with the relations between plants and water. 
Such, briefly summarized, are the five ecological factors 
of the environment—light, heat, water, air, and soil. 
Each is treated in detail in the chapters following. 
