6 BRITISH PLANTS 
known as autotrophic, or self-nourishing ; plants which 
are not green, and all animals, except a few of the very 
lowest, are heterotrophic—that is, they derive their food 
from without. These external sources of food may be 
either living or dead. Organisms living on rotting organic 
matter—e.g., animal or plant remains—are called sapro- 
phytes, those which prey upon living bodies, parasites. 
In either case the ultimate source of food is the green 
plant. 
2. Heat. 
Life is impossible without a certain degree of warmth. 
Within certain limits, the activity of all the vital or 
physiological functions exhibited by plants—viz., assimi- 
lation, respiration, absorption, transpiration, growth— 
increases with a rise of temperature and decreases with a 
fall. It is therefore through the variation in the activity 
of these functions that we observe the effects of heat and 
cold upon plants (Chapter VII.). The general character 
of the vegetation everywhere is largely determined by — 
the climate ; heat is one of the fundamental factors of 
climate (Chapter [.). 
B. THE ATMOSPHERE. 
The air is the second great factor of the environment. 
Land-plants are immersed in it ; it dissolves in water, and 
so reaches water-plants. In dealing with the relations 
between plants and the atmosphere we have to deal with 
1. The chemical effects of air upon plants. 
2. The physical effects of air upon plants. 
1. The Chemical Effects of Air. 
The air is a mixture practically of three gases—oxygen 
(20°8 per cent.), nitrogen (79°10 per cent.), and carbonic 
acid gas (0°035 per cent.)—all of which are, directly or 
indirectly, of vital importance to plants. A varying 
amount of water-vapour is always present, as well as 
dust and impurities and traces of rare gases. 
(a) Water-Vapour.—The other constituents of the air 
remain very nearly constant, but the amount of water- 
