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 hvuig or dead. Organisms Hving 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 hmits, the activity of aU 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 fimdamental factors of 

 chmate (Chapter I.). 



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 (0035 per cent.)— all of which are, directly or 

 indirectly, of vital importance to plants. A var5dng 

 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- 



