3i8 LIFE : OUTLINES OF GENERAL BIOLOGY 



when we think of anaerobic bacteria which do not flourish in ordinary 

 air, where the free oxygen is fatal to them, but can only flourish 

 when it is excluded. And what is true of these microbes is so far 

 true of many tissues or even entire multicellular organisms normally 

 aerobic; that if oxygen be excluded they may continue living anaero- 

 bically for a considerable time. 



In animals that breathe dry air, say mammals, there are familiar 

 arrangements which secure the inhalation and exhalation of air 

 from the lungs; and the out-breathed air has less oxygen and more 

 carbon dioxide than the in-breathed air. In our ordinary talk we 

 speak of the inhaling and exhaling as the breathing or respiration; 

 but the real function of respiration is a much deeper process — 

 namely the oxidation that is associated with the energy-changes that 

 go on in the living matter. Oxygen, brought to the tissues and cells, 

 is utilised in the breaking down or katabolism of the protoplasm, 

 and the energy set free is used in various ways, e.g. in movement 

 and in doing work. This is the essential function of respiration, which 

 is sometimes distinguished, as internal respiration, from the arrange- 

 ments for capturing oxygen and getting rid of the poisonous carbon 

 dioxide, which are summed up in the term external respiration. 

 These arrangements are very diverse in different types of animals, 

 some using gills and others lungs, some using air-tubes and others 

 the skin; but the gist of internal respiration is always the same 

 except in anaerobic organisms. 



Similarly among plants, a distinction is usefully drawn between 

 the aerating system, e.g. the interspaces and channels in the under 

 part of the leaf, and the internal respiration which concerns the 

 breaking down of the protoplasm or of complex proteins associated 

 therewith. The air enters and leaves ordinary flowering plants by 

 the mniute openings or stomata, most abundant on the under 

 surface of the leaf, but there is also diflusion in and out through the 

 delicate epidermis, apart from any openings. 



But the gaseous exchange involved in aeration (corresponding 

 to the animal's external respiration) is to be distinguished from 

 the release of energy by the breaking down of complex carbon 

 compounds which form part of or are closely associated with the 

 protoplasm (corresponding to the animal's internal respiration). In 

 some plants part of the energy released takes the form of heat, but 

 this is not known to be of use in the plant ; and the same may be said 

 perhaps of the lower animals, in which it is not conserved and 

 accumulated as in birds and mammals. In these warm-blooded 

 organisms the animal heat, automatically kept at a constant tem- 

 perature, is quantitatively sufhcient to have an important r61e in 

 increasing the rapidity of chemical reactions and securing a smooth- 

 working metabolism. In plants the role of the oxygen in the internal 

 respiration is essential to growth and to activity, and one of the 



