58 INTRODUCTION TO, GENERAL PHYSIOLOGY 



atmospheric nitrogen. Moreover, there are in the soil what are 

 called " denitrifying " bacteria, which cause a loss in the nitrate 

 unless it is rapidly used up by the plant. This loss involves 

 return of nitrogen gas to the atmosphere. 



Unless, therefore, there were some means of making use of 

 nitrogen from the atmosphere, there would be a continual loss 

 of nitrogen in the form in which alone it can serve as food for 

 plants and animals. The student is probably aware that there 

 are artificial processes by which the oxygen and nitrogen of the 

 atmosphere are made to combine to nitrous and nitric acids, and 

 others which combine nitrogen and hydrogen to form ammonia, 

 which is oxidised to nitric acid by a further process (p., p. 253). 

 But there is a natural process. There are bacteria in the soil which 

 are able to utilise nitrogen from the atmosphere to form the 

 material of their own bodies. When they die, this material serves 

 as a source of ammonia to the soil. The actual chemical reactions 

 by which nitrogen is made use of by these bacteria are not known, 

 but it is clear that a supply of energy is required. This is provided 

 for by oxidation of carbon compounds in the soil. Bacteria, with 

 similar powers, are present in the nodules on the roots of the 

 plants belonging to the order of the beans, clovers, etc., the 

 Leguminosae, and in rare instances in other orders (E., p. 187). 



This last case is one of those known as "symbiosis? where 

 organisms join together for mutual assistance. The leguminous 

 plant supplies the bacteria with a carbon compound to oxidise, and 

 receives in return material which serves it as a source of nitrogen. 

 Readers, of the " Georgics " will remember that Vergil advises 

 farmers togrow vetches on their fields before sowing wheat Another 

 interesting case of symbiosis is that of a marine worm, in whose 

 tissues cells of an alga containing chlorophyll are present. The 

 animal's waste nitrogen serves for the plant cell, and this in turn, 

 by aid of its chlorophyll, supplies carbohydrate to the animal 

 (p., p. 295). We may learn a lesson from this. Much advantage 

 \is to be gained by mutual co-operation in making use of what 

 \s put at our disposal in the outer world. Waste of energy is 

 involved in contest for its possession. Claude Bernard, the great 

 French physiologist, has pointed out how much more inspiring 

 it is to regard living beings as adapting themselves to surrounding 

 conditions, rather than as being in perpetual conflict with them. 

 The life of an animal, as he says, is part of the total life of the 

 universe. 



Sulphur and Phosphorus 



The supply of these elements in organic combination is involved 

 in that of nitrogen, since some of the proteins contain, as parts 



