42 PHYSIOLOGY 



soil free from combined nitrogen, e.g. conifers, but it is in the leguminosse 

 that their presence is most widespread. 



The source of the combined nitrogen, which can be built up by plants 

 into proteins and utilised in this form by animals, is thus not only the am- 

 monium nitrite produced by the agency of electric discharges in the atmo- 



FIG. 15. Section of a root nodule of Dorychnium. (VTTILLEMIN.) 

 a, cortical tissue ; b, cells containing bacteria. 



sphere, but also the free nitrogen of the atmosphere assimilated by various 

 types of bacteria. 



Sulphur is found in all soils in the form of sulphates, generally of lime. 

 As sulphates it is taken up by plants. In the plant cell a process of deoxida- 

 tion takes place at the expense of the energy derived either from the starch 

 or, in the case of bacteria, from other ingredients of their food-supply. It is 

 built up, together with nitrogen, carbon, and hydrogen, to form sulphur 

 derivatives and amino- acids such as cystine, and these, together with other 

 ammo-acids, are synthetised to form proteins. Practically the whole of the 

 sulphur taken in by animals is in the form of proteins. It shares the oxida- 

 tion of the protein molecule in the animal body which it leaves in the form of 

 sulphates. The output of sulphates by an animal can therefore be regarded, 

 like the nitrogen output, as an index of the protein metabolism. It is 

 returned to the soil in the form in which it was taken by the plant, and the 

 cycle can be continuously repeated. 



Iron, although forming but a minute proportion of the material basis 

 of living organisms (the whole body of man contains only six grammes), is 

 nevertheless indispensable for the maintenance of life. It is necessary, for 

 instance, in two important functions, viz. the formation of chlorophyll in the 

 green plant and the respiratory process in the higher animals. Although iron 

 forms no part of the chlorophyll molecule, plants grown in the absence of this 



