46 PHYSIOLOGY 



in the form of proteins. It shares the oxidation 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 



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

 a, cortical tissue ; b, cells containing bacteria. 



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 chloro- 

 phyll molecule, plants grown in the absence of this substance remain 

 etiolated, but form chlorophyll if the smallest trace of iron is added 

 to the soil in which they are growing or even if the leaves are washed 

 with a very dilute solution of an iron salt. In animals iron forms an 

 essential constituent of haemoglobin, the red colouring-matter of the 

 blood, whose office it is to carry oxygen from the lungs to the tissues. 

 It is probable too that the minute traces of iron in protoplasm exercise 

 an important function in the processes of oxidation which are con- 

 tinually going on. Even in the inorganic world iron plays the part of 

 an oxygen carrier. In the earth's crust it occurs as ferrous salts and 

 as ferric oxide. The ferrous silicate, for instance, may be decomposed 



