THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 43 



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 continually 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 by water containing carbon dioxide into silica 

 and ferrous carbonate ; the latter then absorbs oxygen from the atmosphere, 

 liberating carbon dioxide and forming ferric oxide. In the presence of 

 decomposing organic matter, the ferric oxide parts with its oxygen to oxidise 

 the organic substances and is converted once more into ferrous carbonate, 

 and this may be decomposed by the oxygen of the air as before. In the 

 presence of sulphates and decomposing organic matter, ferrous sulphate, 

 which is first formed, undergoes deoxidation to ferrous sulphide, and this may 

 again be oxidised to sulphates and ferric salts on exposure to the atmosphere, 

 so that both the sulphur and the iron act as oxygen carriers between the 

 atmosphere and the organic matter. Iron is obtained by plants from the 

 soil as ferrous or ferric salts. In the protoplasm it is built up into highly 

 complex organic compounds, and in this form is taken up by animals. It is 

 probable that the main requirements of the animal for iron, which are very 

 small, may be satisfied entirely at the expense 'of these organic compounds, 

 but there can be little doubt that the animal can, if need be, also utilise the 

 iron salts present in its food. The animal proceeds extremely economically 

 with its supply of iron. Any excess of iron above that needed to supply 

 the iron lost to the body is excreted almost entirely with the faeces in the 

 form of sulphide. In the soil this undergoes oxidation and returns once more 

 to the form in which it was originally taken up by the plant. 



Phosphorus is absorbed by the plant as phosphates. In the cell proto- 

 plasm it is built up with fatty acids and other organic radicals to form com- 

 plex compounds such as lecithin, a phosphorised fat, and nuclein, a com- 

 bination of phosphorus with nitrogenous bases of great variety. Both leci- 

 thin and nuclein are essential constituents of living protoplasm. Practically 

 the whole of the phosphorus income of animals is represented by these 

 lecithin and nuclein compounds. After absorption into the animal body they 

 are broken down by processes of dissociation and oxidation, with the pro- 

 duction, as a final result, of phosphates, which are excreted with the urine 

 or faeces and return to the soil. 



Chlorine, potassium, sodium, calcium, and magnesium are taken up by 

 the plants in the form of salts. Although playing an essential part in all 

 vital processes, they do not seem to be built up into organic combination 

 with the protein, and other constituents of the cell protoplasm. They are 

 therefore taken up also by animals in the form of salts, and as such are again 

 excreted with the urine. 



