THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 47 



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, as well as the 

 latter, is excreted almost entirely with the faeces in the form of sul- 

 phide. 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 

 protoplasm it is built up with fatty acids and other organic radicals 

 to form complex compounds such as lecithin, a phosphorised fat, and 

 nuclein, a combination of phosphorus with nitrogenous bases of great 

 variety. Both lecithin 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 pro- 

 cesses of dissociation and oxidation, with the production, as a final 

 result, of phosphates, which are excreted with the urine or fa3ces 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 proto- 

 plasm. They are therefore taken up also by animals in the form of 

 salts, and as such are again excreted with the urine. 



Little is known about the significance, if any, of the other elements 

 which I have mentioned as occasional constituents of living beings. 

 Silicon, which is of universal distribution, is assimilated as silica, 

 probably in colloidal solution, and is distributed in minute quantities 



