218 NUTRITIVE METABOLISM 



fungi. It accompanies carbon in the construction of the more 

 important compounds of protoplasm. 



Nitrogen is generally absorbed from the soil in the form of 

 nitrates and, ammonium salts by green plants, although phanero- 

 gams and algae may absorb minute proportions of such substances 

 as urea, glycocoll, asparagin, leucin, tyrosin, guanin, kreatin, hip- 

 puric acid, uric acid, acetamide, and propylamine, and obtain some 

 nitrogen in this manner. Some independent organisms inclusive 

 of Clostridium and bacteria of leguminous tubercles have the 

 power of fixing the free nitrogen of the air. Nitrogen enters into 

 the composition of the proteids, and is equally important with 

 carbon in the construction of living matter. 



Sulphur is absorbed in the form of sulphates and enters into 

 the composition of many proteids, and some volatile oils. 



Phosphorus is absorbed in the form of the phosphates, and it 

 may also be taken up in some of its organic compounds such as 

 lecithin. It enters into the composition of nuclein, and plastin 

 which have an important place in the organized structure of the 

 cell and also enters into lecithin, the function of which is in some 

 doubt. 



Calcium is absorbed in the form of the phosphate, nitrate, sul- 

 phate and carbonate, undergoing decomposition in the last named 

 compound during the process. Calcium is not found in embry- 

 onic tissues, but is abundant in adult cells in which, especially, it 

 is infiltrated in the wall. It is difficult to attribute any direct 

 function to this element ; although its absence occasions serious 

 disturbances in the higher plants, yet fungi may carry on normal 

 development without it. The presence of calcium salts in a cell 

 results in the formation of insoluble precipitates when oxalic acid 

 is formed, which is probably one of the important uses of this 

 element. Its connection with pectic acid in forming cell-mem- 

 branes may prove to be its most important purpose. 



Potassium is absorbed as sulphates, phosphates, silicates and 

 chlorides. It is abundant in embryonic tissues, is found associ- 

 ated with reserve food and material in transit. It is also possible 



