348 PHYSIOLOGICAL CHEMISTRY. 



Plant food. Waste products of animal life. 



Carbon dioxide. Carbon dioxide. 



Water. Water. 



Ammonia, NH 3 . Urea, CO(NH 2 ) 2 . 



Nitrates, M X NO 3 . Urates, M S C 5 H,N 4 S . 



f Calcium. f Calcium. 



Phosphates 1 | M ium . Phosphates Magnesium. 



Sulphates \ of j Sodium Sulphates r of j Sod}jm 

 Chlorides I Potassium . [ Potassium . 



Formation of organic substances by the plant. As shown in 

 the above table, plants take up the necessary elements for 

 organic matter from a comparatively small number of com- 

 pounds. All carbon is derived from carbon dioxide ; hydrogen 

 chiefly from water; oxygen from either of the two substances 

 named, as well as from the various salts; nitrogen either from 

 ammonia, nitrates or nitrites; while sulphur and phosphorus 

 are derived from sulphates and phosphates respectively. These 

 substances are taken into the plant chiefly by the roots, the 

 assimilation of the necessary mineral constituents being facili- 

 tated by an acid secretion (discharged from the roots) which has 

 a tendency to render these salts, present in the soil and sur- 

 rounding the roots, soluble. 



Water having absorbed more or less of carbon dioxide, of 

 ammonia or ammonium salts, and of nitrates, phosphates, and 

 sulphates of potassium, calcium, etc., enters the plant through 

 the roots by a simple process of diffusion, and is carried to the 

 various green parts of the plant (chiefly to the leaves), where, 

 under the influence of sunlight, a chemical decomposition and 

 the formation of new compounds take place, the liberated 

 oxygen being discharged directly through the leaves into the 

 atmosphere. 



It is difficult to explain fully the process of the formation of 

 highly complex organic compounds in the plant, because we 

 know so little in regard to the intermediate products which are 

 formed. It is, however, fair to assume that the various com- 

 pounds above mentioned as plant food, are first decomposed 

 (with liberation of oxygen) in such a manner that residues or 

 unsaturated radicals are formed, which combine together. From 

 these compounds, produced at first, more complicated ones will 



