640 PHYSIOLOGICAL CHEMISTRY. 



Plant food. Waste products of animal life. 



Carbon dioxide. Carbon dioxide. 



Water. Water. 



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



Nitrates, MzNO 3 . Urates, MzC s H 2 N 4 O 3 . 



f Calcium. f Calcium. 



Phosphates , | M ium _ Phosphates | , M ium . 



Sulphates - of Sodi(]m Su phates . of godium 



Chlorides ) potassium ^ Chlondes J Pw^ium. 



It should be remembered that no sharply defined line of demarca- 

 tion can be drawn between plants and animals. Synthetic processes 

 occur in the body of animals, and cleavage processes take place in 

 some plants. However, in the animal organism the processes of oxi- 

 dation and cleavage are predominant, while in plants those of deoxi- 

 dation and synthesis are prevalent. 



Formation of organic substances by the plant. As shown in 

 the preceding table, plants take up the necessary elements for organic 

 matter from a comparatively small number of compounds. 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, or from nitrates or nitrites ; 

 while sulphur and phosphorus are derived from sulphates and phos- 

 phates respectively. These substances are taken into the plant chiefly 

 by the roots, the assimilation of the necessary mineral constituents 

 being facilitated by an acid secretion (discharged from the roots) 

 which has a tendency to render these salts, present in the soil and 

 surrounding 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. However, it is fair to assume that 

 the various compounds 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, pro- 



