114 GROWTH AND WORK OF PLANTS 



these into nitrates by these bacteria, gives them the energy \o fix the carbon 

 from the carbon dioxide. 



181. Metabolism. Metabolism means change, or changing around, and 

 in the life processes of animals and plants refers to the chemical changes 

 taking place. The building up processes, the different kinds of assimilation, 

 are constructive metabolism (anabolism), while the breaking down processes, 

 respiration, fermentation, decay, etc., are destructive metabolism (katabolism). 



182. The building up of proteins in the plant. The pro- 

 tein substances in (paragraph 19) plants have a more compli- 

 cated molecule than the carbohydrates since they contain carbon, 

 hydrogen, oxygen, nitrogen, sulphur and sometimes phosphorus. 

 These are largely formed in the leaf, but are formed in other parts 

 of the plant also, and perhaps can be formed in limited extent in 

 any living plant cell. Some plant physiologists claim that they 

 are formed only in sunlight and in the presence of chlorophyll.* 



183. The leaf as an organ of assimilation. The formation 

 of carbodydrates, the proteins and some other substances, while 

 taking place in all green parts of plants, is predominant in the 

 leaf because of its adaptition to the process of photosynthesis, 

 and the close relation of the formation of other products closely 

 dependent on the carbohydrates. The sulphates, phosphates, 

 nitrates, etc., absorbed by the roots meet the carbohydrates, 

 (see footnote *) in the leaf and are here assimilated, 



IV. DIGESTION.! 



184. Digestion in plants includes those processes in which 

 food products and food substances either in the plant or outside 

 of it are changed chemically into a condition in which they can be 



* One reason for this belief is that in some plants it has been found that 

 the nitrates (salts of nitric acid=HNO 3 ) which are absorbed by the roots 

 accumulate in the leaves during the night, and disappear during the day. 

 During the day the nitrates unite with carbohydrates (sugar) and some 

 sulphur compounds to form proteins. Those who believe that proteins are 

 also formed during the night concede that they are formed more actively 

 during the day because photosynthesis is then going on, since the formation 

 of carbodydrates is then active and in their elemental condition may more 

 easily unite with the nitrates. 



j" See also paragraph 208. 



