114 STRUCTURE AND FUNCTIONS OF LEAVES 



Probably diastase or some other ferment in the green parts of 

 the plant transforms the newly made starch into sugar, and some 

 of this is apparently combined on the spot with nitrogen, sul- 

 phur, and phosphorus. These elements are derived from nitrates, 

 sulphates, and phosphates, taken up in a dissolved condition 

 by the roots of the plant and transported to the leaves. The 

 details of the process are not understood, but the result of the 

 combination of the sugars or similar substances with suitable 

 (very minute) proportions of nitrogen, sulphur, and phosphorus 

 is to form complex nitrogen compounds. These are not precisely 

 of the same composition as the living protoplasm of plant cells 

 or as the reserve proteids stored in seeds (Sees. 8, 12), stems 

 (Sec. 66), and other parts of plants, but are readily changed 

 into protoplasm or proteid foods as necessity may demand. 



Assimilation is by no means confined to leaves ; indeed, most 

 of it, as above suggested, must take place in other parts of the 

 plant. For instance, the manufacture of the immense amounts 

 of cellulose, of cork, and of the compound (lignin) characteristic 

 of wood fiber, which go to make up the main bulk of a large tree, 

 must be carried on in the roots, trunk, and branches of the tree. 



137. Metabolism. It is convenient to have a single word to 

 express all the chemical changes which are controlled by the 

 living protoplasts. Such a word is metabolism. It embraces all 

 the nutritive processes mentioned in Sec. 126, as well as respi- 

 ration and the chemical changes concerned in the excretion of 

 waste materials. 



There are two principal types of metabolic processes, con- 

 structive metabolism (such as photosynthesis), which unites 

 simpler compounds into more complex ones, and destructive 

 metabolism (such as respiration), which breaks up complex 

 substances into simpler ones. 



Digestive metabolism, performed by means of various ferments, 

 begins, as already mentioned, in the seed during germination and 

 is carried on in most parts of the higher plants during all active 

 periods of their lives. It is especially energetic in removing 



