MINUTE STRUCTURE OP LEAVES. 123 



known by the general name of metabolism. The change of 

 starch into grape sugar or maltose is a characteristic instance 

 of the non-constructive kind of metabolism. It is essential to 

 the growth of the plant that many and complicated transfor- 

 mations of material should occur within it; for example, 

 starch, oil, and such insoluble proteids as are deposited in the 

 outer portion of the kernel of wheat and other grains are 

 extremely well adapted to serve as stored nutriment, but, on 

 account of their insoluble nature, are quite unfit to circulate 

 through the tissues of the plant. The various kinds of sugar, 

 on the other hand, are not well adapted for storage, since they 

 ferment easily in the presence of warmth and moisture. 



By metabolic processes the tissues of the plant and their 

 contents are all constructed out of certain formative materials. 

 From starch, sugars, or fats, cellulose, the material of ordinary 

 cell walls, is made, and from various proteids protoplasm and 

 the chlorophyll bodies are produced. 



Two important differences between fixation of carbon and 

 the non-constructive or destructive type of metabolism should 

 be carefully noticed. Destructive metabolism goes on in the 

 dark as well as in the light, and it does not add to the total 

 weight of the plant. 



157. Excretion of Water and Respiration. — Enough has 

 been said in § 145 concerning the former of these processes. 

 Respiration or breathing in oxygen and giving off carbonic acid 

 gas is an operation which goes on constantly in plants, as it does 

 in animals, and is necessary to their life. For, like animals, 

 plants get the energy with which they do the work of assimila- 

 tion, growth, reproduction, and performing their movements, 

 from the oxidation or burning up of such combustible substances 

 as they can use for that purpose ; for instance, starch and sugar.* 



1 The necessity of an air supply about the roots of the plant may be shown by 

 filling the pot or jar in which the hydrangea was grown, for the transpiration exper- 

 iment, perfectly full of water and noting the subsequent appearance of the plant at 

 periods 12 to 24 hours apart. 



