GENERAL ,,- 



443 



may have already been submitted to various preparatory operations to 

 render it available to the plant, as, for example, when nutrient substances 

 are absorbed after undergoing extracellular digestion by the aid of an 

 excreted enzyme. Similarly the assimilation of carbon dioxide is an 

 example of a remarkable photosynthesis by which a supply of organic 

 food is obtained, while nitrate bacteria can carry on a similar synthesis 

 by means of chemical energy, and when a fungus grows upon formic 

 or acetic acid and is supplied with inorganic salts, the chemical energy 

 obtained by the oxidation of these acids enables the plant to synthesize 

 from these simple materials all the substances, proteids, fats, carbohydrates, 

 &c. which it needs. Examples of preparatory changes are also afforded in 

 the storage and translocation of reserve food-material, and frequently far- 

 reaching chemical modifications are induced in order to provide substances 

 suitable for prolonged storage, while in rendering such substances available 

 for transport to the regions where they are required, equally profound 

 changes may be necessitated. 



Even the most complete knowledge of the intimate processes of 

 metabolism would not necessarily enable us to distinguish clearly in all 

 cases between the preparatory stages and the final act in a metabolic 

 process. Moreover, a substance which normally remains intact may be 

 drawn into metabolism when the plant is starved or when the external 

 conditions are altered in some way or other. The processes of photo- 

 synthesis and proteid-synthesis have already been dealt with in con- 

 nexion with the sources of organic food, and a few general observations 

 have been made upon the meaning and importance of metabolism in 

 general, which may serve as an introduction to this and the following 

 chapters (Sects. 50 and 5 1 ). Respiration and fermentation, though simply 

 special forms of metabolism, may for didactic reasons be treated as a subject 

 apart, and it is mainly owing to the profound decompositions which these 

 two processes induce that the chief supply of the energy is obtained for 

 vital activity and metabolism in general. The respiratory activity is made 

 evident and directly measurable by means of the excretion of the products 

 of decomposition, which must be continually removed so that an injurious 

 or inhibitory accumulation may be avoided. 



The purpose of methodical research is to investigate the exact progress 

 of each metabolic activity and to establish its importance in the plant's 

 economy, but at the same time attention must be paid to the inter- 

 acting relationships between the different processes and their mutual depen- 

 dence, for otherwise incomplete and therefore inaccurate ideas must 

 necessarily be obtained. Thus the insufficient or abnormal performance of 

 any functional activity must ultimately influence all the others, while the 

 cessation of a single metabolic activity will gradually cause the rest to 

 cease one after the other, death ultimately ensuing. Only when a much 



