45] CHAP. II. PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 197 



4. Catabolism. Under this term are included all the chemical 

 processes going on in the plant which lead to the formation of 

 simple substances from more complex ones. 



The chief physiological importance of the catabolic processes is 

 this : that, inasmuch as they consist in the decomposition of 

 relatively complex and unstable substances into others which are 

 relatively simple and stable, they necessarily involve a conversion 

 of potential into kinetic energy ; and it is by means of the kinetic 

 energy so evolved that the plant exhibits those phenomena, such as 

 growth, movement, etc., which characterise it as a living organism. 

 The degree of activity of life depends directly upon the degree of 

 catabolic activity ; when catabolism ceases, life ceases ; the organ- 

 ism is dead. A good illustration of this is afforded by the scarcely 

 perceptible catabolism of seeds, bulbs, etc., when quiescent, and 

 their very active catabolism when they begin to germinate. 



The catabolic processes of the plant are carried on either by the 

 living protoplasm itself, or by means of certain substances formed 

 by the protoplasm, which are termed enzymes. 



The catabolic processes carried on by the protoplasm are mainly 

 such as depend upon the absorption of free oxygen from without, 

 and are accompanied by an evolution of carbon dioxide ; in fact this 

 gaseous interchange between the plant and its environment, termed 

 Respiration, is the external manifestation of the performance of 

 these catabolic processes. The seat of these processes is the 

 protoplasm, and it is mainly the molecules of protoplasm that 

 are decomposed ; in other words, just as the construction of the 

 protoplasm-molecule is the ultimate result of anabolism, so the- 

 decomposition of the protoplasm-molecule is the central fact o 

 catabolism. 



The reason, then, why most plants die when they are deprived 

 of free oxygen, is that they are unable to carry on, under these- 

 circumstances, those catabolic processes by which the energy 

 essential to the maintenance of life is evolved ; just as a fire goes 

 out, that is the oxidation of the coal stops, under the same con- 

 ditions. 



Though it may be generally stated that living plants at all 

 times absorb free oxygen, and that the maintenance of life depends 

 upon a constant absorption of free oxygen, yet there are excep- 

 tions. There are, for instance, certain Fungi, such as Yeast and 

 Bacteria, which can live in the absence of free oxygen. Under 

 these conditions they carry on other processes of decomposition into 



