722 PART IV. THE PHYSIOLOGY OF PLANTS. 



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 unorganised ferments or 

 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 pro- 

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

 decomposition of the protoplasm-molecule is the central fact of 

 catabolism. 



The significance of the absorption of free oxygen in connexion 

 with catabolism appears to be this : the chemical decompositions 

 which constitute catabolism involve a certain expenditure of 

 energy, though the amount thus expended is very much less than 

 the amount evolved by the decompositions ; the smaller, then, the 

 the amount of the energy expended, the greater will be the amount 

 of available kinetic energy in the plant : now the entrance of free 

 oxygen into the decompositions facilitates their performance, so that, 

 under these circumstances, the largest amount of kinetic energy is 

 evolved at the smallest initial expenditure. 



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 kinetic 

 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 conditions. 



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. They are 

 unable, under these conditions, to carry on what may be termed 



