128 VEGETABLE PHYSIOLOGY 



much more easily carried on, and its products are more 

 numerous and simpler. The decomposition and recompo- 

 sition go on side by side, simpler bodies being gradually 

 produced, either by their splitting from the protoplasm 

 directly, or by their being formed at the expense of the 

 more complex decomposition-products during processes of 

 slow oxidation in the substance of the protoplasm, till 

 finally a certain production of carbon dioxide and water is 

 arrived at. So long as the protoplasm remains alive the 

 amount of these is relatively small, reconstruction con- 

 tinually taking place. When, however, the protoplasm 

 dies, simple bodies, such as carbon dioxide, water, and 

 possibly ammonia, in addition, are produced abundantly 

 from the decomposition which attends its death. 



It is possible that such processes as these are supple- 

 mented by others, in which the living substance is not 

 so intimately concerned. The protoplasm may combine 

 loosely with oxygen, and transfer it to other substances 

 present in it, which are consequently more directly oxi- 

 dised, the protoplasm in some way inducing the oxidation 

 of compounds which oxygen alone cannot attack. In this 

 way the protoplasm may act as an oxidising agent, without 

 itself undergoing the decompositions suggested. 



The carbon dioxide is thus the final term in a series 

 of decompositions, of which the living substance is the seat 

 and into which it may actually enter, the decompositions 

 themselves being promoted by the access of oxygen. In 

 some cases, such as those of the succulent leaves of the 

 CrassulacecB and the tissues of the Cactus already alluded 

 to, this final term is not reached, no carbon dioxide being 

 exhaled. We have no reason to think that in these cases 

 a fundamentally different series of changes is set up. De 

 Saussure found that a piece of stem of Opuntia absorbed a 

 quantity of oxygen, which could not be extracted from it 

 by the air-pump. The fate of this oxygen must have been 

 similar to that which is absorbed by other plants ; it must 

 have entered into some form of combination, probably with 



