Lecture VII. 51 



hydrate, it is under all circumstances solar energy which is 

 absorbed in photosynthesis and made available in respiration. 

 Therefore all the work done by plants is referable to this source 

 alone. We can generalise still further and state that all the work 

 done by other organisms also even by the higher animals is 

 ultimately to be credited to this source ; for we have already seen 

 that all organic nature is dependent on the chlorophyll of green 

 plants for its supplies of food and that it is from the substance of 

 these plants all other living beings draw their supplies of energy, 

 which reappears in their work. 



As in photosynthesis so in respiration, we only know the be- 

 ginning and the end of the process. The stages of the reaction 

 are unknown. Thus it is quite possible that the absorbed oxygen 

 does not directly oxidise the soluble carbohydrates of the cell, 

 but that it combines with the protoplasm and oxidises, in the first 

 instance, some atomic group in it, which is replaced subsequently 

 by the available carbohydrates. The end result is the same. 

 For the carbohydrate is drawn on to rebuild the protoplasm and 

 disappears just as if it were directly oxidised. 



This view is rather supported by the fact that during life proto- 

 plasm is continually disintegrating, breaking down into simpler 

 substances. These wasting processes, of which respiration is one, 

 are classed together as catabolism. The products of catabolism 

 cannot always be distinguished from stages in the building up 

 processes of the cell, namely, its anabolism, and probably many of 

 the substances found in the cell are sometimes products of cata- 

 bolism and sometimes stages in anabolism. This is certainly the 

 case with carbon dioxide and water. Evidently such an arrange- 

 ment is very economical in material, but it seems only possible in 

 those organisms which do not liberate large quantities of energy 

 at a time. Thus it is only in the more active animal kingdom 

 that we find special organs developed for the elimination of the 

 waste products of catabolism. 



Some view the contractile vacuoles of such plants as Chlamy- 

 domonas in this light and hold they are organs for the elimination 

 of waste nitrogenous products from the cell. They may also 

 evacuate dissolved carbon dioxide, which however, without their 

 intervention, diffuses out of the cell. Certainly they act in this 

 way ; and in conformity with this view, contractile vacuoles are 

 only found in the vegetable kingdom in motile cells in which 

 destructive catabolic processes setting free energy are presumably 

 very active. 



It seems indeed more philosophical to regard the contractile 



