296 VEGETABLE PHYSIOLOGY 



is employed in the reconstruction of the protoplasm from 

 these products and the new food supplied. As, however, 

 the final result is the formation of a certain quantity of the 

 simpler bodies mentioned, there is always a balance of 

 energy set free. 



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 

 Crassulacece 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 

 the living substance. The resulting decompositions, 

 though taking at first the same course as in other cases, 

 did not go so far. Instead of the liberation of carbon 

 dioxide, there was found a considerable increase in the 

 amount of certain organic acids, chiefly malic and oxalic 

 acids, which remained in the cells, and which probably 

 represented the ultimate products of the decomposi- 

 tions. 



Though respiration is always proceeding wherever 

 there is living protoplasm, the activity of the process 

 is modified by different physical conditions. Of these, 

 temperature is one of the most important. There is a 

 lower limit, beyond which it appears to be suspended, 

 though life is not destroyed. This limit varies in different 

 plants, but is generally one or two degrees below the 

 freezing point of water. In a few cases, such as Conifers 

 and Lichens, it may even be -10 C., but this is rare. 

 As the temperature rises from this minimum point, the 



