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 
Crassulacee 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 Opwntia 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 
