296 VEGETABLE PHYSIOLOGY 
the fact that the oxygen absorbed and the carbon dioxide 
exhaled are approximately equal in volume, carbon dioxide 
being perceptibly heavier than oxygen. Besides the 
carbon dioxide, however, there is always also a certain 
exhalation of watery vapour, which takes place quite 
independently of any supply from the root or the cut end 
‘of the stem. The nature of the metabolism, or the vital 
processes, 1s such that the living substance gives off both 
water and carbon dioxide, while it coincidently absorbs 
oxygen. This is quite independent of any constructive 
processes, for it can be observed when no nutritive material 
of any kind is supplied to the plant. 
Though respiration is constantly proceeding wherever 
living substance is found, the activity of the process is 
by no means uniform. With care it can be detected in 
such quiescent parts of plants as resting seeds, or buds 
during their winter suspension of development, but in 
such cases the gaseous interchange is reduced to a minimum. 
In growing shoots or germinating seeds in which vital 
processes such as the growth of protoplasm are going on 
rapidly, and life is very active, it reaches a maximum. 
In ordinary adult leaves and branches the activity of 
respiration is intermediate between the other two condi- 
tions. It is more intense, again, in the floral organs during 
the time of their maturation. We may say in general 
terms, wherever protoplasm is abundant, and the chemical 
processes connected with the manifestation of its life are 
going on most vigorously, there respiration is most active. 
It is connected especially with the vital processes, and is 
not associated directly with the presence of food materials. 
A proof of this is afforded by an estimation of the activity 
of respiration in seedlings, which, in the case of wheat, has 
been found to increase steadily for about a fortnight, and 
then to decline. Further evidence is afforded by the fact 
that if seeds are thoroughly dried they do not respire. In 
this condition the protoplasm is completely quiescent, so 
far as we can ascertain. If, however, only a little water is 
