290 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, is such that the living substance gives oft' 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 mini- 

 mum. 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 



