114 VEGETABLE PHYSIOLOGY 
percentage of nitrogen in a volume of gas obtained from a 
plant may not correspond with the percentage in an equal 
volume of air, but this will result from an interference 
with the amount of oxygen and carbon dioxide, and not be 
due to an absorption or exhalation of nitrogen, neither of 
which takes place to an appreciable extent. 
The variations in composition which are noticeable are 
due to two processes which are characteristic of the vital 
processes of green plants. As we shall see in a subsequent 
chapter, all the green parts of plants are during daylight 
engaged in absorbing carbon dioxide from the air, and 
exhaling oxygen into it. In such parts this interchange 
takes place with considerable energy, and the composition 
of the air in their intercellular spaces varies accordingly, 
becoming relatively much richer in oxygen than itis in the 
deeper parts which are not illuminated, and which contain 
no green colouring matter. An interchange in the opposite 
direction goes on continually wherever there is living 
protoplasm, for this is always absorbing oxygen so long 
as it lives, while a good deal of carbon dioxide is simul- 
taneously exhaled. This process, unlike the other one, is 
not confined to any particular part of the plant, nor is it 
ever in abeyance. Thus the plant shows a continuous and 
universal production of carbon dioxide, and a partial and 
local consumption of this gas. At the same time it 
exhibits a constant demand for oxygen everywhere, and a 
temporary production of it in places. The composition of 
the air in the intercellular spaces must therefore vary 
from time to time, and from place to place, according to 
the intensity and the localisation of these changes. 
The process of diffusion, which is one of the phenomena 
characteristic of gases, leads to a constant occurrence of 
gaseous currents in plants. These currents may be influ- 
enced by various properties of the gases concerned, and by 
other factors, both internal and external. The rate at which 
carbon dioxide is absorbed by the cell-wall is very different 
from the rate of absorption of oxygen. If an atmosphere 
