120 VEGETABLE PHYSIOLOGY 
The amount of nitrogen varies but little. This gas 
has a certain feeble solubility in water, and a small quantity 
goes into solution in the water which saturates the cell- 
walls ; but as such nitrogen is not made use of in the cells, 
its absorption very speedily ceases, the cell-sap not being 
able to contain more than a trace of it. The 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 absorp- 
tion 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 it is 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 tem- 
porary 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 
