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 



