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 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 

 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 



