74 LECTURE V. 



is at a low pressure (in his experiment O'l/ mm.) they can 

 do so. Results of a similar nature have been obtained by 

 Bert with reference to the oxygen of the air: they may 

 be briefly stated thus, that increase or diminution of the 

 atmospheric pressure is prejudicial to plant-life, but this 

 prejudicial effect is not produced when the experiment is 

 so arranged that the oxygen present exerts a pressure ap- 

 proximately equal to that which it exerts in ordinary air. 

 These facts fully illustrate the relation of the pressure of a 

 gas to its absorption. 



We may now enquire what are the gases which are 

 absorbed by the leaves of plants, without, however, entering 

 upon a discussion of their relative importance to the well- 

 being of the plant ; that subject will be treated of in subse- 

 quent lectures. 



The air, the medium by which the leaves of land-plants 

 are surrounded, is a mixture of gases having the following 

 average quantitative composition. 



Oxygen 2O'6i per cent, by volume. 



Nitrogen 77*95 



Carbon dioxide 0*04 



Aqueous vapour 1*40 



Ammonia ) 



Nitric acid} traces ' 



Since all these gases are soluble in water, it may fairly 

 be concluded that they can be absorbed by the cells of the 

 leaves, and this is in fact the case. Let us imagine for 

 instance a living cell, the cell-sap of which contains no gases 

 in solution, exposed to air for a short time. In this case, the 

 proportion of each gas absorbed will depend upon its solu- 

 bility in the cell-sap. Now the solubilities of the gases of 

 which air is composed bear the following relations to each 

 other : 



i vol. of water, at I5C. dissolves 



Oxygen 0*030 



Nitrogen 0^015 



Carbon dioxide . ..roo2 



