[ '^S2 ] 



CHAPTER XX 



GASEOUS EXCHANGE IN WATER PLANTS 



THE problems which a water plant has to solve, in 

 connexion with its assimilation and respiration, differ 

 widely from those which confront a terrestrial plant, since, 

 instead of being surrounded by atmospheric air, it passes its 

 life in water holding only a certain amount of air in solution. 

 Owing to the varying solubility of the atmospheric gases, the 

 dissolved air differs from free air in composition. At 15 C, 

 the proportions in which the constituents should occur have 

 been calculated to be as follows ^r 



FREE AIR DISSOLVED AIR 



Carbon dioxide 0-04% 2-19% 



Oxygen 20-8o% 33-98% 



Nitrogen 79-16% 63-82% 



In practice, however, the air dissolved in the surface layers of 

 the water of lakes and streams, under natural conditions, yields 

 varying figures when analysed, but all observers appear to agree 

 that, as regards carbon dioxide it is supersaturated, sometimes 

 highly so^. It seems clear that the excess cannot be obtained 

 by diffusion from the air, for an American writer"^, who has 

 experimented with Elodea canadensis^ has shown that sufficient 

 carbon dioxide to keep this plant growing, or even alive, does 

 not diffuse into water exposed to atmospheric air at Baltimore 

 during the winter months. He demonstrated that all the carbon 



^ Devaux, H. (1889). The proportion of nitrogen given in this table 

 naturally includes the other inert gases which were not distinguished in 

 Devaux's time; the amount would be more correctly stated as including 

 approximately 78 per cent, of Nitrogen and i per cent, of Argon. 



2 Forel, F. A. (i 892-1904); Regnard, P. (1891). 



3 Brown, W. H. (19 13}. 



