1896.] water currents on the assimilation of aquatic plants. 89 



The oxygen which arises as a bye-product of assimilation is less 

 diffusible than the C0 2 absorbed, and this necessarily leads to a 

 rise of gas-pressure in the intercellular spaces, and to an escape of 

 the internal atmosphere at any openings which may exist. It 

 therefore follows that the more vigorous the assimilation becomes 

 the more rapid is the escape. 



We have shown that in some cases at least stirring the water 

 produces an increased yield of gas, there is therefore prima facie 

 evidence that assimilation is actually increased. But unless we 

 can ascertain in what way the amount of oxygen yielded as free 

 gas and by diffusion is affected by stirring, we cannot be sure 

 of the point. 



We have made a few analyses of the gas collected over water 

 from Elodea, using water which had stood for twenty-four hours as 

 well as freshly-drawn water, so as to avoid errors from bubbling 

 due to gas-pressure. 



Elodea, Feb. 13, 1896. 



Fresh Tap-water. 

 per cent. 



9.58 to 11.34 a.m. 



11.38 a.m. to 12.43 p.m. 



still 

 stirred 



22 

 20 



Feb. 17, 1896. Fresh Tap-water. 



Feb. 18, 1896 



10 to 11.23 a.m. 



11.26 a.m. to 12.45 p.m. 



stirred 

 still 



19 

 20 



The percentage of oxygen is remarkably low in these cases. 

 Devaux found the percentage of oxygen yielded by Elodea in 

 the light to be about 25 p.c, and we have, in earlier experiments, 

 found a similar proportion. On the other hand we have also 

 found Elodea yielding no more than 20 p.c. when the evolution 

 of gas was certainly nearly all due to assimilation since it was 

 diminished to a quarter of its former amount by darkening the 

 plant. 



Moreover, Devaux has shown that in darkness Elodea may 

 give off gas containing only 16 p.c. of oxygen, and we have found 

 that Hottonia may only give off 14 p.c. of oxygen. It seems 

 possible therefore that a yield of 19 p.c. of oxygen may be an 

 index of assimilative activity. 



