88 Mr Darwin and Miss Pertz, On the effect of [Feb. 10, 



Cause of the 'phenomenon in Elodea. 



There can be no doubt that the increased yield of gas when 

 the water is stirred is simply due to the movement of the water 

 bringing the leaves rapidly in contact with fresh layers of water 

 and thus hastening the process of diffusion. It illustrates in fact 

 the same phenomenon as the well-known experiment in which a 

 strong solution of a coloured salt {e.g. potassium bichromate) is 

 covered by a layer of water in order to demonstrate the extreme 

 slowness with which the yellow colour spreads to the upper layers 

 if the jar is undisturbed. 



It is clear that the effect of stirring on water-plants is a 

 physical phenomenon in the sense that it is not necessarily con- 

 nected with the act of assimilation. 



This is proved by taking advantage of the fact that aquatic 

 plants placed in water highly charged with gas give off bubbles in 

 the dark 1 . 



We observed this form of bubbling by using a feeble illu- 

 mination just enough to enable us to count the bubbles, but quite 

 insufficient for assimilation. When the water was stirred the rate 

 of bubbling was clearly increased. In other cases where the dark- 

 bubbling had ceased, it was started again by means of stirring. 



Assimilation. 



The question whether the increase in the yield of free gas 

 produced by stirring necessarily implies increased assimilation 

 is one to which we cannot give a complete answer. 



The solution involves a more complete control of the physical 

 conditions than obtained in our experiments, as well as analytical 

 data which we cannot at present supply. 



The gaseous exchange of an aquatic plant does not necessarily 

 involve any yield of free gas, it depends primarily on diffusion 

 of dissolved gases both into and out of the plant. The escape 

 of bubbles is, as Devaux and others have pointed out, an accidental 

 occurrence depending on the existence of fortuitous openings into 

 the intercellular spaces. It is clear therefore that the yield of 

 free gas cannot, without further inquiry, be assumed to represent 

 assimilation. 



There is, moreover, as already pointed out, an evolution of 

 gas quite independent of the action of light, depending on the gas- 

 pressure in the water. Devaux has explained how it is that, in 

 spite of the above considerations, the rate at which gas is 

 given off does fairly well represent the activity of assimilation. 



1 See Devaux, Annates des Sc. Nat., 1889, p. 38, where the literature of the 

 subject is given. 



