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



(3) On the effect of water currents on the assimilation of 

 aquatic plants. By Francis Darwin, M.A., F.R.S., and D. F. M. 

 Pertz. 



The principal fact established by our research is the notable 

 increase in the gas given off by certain aquatic plants in the 

 process of assimilation, when the water in which they are culti- 

 vated is disturbed by stirring. 



The fact is of interest as bearing on our general knowledge of 

 the gaseous interchange in aquatic plants. It also bears on the 

 nutrition of these plants in still and running water. Moreover it 

 is of interest on the question of method, i.e. on the question of 

 how far the yield of gas represents assimilative activity. 



This last point is illustrated by the history of our research 

 which began with a chance observation made a few years ago, 



If a small quantity (about 5 p.c.) of alcohol is added to the 

 water in which a sprig of Elodea canadensis is bubbling under 

 the influence of light, the rate of bubbling at once shows a con- 

 siderable temporary increase. It becomes about twice as quick, 

 and returns to its normal pace in four or five minutes. 



It seemed possible that the plant was stimulated by the alcohol, 

 but it appeared more probable that the result was physical rather 

 than physiological. We therefore fitted up a simple apparatus 

 by means of which air was forced by the pressure of a column 

 of" water to escape through a capillary tube (0'2 to 0'3 mm.) 

 into a beaker of water. The air ceased to escape from the tube 

 while the column of water showed a pressure of about 20 cm. 

 When alcohol was added in such a way as to mix with the water in 

 the beaker close to the opening of the tube, air at once escaped 

 from the capillary and the water column fell several cm. This 

 result obviously depends on the fact that the surface-tension of 

 a mixture of alcohol and water is less than that of water. 



To discover whether the effect of alcohol on water-plants 

 also depended on diminished surface-tension at the point of 

 escape of the bubbles, it was necessary to eliminate the possible 

 stimulative effect of alcohol on the leaves. This was done by 

 Devaux's method 1 . The tube of a funnel was cut short below the 

 conical part, and in this opening the stalk of a Hottonia was held 

 by a plug of cotton- wool, so that the cut end projected into the 

 expanded part of the funnel. Melted gelatine was then poured in 

 from above, which became solid rapidly and thus closed the funnel 

 below with a water-tight plug through which the stalk passed. 

 The main body of the plant could now be immersed in a beaker 

 of water, while the bubbles from the cut end of the stalk escaped 

 into water contained in the funnel. When alcohol (5 p.c.) was 



1 Annales des Sciences Naturelles, ix. 1889. p. 67. In our experiments the 

 plant is in a reversed position to that figured by Devaux. 



