46 



Prof. H. H. Dixon. 

 Table I. 



[Nov. 7, 



Transpiration per hour. 



Date. 



Temperature. 



grammes. 





°0. 



2 65 



July 24 



18-5 



2-14 



„ 25 



20-5 



3'25 



„ 26 



22-0 



0-87 



„ 27 



19-5 



0-97 



„ 28 



21-5 



Watered — 







2 23 



» 28 



24-0 



"Watered — 







1-59 



„ 30 



19-0 



3-05 



,. 30 



22-5 



The dependence of transpiration on the supply is prettily illustrated by 

 Darwin's experiment with the horn hygroscope.* Darwin records that when 

 the hygroscope is applied to the leaves of a branch severed from a plant, it 

 indicates a gradually diminishing rate of transpiration as the store of water 

 in the branch is gradually exhausted. A diminished rate of transpiration is 

 also indicated by the hygroscope when the supply to a branch is reduced by 

 the application of a clamp constricting the water conduits leading to its leaves. 



The diminution in the rate of transpiration in these experiments is no 

 doubt due to a rise in the tension of the water columns supplying the 

 evaporating cells. The tension rises in the first case as the limited water 

 store is drawn upon, and the water surface is dragged into the cut surface of 

 the branch. The surface-tension forces developed there then oppose the 

 transpiration current. In the second experiment the tension is increased by 

 the rise of the resistance brought about by the reduction of the calibre of the 

 conduits. Further reduction of transpiration is occasioned by the closing of 

 the stomata and by other phenomena, which are themselves direct or indirect 

 consequences of the rise of tension in the water. 



According to this view an interesting observation of Darwin'sf receives a 

 ready explanation. There is a momentary increase of transpiration in these 

 experiments immediately after the separation of the branch and immediately 

 after the application of the clamp. In the first case we may assume that the 

 tension in the water supply is reduced by fracture, in the second case by the 

 compression applied when screwing up the clamp. Transpiration obviously 

 soon re-establishes tension, which becomes greater in both cases than it was 

 originally. 



* Darwin, " Observations on Stomata," ' Phil. Trans.,' B, 1898, pp. 539 ff. 

 t Darwin, loc. cit., pp. 548 ff. 



