i2 4 TRANSPIRATION AND ASCENT OF SAP ch. 



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 trans- 

 piration 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's receives a ready explanation. There is a 

 momentary increase of transpiration in these experiments 

 immediately after the separation of the branch, and im- 

 mediately 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. Trans- 

 piration obviously soon re-establishes tension, which be- 

 comes greater in both cases than it was originally. 



In many ways, then, we see it is established that supply 

 largely controls transpiration, and in neglecting this factor 

 Ewart considerably exaggerated the maximum velocities of 

 the transpiration through the stems of trees. 



Determination of the resistance. We come now 

 to consider the amount of resistance experienced by the 

 transpiration current in its passage through the water 

 conduits. 



