xi ENERGY AVAILABLE FOR RAISING SAP 211 



the wood traversed. Hence the tension applied to the 

 upper end of the water columns, which will be able to 

 raise the transpiration stream in a tree, must equal the 

 pressure produced by a head of water twice the height 

 of the tree. In a tree 100 m. high, therefore a tension 

 of 20 atm. must be produced. 



The cohesion of sap amounting, as it does, to at least 

 200 atm. is in no way taxed by this tension. 



The transpiring cells of the mesophyll normally remain 

 turgid during transpiration ; accordingly we would expect, 

 if our line of reasoning is correct, that in high trees the 

 osmotic pressure keeping them distended must correspond 

 in magnitude to the tensions necessary to raise the sap. 



This surmise has been confirmed by determinations of 

 the osmotic pressures of the saps of various leaves. These 

 pressures have always been found adequate to resist the 

 transpiration tension ; but in many cases other factors 

 enter in, and the pressures developed are much in excess 

 of those demanded by transpiration. 



Finally, it has been shown that the stored energy set 

 free by respiration in leaves is quite sufficient to do the 

 work of secretion against the resistance of the transpira- 

 tion stream ; while, when the vapour pressure of water 

 in the surrounding space is low, and when evaporation is 

 doing the work of raising the sap, the expenditure of 

 energy in this process will reduce the quantity of water 

 evaporated only by an imperceptible amount. 



Literature. 



Aubert, E., " Recherches sur la respiration et l'assimilation des plantes 

 grasses," Revue general de Botanique, 1892, Tom. 4, p. 373. 



Brown, H. T., and Escombe, F., "On the Physiological Processes of 

 Green Leaves," Proc. Roy. Sac. London, 1905, vol. 76 B, p. 29. 



Dixon, H. H., "On the Physics of the Transpiration Current," Notes from 

 the Botanical School of Trinity College, Dublin, 1897, vol. 1, p. 57. 



Id. "Transpiration and the Ascent of Sap," Progressus Rei Botanicae, 

 1909, Bd. iii, s. 1. 



