[ 76 



7 1 



XLVI. 



NOTE ON THE EOLE OE OSMOSIS IN TRANSPIRATION. 

 Bx HENRY H. DIXON, B.A., Assistant to the Professor of 

 Botany, Trinity College, Dublin. 



[cOlIjilUNICATED BX DR. E. P. WEIGHT.] 



[Eeacl January 13, 1896:1 



Ix a paper^ on tlie Ascent of Sap, read before the Royal Society, the 

 joint publication of Dr. J. Joly and myself, we attribute the raising 

 power of the sap, when root-pressure is not acting, either to surface- 

 tension forces developed in the substance of the walls of the 

 evaporating cells, or to the osmotic action of the cells adjoining and 

 in proximity to the upper terminations of the water conduits. The 

 present note is occupied with a discussion of this question regarded 

 both from an experimental standpoint and also from a structural 

 point of view. Erom various considerations Ave think it most 

 probable that under normal conditions the force which establishes 

 the tension in the sap in water-conduits is to be referred to the 

 osmotic properties of the cells of the leaf, while we do not deny 

 that under certain conditions the raising force is to be attributed 

 to the evaporation proceeding at the surface film formed in the 

 evaporating cell-walls. 



Firstly, with regard to the experimental evidence : In transpiration 

 experiments in which colouring materials {e.g. eosin) or other sub- 

 stances poisonous or non-poisonous {e.g. copper sulphate, picric acid, 

 tartaric acid,- sodium bicarbonate, ferricyanide of potassium, &c.), are 

 supplied in watery solution to the cut surface of a transpiring branch, 

 it is found that the rate of transpiration continues without much 

 diminution till the solution can be detected in the leaves, but then 

 suddenly falls off. This may be seen either by direct measurement^ 

 of the water taken up or by the fact that the leaves of these branches 

 usually remain fresh until the arrival of the salt solution in them, 



1 PhU. Trans, of Eoy. Soc, London, vol. 186 (1895), B. 



^ Cf. Annals of Botany, Sept., 1895. 



3 Cf. Strasburger, " Ueber das Saftsteigen," p. 12. 



