770 Proceedings of the Royal Irish Academy. 



transpired is doubtless due to some action of a protective nature in the 

 living branch checking the transpiration when the leaves droop, 

 which does not occur in the branch killed by the high temperature. 

 Prom these experiments it is probable that the change in the rate of 

 transpiration after the introduction of various solutions into the upper 

 terminations of the tracheal tissue is clue to a diminution of the 

 turgescence of the cells which adjoin these terminations, rather than 

 to an alteration in the surface tension of the evaporating films. The 

 fact that branches, even when the cells of their leaves have lost their 

 turgescence owing to exposure to high temperatures, are still able to 

 draw up water in diminished quantities, is to be explained by 

 assuming that the raising of the water in these cases is due simply to 

 the tractional forces developed by evaporation taking place at the 

 surface-films of the evaporating cells. In this case and in the case of 

 the raising of water in the j)ieces of dead wood as described by 

 Strasburger,! the process is strictly analogous to the raising of water 

 by evaporation at the surface of a porous vessel. 



Again the fact that the leaves even of the highest trees remain 

 turgescent during the time of transpiration may, taken in conjunction 

 with the foregoing observations, be used as an argument in su]3port of 

 the view that it is the osmotic properties of the cells of the leaf which 

 directly put the water in the tracheal system in tension. Por unless 

 water was supplied by osmotic pressure through the walls of the 

 evaporating cells as quickly as it evaporates from their outer surface, 

 it is evident that instead of a pressure being maintained in the cells a 

 tension would be set up by the evaporation, and the cells would no 

 longer remain turgescent and the flagging of the leaves would result. 

 This amounts to saying that if the tension set up by evaporation tend- 

 ing to collapse the cells, which intervene between the evaporating 

 surfaces and the upper terminations of the water conduits, be not 

 opposed by a greater force exerted by the turgescence of these cells, the 

 leaves will become flaccid, and as we know the leaves do not normally 

 become flaccid during transpiration we may conclude that during 

 normal transpiration the tractional force is exerted by the osmotic 

 properties of the turgescent cells in the leaf. 



It appears quite warrantable to assume that the osmotic attraction 

 of the cells of the leaf is capable of exerting a tension in the conduits 

 adequate to raise the sap in the highest trees ; for osmotic pressures 



1 Strasburger : ' ' Ueber den Bau und Verriclituiigen der Leitungsbabnen in den 

 Pflanzen." 



