768 Proceedings of the Royal Irish Academy. 



but then rapidly lose their turgescence and flag and finally become 

 dry and crisp.^ These experiments are easily explained if we may 

 assume that at first the solution is drawn up in the conduits under 

 the tension set up by the osmotic forces of the cells of the leaves, but 

 afterwards, when the upper portion of comparatively pure water has 

 been eliminated from the plant by transpiration, the solution comes 

 in contact with the osmotic cells and modifies or destroys their 

 osmotic properties. These latter soon lose theii' turgescence, and the 

 leaf di'oops. That the loss of turgescence will necessarily be attended 

 by a diminution of the amount of water evaporated from the leaf will 

 appear evident when it is considered that : Firstly, the transprdng 

 surface of the evaporating cells will be diminished owing to the con- 

 traction of the cells when they cease to be turgescent. Secondly, the 

 diffusion of the water-vapour from the inner tissues of the leaf will 

 be hindered by the collapse of the intercellular passages caused by 

 the drooping of the leaf ; and, finally, the evaporating films which on 

 the surface of the wall of the turgescent cell may be supposed to be 

 bulged outwards, will under the new conditions tend, by retreating 

 inwards into the substance of the cell-walls, to form a series of more 

 concave menisci from which as is known evaporation will proceed 

 more slowly. However, as the walls of these cells are imbibed with 

 water, and a continuous column of water extends back from the outer 

 evaporating walls down through the conduits into the plant hanging 

 from the surface film fonned on the outside of the evaporating cell- 

 wall, a slow movement upwards will take place of the solution in 

 the conduits even after the loss of turgescence by the osmotic cells. 

 The supply is in most cases inadequate to prevent the di'ying of the 

 leaf.2 



To the evaporation at the surface film formed on the cell-walls 

 must also be referred the ascent of water in pieces of dead wood 

 which have been soaked and injected with water, as described by 

 Strasburger f for in this case, of course, the intervention of osmotic 

 forces is excluded. It is to be noticed that in this experiment also 

 the wood dries downwards from above. 



It may be suggested that in the case of the experiments with 

 solutions just referred to, the changes in the rate of transpiration was 



^ Strasburger, " Ueber den Bau und Verrichtungen der Leitungsbahnen in den 

 Pflanzen." Dixon and Joly, "On the Course of the Transpiration Current," 

 Annals of Botany, September, 1895. 



- Strasburger, " Ueber den Bau," &c. 



^ Strasburger, loc. cit., p. 662 ff. 



