772 Proceedings of the Royal Irish Academy. 



the concentration in the sap of the turgescent cells necessary to attract 

 the water from the conduits is maintained. This consideration also 

 explains the fact that the conditions favourable to rapid evaporation 

 are those favourable to i-apid transpiration. 



This fact, namely, that the upward movement of the transpiration- 

 current is ultimately due to the evaporation taking place at the sur- 

 faces of the cell-walls in the leaf, leads us to consider what advantages 

 are obtained by the intervention of the osmotic-cells between the 

 evaporating surfaces and the conduits. 



In the first place it is probable that without the internal pressure 

 set up by the turgescence of the cell, that the cell-wall would dry 

 back and cease to be able to draw up water from below, as is found to 

 be the case in those experiments in which the osmotic action is elimi- 

 nated : in addition to this, the arrangement as it is found in the leaf, 

 is a most beautiful automatic mechanism, by which in times of 

 excessive evaporation the osmotic power of the cell is increased by the 

 greater concentration of the sap contained in it. This simultaneously 

 increases its attractive forces on the water on the conduits, and tends 

 to diminish the rate of the loss of water from it due to evaporation ; 

 since evaporation takes place more slowly from concentrated solu- 

 tions. 



Another advantage derived from the presence of the turgescent- 

 cells is the great rigidity which they confer on the leaf. It is indeed 

 wonderful to see how completely a stiff leaf collapses and hangs flabby 

 after it has been robbed of its turgescence by exposure for a few 

 minutes to steam. The cells of the leaf acting like minute Bourdon 

 tubes, keep the leaf expanded as long as they are turgescent, but when 

 their turgescence is destroyed, are no longer able to support its weight. 

 Not only is the distension of the cells due to turgescence favourable 

 to transpiration by exposing greater external leaf-surface, and conse- 

 quently stomata, to the air, but also when the leaf is held out rigidly 

 by turgescence, the internal air-passages and smaller intercellular 

 spaces are kept open, and a free circulation of vapour is permitted to 

 take place through them. In this relation of turgescence to trans- 

 piration, we have again an automatic contrivance of extreme beauty. 

 For if it should come to pass that the external conditions were so 

 favourable to evaporation that the leaf should lose more water than 

 it can derive from the conduits, it is evident that the turgescence of 

 the transpiring cells will be diminished. This will, by the mechanism 

 described, bring about a diminished freedom in the passage of water- 

 vapour through the spaces in the leaf and through the stomata, owing 



