24 TRANSPIRATION AND ASCENT OF SAP ch. 



The stimulating action of the light is indirect, and pro- 

 bably is effective by the increased supply of oxygen set free 

 by assimilation. The upward movement of the eosin is most 

 rapid when bubbles of oxygen are being evolved in quan- 

 tity at the surface of the leaves. This observation, then, 

 constitutes another proof that the lifting action is due to 

 a vital process, and decreases when the supply of oxygen 

 is diminished. In this respect the action resembles other 

 vital phenomena, such as growth, irritability, etc. The fact 

 that a small rise does take place in the dark is explained 

 by the presence of oxygen in the water, and also of that 

 derived by intra-molecular respiration. 



The combination of this oxygen will of course lead to a 

 minute rise in temperature which will favour a distilla- 

 tion of water from the leaves. This effect, however, would 

 probably be so small that it could not account for the 

 rapid rise of water in plants in a saturated space, as has 

 been just described. 



Over short periods osmosis may, however, be adequate 

 to draw up water into leaves even though they are sur- 

 rounded by a saturated space ; but this seems only possible 

 if we start with the leaf-cells incompletely distended. 

 Until they are fully turgid they will absorb water on every 

 side, and naturally some of this will come from the tracheae, 

 while some may at the same time be derived from water 

 in contact with their outer surfaces. 



Summary. From what has been here detailed, I 

 think we may with great confidence assert that the 

 elevation of the sap, when plants are situated in satu- 

 rated places, is effected by directed actions taking place 

 in the living cells of the leaves. Simple osmotic and 

 evaporative forces cannot be continually effective in 

 raising the water in the conduits in these circum- 

 stances. With regard to the elevation of water, when 

 the leaves are surrounded by an unsaturated atmosphere, 

 we cannot as yet be dogmatic. But the fact that, 



