634 Proceedings of the Royal Irish Academy. 



The stimulating action of the light is indirect, and probably is 

 effective by the increased supply of oxygen set free by assimilation. 

 The rising of the eosin is most vigorous Tvhen bubbles of oxygen are 

 being rapidly evolved at the surface of the leaves. This observation, 

 then, constitutes another proof that the raising action is due to a 

 vital process, and ceases when the supply of oxygen is cut off. In 

 this respect the action resembles other vital phenomena, such as 

 growth, irritability, &c. 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 distillation 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 described in this note. 



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 saturated spaces, is effected by directed actions taking 

 place in the living cells of the leaves, and to some extent perhaps in 

 those of the stem. Simple osmotic and evaporative forces cannot be 

 effective in raising the water in the conduits under 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, when the leaves of plants are killed, 

 they dry up and are unable to furnish themselves with sufficient water 

 from an unlimited supply at the base of their stem, argues that 

 surface tension and evaporation forces at their surfaces are in them- 

 selves inadequate. And when we couple with this the observations 

 on the directed vital actions taking place in the leaf-cells when they 

 are surrounded with a saturated atmosphere, I think we may, with 

 great probability, assume that these directed vital actions are 

 responsible to a great extent for raising of water in plants even in 

 unsaturated spaces. In any case the present experiments show that 

 these directed vital actions are capable of replacing and supplement- 

 ing the more simple physical actions, e.g. evaporation and osmosis. 



These conclusions are in no way at variance with Strasburger's 

 famous experiments on dead trees.* In them, the water, in a tensile 

 state, was raised by evaporation taking place at the water-surface in 

 the permeable cell-walls. 



' Ueber den Bau und Terrichtungen der Leitungsbahnen. 



