i THE NATURE OF TRANSPIRATION 25 



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 sur- 

 faces are in themselves 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 pro- 

 bability, assume that these directed vital actions are 

 responsible to a great extent for the raising of water in 

 plants even in unsaturated spaces. In any case the present 

 evidence shows that directed vital actions are capable 

 of replacing and supplementing the more simple physical 

 actions, e.g., evaporation, capillarity, and osmosis. 



When, however, the pressure of water vapour is further 

 reduced and evaporation is so rapid that secretion cannot 

 keep the supply equal to the demand, then it must be sup- 

 posed that capillarity (imbibition) and evaporation draw 

 the water to the surface of the cells. Whether this state 

 of affairs can be prolonged indefinitely without injury to 

 the leaves is not at present known. 



Literature. 



Brown, H. T., and Escombe, F., "Static Diffusion of Gases and Liquids 

 in Relation to the Assimilation of Carbon and Translocation in Plants," 

 Phil. Trans. Roy. Soc. London, vol. 193, B., p. 223. 



Dixon, H. H., "Note on the Role of Osmosis in Transpiration," Proc. 

 Roy. Irish Acad., vol. iv, ser. 3, p. 61, and Notes from the Botanical School 

 of Trinity College, Dublin, vol. i, p. 35. 



Id. " On the Effects of Stimulative and Anesthetic Gases on 

 Transpiration," Proc. Roy. Irish Acad., vol. iv, ser. 3, p. 618, and Notes 

 from the Botanical School of Trinity College, Dublin, vol. i, p. 97. 



Id., "Transpiration into a Saturated Atmosphere," Proc. Roy. Irish 

 Acad., vol. iv, ser. 3, p. 626, and Notes from the Botanical School of Trinity 

 College, Dublin, vol. i, p. 106. 



Id., "On the Physics of the Transpiration Current," Notes from the 

 Botanical School of Trinity College, Dublin, vol. i, p. 57. 



Id., "A Transpiration Model," Proc. Roy. Dublin Soc, 1903, vol. x 

 (N. S.), p. 114, and Notes from the Botanical School of Trinity College, Dublin, 

 vol. i, p. 217. 



