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XXVII. 



TEANSPIRATION INTO A SATURATED ATMOSPHERE. 

 By henry H. DIXON, D.Sc, Assistant to the Professor 

 of Botany, Trinity College, Dublin. 



(COMMTHSTICATED BY E. P. WEIGHT, M.D.) 



[Eead January 10, 1898.] 



Many observations and considerations^ bave led me to suspect that 

 the tension transmitted in the water-containing capillaries of tran- 

 spiring plants, is not simply referable to evaporation taking place at 

 the liquid surface in the imbibed membranes of the leaf-cells. And 

 it seemed probable that pumping actions proceeding in these cells 

 (analogous to the pumping actions of root-cells manifested during the 

 "bleeding" of plants) may be directly responsible for the elevation 

 of the transpiration current.^ 



If this be true, the elevation of water in the capillaries during the 

 transpiration of living plants is effected by a " vital '' process, and in 

 this respect resembles the raising of water in plants by root-pressure, 

 and the transference of water in ccenocytic fungi. Of course the 

 water of the transpiration-current must be drawn up by the leaf -cells, 

 and so the process differs from the other examples, in which the water 

 is pushed up by the living cells. 



In thus ascribing the lifting of water to a "vital" and not a 

 "physical " process going on in the transpiring cells, it must not be 

 thought that any ultraphysical phenomena are presupposed. To 

 guard against such a misapprehension, it may be well to state that, 

 by "vital" processes are here meant, processes which cannot be 

 accounted for by the immediate energy -relations of the organism to 

 the external world, but those in which energy previously stored by 

 the organism, e.g.^ as oxidisable materials, is utilized, and which only 

 take place during the life of the organism. 



1 " Report of a Discussion on the Ascent of "Water in Trees," Ann. of Botany, 

 December, 1896, p. 651. 



-"On the Physics of the Transpiration Current," Notes from the Botanical 

 School, T. C. D., No. 2, 1897, p. 88. 



