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III. 



VITALITY AND THE TRANSMISSION OF WATEE THKOUGH 

 THE STEMS OF PLANTS. 



By HENEY H. DIXON, Sc.D., F.E.S., 

 Professor of Botany in the University of Dublin. 



[Read November 24. Ordered for Publication December 8, 1908. Published March 1.5, 1909.] 



When tlie transpiring cells of the leaves abstract water from tlie traehese a 

 tensile stress is set up in the water remaining in the tracheae, inasmuch as 

 the latter are normally completely filled with water. This stress is resisted 

 by the walls of the tracheae, to whioli the water adheres, and is transmitted 

 downwards through the water of the transpiration current permeating and 

 filling a greater or less number of the tracheae composing the wood of the 

 plant. 



Exception has been taken^ to this view of the mechanism of the ascent 

 of sap on the ground that the resistance of the wood is so great — whoTi the 

 length of stem traversed by the water is considerable — that we cannot 

 imagine either the transpiratory forces to be equal to the task of dragging 

 the water-column upwards, or the column to have the tensile strength to 

 sustain the stress. 



1 have elsewhere shown" that these objections are based upon an over- 

 estimate of the amount of water transpired, and consequently of the velocity 

 of the transpiration-current, and also on determinations of resistance which 

 are excessive. 



In order to account for the manner in which the transpiration current 

 is relieved of this supposed resistance, Ewart^ has endeavoured to show 

 that the cells in the neighbourhood of the capillaries of the wood exert a 

 lifting force on the ascending water-column. He has invented several 

 mechanical schemes to illustrate how this may be effected, none of which, 



lEwart, Roy. See. Phil. Trans., B., 1905, and Ibid., 1908. 



2 Roy. Soe. Proe., B, 1907, p. 42, et seq. 



^ Roy. Soc. Phil. Trans., B, 1905, and Ann. of Botany, 1907, p. 443. Roy. Soo. Phil. Trans., B, 

 1908. 



SCIENT. PROC, R.D.S., VOL. XII., NO. Ill, E 



