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IX. 



A TKANSP1RATI0N MODEL. By HENRY H. DIXON, Sc.D., 



Assistant to the Professor of Botany, University of Dublin. 



[Read, June 16 ; Received for Publication, June 23 ; Published, October 15, 1903.] 



It is a matter of common observation that the leaves of tall trees 

 remain turgid during active transpiration. This turgidity is due 

 to the osmotic pressures of the solutions distending the proto- 

 plasmic membranes of the cells of the mesophyll of the leaves. 

 Pressures ranging from 6 to 16 atmospheres have been measured in 

 these cells. 1 The cells distended by these pressures adjoin directly 

 the upper extremities of the water-conducting tracts of the plant, 

 in which it has been shown elsewhere that the water of the 

 transpiration current is in a state of tension. 2 During transpira- 

 tion the turgid cells lose water on their outer side by evaporation. 



The intervention of osmotic pressure between the evaporat- 

 ing surfaces and the stressed water makes the process somewhat 

 more difficult to conceive than if the evaporation from the meso- 

 phyll-cells directly stressed the liquid in the tracheidal elements of 

 the vascular^ bundles of the leaves. 



In order to make clear the part played by osmotic pres- 

 sures in raising the transpiration current, according to the view 

 advocated by Dr. Joly and myself, I have elsewhere compared 3 

 the actions taking place in the cells of the leaf with those 

 which would proceed in an osmotic cell formed of a semi- 

 permeable membrane containing a solution, and placed in 

 contact with the upper end of a vertical tube filled with 

 water, the lower end of which dips into a reservoir of water. 

 The arrangement was supposed to function as follows : — The 



1 Maquenne : Compt. rend. 1896, p. 898. On the Osmotic Pressure in the Cells 

 of Leaves. Henry H. Dixon: Proc. Roy. Irish Acad., 1897. — Sutherst, Chemical 

 News, 1901, p. 234, Heald : Bot. Gazette, 1902, p. 81. 



2 On the Ascent of Sap. H. H. Dixon and J. Joly : Proc. Roy. Soc, 1894. 



3 Note on the Role of Osmosis Transp ration, Proc. Roy. Irish Acad., 1896, 

 p. 774.1 



