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IV. 



THE COHESION THEOBY OF THE ASCENT OF SAP. A 

 Eeply by HENEY H. DIXON, Sc.D., Assistant to the Professor 

 of Botany, Trinity College, Dublin. 



February 17 ; Received for Publication February 20 ; 

 Published June 5, 1903.] 



In a series of short papers in the Berichte der Deutschen Bota- 

 nischen Gresellschaft for 1900, C. Steinbrinck has shown that 

 cell- walls, lignified or not, imbibed with water or dry, are to a con- 

 siderable extent permeable to air under a difference of pressure 

 of less than one atmosphere. 



This permeability to air of the walls of the conducting tracts 

 he regards as rendering untenable the Cohesion Theory of the 

 Ascent of Sap — a theory which Dr. Joly and I published in 

 1894. 1 



The fact observed by Steinbrinck is, however, by no means 

 antagonistic to our views. In our paper, read before the Royal 

 Society, we pointed out and showed by direct experiment that 

 water containing large quantities of air is as capable of trans- 

 mitting tension as air-free water. 2 Consequently air, diffusing 

 through the moist lignified walls of the conducting capillaries, 

 necessarily being in solution, would not break, or tend to break, 

 the continuity of the water-columns within them. 



The interesting fact which Steinbrinck has established — viz., the 

 permeability of these membranes to air — only affects the problem 

 of the Ascent of Sap so far as to show that we have, in the water 

 of the transpiration current, to deal with water containing air, and 

 not with an air-free liquid. At the outset, Dr. Joly and I had 

 anticipated the probability, almost amounting to a certainty, that 



1 Proc. Roy. Soc, Nov. 15, 1894; p. 3, vol. lvii. "Nature," Nov. 22, 1894, 

 p. 93, vol. li. 



2 Proc. Roy. Soc, he. cit. Trans. Roy. Soc, vol. exxvii., p. 568. 



