Dixon — The Cohesion Theory of the Ascent of Sap. 49 



the water in the capillaries contained appreciable quantities of air. 

 This conclusion we arrived at from the observation that the fluid 

 exuded during " bleeding " holds a considerable amount of air 

 dissolved in it. It was for this very reason that we thought it 

 necessary, and indeed essential, before publishing our Cohesion 

 Theory of the Ascent of Sap, to investigate the possibility of 

 tension existing in air-saturated water. 1 



The air which penetrates into the capillaries of the conducting 

 tracts has first to diffuse through the water-imbibed walls. The 

 passage through these wet walls, of course, secures that it can only 

 enter the contained water in a state of solution. In this state, 

 it has been experimentally shown that air does not lead to the 

 rupture of tensile water. Only if free gas (i.e. undissolved gas) 

 could, as such, pass through the imbibed walls would the water in 

 the capillaries be rendered incapable of withstanding tension. 



But even with regard to free gas, I venture to think that 

 neither Askenasy nor Steinbrinck thoroughly realises how limitedly 

 its presence, in moderate quantities, affects the transmission of 

 tension in the water of the conducting tracts. One quotation 

 from Steinbrinck will suffice : — 



" Bekanntlich 2 hat Askenasy zugestanden, dass die Gegenwart 

 von Luftblasen in den Leitungsbahnen seiner Theorie 3 Schwier- 

 igkeiten bereitet, da diese Blasen den Zusammenhang der Wasser- 

 faden unterbrechen. . . . Stellen wir uns aber, um dieser thatsach- 

 lichen Schwierigkeit aus dem Wege zu gehen, die Leitungsbahnen 

 eines etwa 20 m. hohen lebenden Baumes fur den Moment einmal 

 wirklich ganz blasenfrei und wassererfiillt vor, so wiirde . . . die 

 Bejahung des erwahnten Satzes (viz., the permeability of the 

 walls for air) sofort das Auftreten solcher Blasen am Gipfel der 

 Leitungsbahnen und damit den Stillstand der Aufwartsbewegung 

 verlangen." 



As a matter of fact, however, the permeability of the walls 

 to air does not necessitate the liberation of free gas in the form of 



1 Trans. Roy. Soc, loc. cit. 



2 Ber. d. deutsch. tot. Gesell., 1900, p. 392. 



3 It may be noticed that, although Steinbrinck in his earlier Papers attributes the 

 authorship of the Cohesion Theory to Dr. Joly and myself in the first place, and to 

 Askenasy in the second, in this latter Paper he patriotically assigns it to Askenasy 

 alone. Cf. Ber. d. Deutscb. Bot. Gesell., 1897, p. 87, with quotation given here. 



