I902J RISE OF THE TRANSPIRATION STREAM 185 



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strated by Naegeli and interpreted with an insight in advance of 

 his time. He says (1866 : 498) '*Viele Versuche zeigten, dass 

 in frisch gezogenen Capillarrohren die Spannung negativ warden 

 kann,ohne dass In dem ausgekochten Wasser Dampfbildung ein- 

 tritt;'' thus in one experiment the cohesion of the water sus- 

 tained a mercury column 44"^"^ in excess of that in the barome- 

 ter. Two other sentences outline Naegeli's interpretation : 

 '* Dieses Entzweireissen einer VVassersaule ist im Grunde nichts 

 anderes als Dampfbildung in derselben^ ;'* and concerning the 

 amount of resistance to such pulling apart, '* Es ware selbst 

 moglich, das es dafiir keine Grenze gabe." Bohm cites Helm- 

 holtz on this same point, the passage referred to (1874:492) 

 being on cohesion by very dilute sulphuric acid. Bohm himself 

 in 1890 (Bot. Centralb. 268) found that Salix with boiled roots 

 would absorb water and draw up mercury '* Stets bis zur Bar- 

 ometerhohe," when the total pressure of mercury in the water 

 must have been above one atmosphere. Strasburger (1S91 : 792) 

 got the same result with branches of Taxus and Tsuga. Bohm 

 then (1893 : 209) reported the lifting of mercury columns 22°^"^ 

 above that of the barometer by Salix, and 161"'"' above it by 

 Thuja. In this paper Bohm seems to refer repeatedly to cohe- 

 sion when he speaks of capillarity. 



A little later than this, Askenasy (1895) published an exceed- 

 ingly clear theory on the ascent of sap, in which the chief 

 role in making the pull set up by evaporation in the leaves ope- 

 rative at indefinite distances down the trunk was ascribed to the 

 cohesion of the water. The priority of the publication of 

 most of this theory belongs to Dixon and Joly, who published 

 abstracts of their work in 1894. The general acceptance of it 

 is due more largely to Askenasy, whose name is usually asso- 

 ciated with it. So is Strasburger's name, though he has not 

 publicly subscribed to it, and it is not in harmony with all of his 

 work. Like the imbibition theory, this one was complete in 

 that the premises, proven and assumed, were an adequate 

 foundation for the conclusion ; the factors, as the theory used 



s Physicists are not agreed on this. 



