igo BOTANICAL GAZETTE [seftember 



alone (according to Lehmann) in reporting that a jar did not 

 instantly release any negative tension, and his tension was only 

 that of one meter of water. Trees are almost never absolutely 

 at rest ; their very agitation by the wind has served once 

 (Spencer) to explain the rise of water. Kamerling, in the same 

 mathematical argument showing that the cohesion of pure water 

 at rest is not limited, proves that the kinetic energy of moving' 

 water may easily pull it apart. The transpiration stream not 

 only moves, but moves irregularly, which no negative tensioa 

 could survive. Because trees are not at rest and the transpiration 

 stream is in unsteady motion, we must regard any negative 

 tension in it as a priori impossible. 



•While Dixon and Joly (1895:571) regard a continuous 

 column of water as an essential assumption in their theory, and 

 base it on Strasburger's statement that there is very little air it> 

 the youngest wood, Askenasy, adhering more closely to Stras- 

 burger, recognizes that the presence of some air is not an effective 

 bar to the movement of water in the same tracheae, and seems to 

 expect cohesion to operate around and past bubbles. I cannot 

 see the possibility. We believe cohesion to depend on the fact 

 that the first break in a column of w^ater must be an infinitely 

 small bubble, whose surface is a meniscus of infinitesimal radius 

 and a correspondingly great surface tension. In the wood 

 bubbles are present of a very appreciable size. When trans- 

 piration is most active, and cohesion is supposed to be most 

 necessary, these bubbles stretch and become practically cylinders. 

 Midway up its side the surface of such a bubble is very much 

 nearer that of a cylinder than of a sphere. The surface tension 

 of such a meniscus hardly deserves the name of capillarity ; 

 cohesion is out of the question. The low tension of the bubble 

 still further depresses the surface tension. ^ 



Capillarity and cohesion are usually names of manifestations- 

 of surface tension under different conditions. Applied to its 



7 In a glass tube open at one end, the shape of the meniscus itself is a function o 

 the tension of the gas ; in the case we are considering this is not quite true, because 

 shape of the tracheae helps to determine that of the bubble. 



d^ 





