Transpiration and the Ascent of Sap. 39 



limb. If the break opened by the shock is so small that its surface 

 tension forces can withstand the tension in the liquid, the bubble 

 will close again. But if once the bubble formed is so large that its 

 surface tension is overcome by the tension of the liquid, an unstable 

 condition is entered on, and the bubble is continually enlarged till 

 the tension of the liquid is nil. It is however evident that if at 

 any moment we could confine the bubble and prevent it from en- 

 larging, the liquid would again pass into a state of tension due to 

 the weight of the lower parts. 



Quite recently the author ^) has been able, by using Bert helot's 

 method, to show that the cohesion of water amounts at least to 

 150 atmospheres and that water, even when subjected to a tension 

 of this magnitude, refuses to be severed from the walls of the con- 

 ducting tracts of plants. The water used in these experiments was 

 saturated with air and contained in it pieces of the conducting tracts 

 of plants. The range of temperature over which this cohesion was 

 exhibited lay between 25 **— 80 " C. 



The theory of the Ascent of Sap, which Dr. Joly and the author 

 advocate, assumes that the water in the conducting tracts of high 

 trees hangs there by virtue of its cohesion just in the same way as 

 the water hangs in the J-tube. The adhesion of water to the walls 

 of the tracheae may be shown to be very great. Thus, if a fresh 

 piece of wood from the conducting tracts is enclosed in a vessel filled 

 with water in a state of tension, it will be found that in every case 

 rupture will occur at the surface of the glass rather than at the 

 walls of the tracheae. The adhesion of water to the walls of the 

 conducting tubes is thus probably always greater than the adhesion 

 of water to glass. This is quite to be expected, when we take into 

 account the manner in which water permeates the substance of the 

 walls of the tracheae when it is brought into contact with it. 



The teaching of all these experiments is obviously that water 

 under suitable conditions can transmit a pull just like a rigid solid. 

 In the liquid,' however, the stress is hydrostatic, and, like hydrostatic 

 pressure, is transmitted equally in all directions. It is not sustained 

 consequently by a single point but alfects the whole internal wetted 

 surface of the containing vessel. In another particular the stressed 

 liquid differs greatly from the stressed solid. It is much more unstable. 

 A small flaw (i. e. a bubble) in the tensile liquid rapidly spreads and 

 almost instantaneously severs the whole column; it matters not how 

 large the cross section of the unbroken part may be, a comparatively 

 feeble tension will tear it across. In the solid — a metal wire for 



^) H. H. Dixon, Note on the tensile Streng-th of Water. Proc. Roy. Dublin Soc, 

 1909 and Notes from the Botanical School, Trinity College, Dublin, Vol.11 No.l. 1909. 



