iv COHESION THEORY OF ASCENT OF SAP 87 



in these experiments is overcome. The rupture starts as 

 an extremely small discontinuity in the water. Surface 

 tension forces develop immediately at the surface of 

 this bubble. At its inception, being extremely small, 

 these forces are very great, but if the bubble enlarges, the 

 surface tension forces tending to close it rapidly diminish. 

 In our experiments the forces tending to open it are (1) 

 the momentum of the water conferred on it by the shock, 

 and (2) the gravitational pull giving rise to the tension in 

 the liquid. We may neglect the vapour pressure of the 

 bubble, since it is balanced by the vapour in the other 

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

 surface tension can withstand the tension in the liquid, 

 it will close again. But if once the bubble formed is so 

 large that its surface tension is overcome by the inertia 

 and weight of the liquid, an unstable condition is entered 

 on, and the bubble is continually enlarged till the tension 

 in the liquid is nil. It is, however, evident that if at any 

 moment we could confine the bubble and prevent it from 

 enlarging, the liquid would again pass into a state of 

 tension due to the weight of the lower parts. 



Cohesion theory. 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 

 experiment with the J -tube described above. The ad- 

 hesion of water to the walls of the tracheae we have shown 

 to be very great. For, as will be seen, if a fresh piece 

 of wood from the conducting tracts is enclosed in a 

 vessel filled with water in a state of tension, in every 

 case rupture will tend to occur at the surface of the 

 glass rather than at the walls of the tracheae, showing 

 that the adhesion of water to the walls of the conducting 

 tubes is thus always greater than the adhesion of water 

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



