v TENSILE STRENGTH OF SAP OF TREES 113 



mated to 207 atmospheres ; while in the second a tension 

 of about 132 atmospheres was attained before rupture 

 occurred. 



The former of these is, I believe, the highest tension yet 

 experimentally produced in any liquid. Possibly this very 

 good cohesion possessed by unboiled sap is due to the pres- 

 ence of colloids in it. It seems probable that when the 

 tension is just adequate to start a rupture, if the latter 

 remains sufficiently small, its surface tension will be able to 

 withstand the stretching action due to the contraction and 

 cohesion of the liquid. Thus, if the rupture, at its first 

 inception, can be delayed in spreading, it may be obliterated 

 and cohesion re-established. The presence of the colloid 

 may bring about the necessary delay. The appearance 



Table 11. 



exhibited occasionally in these sap-containing tubes may 

 be interpreted as favouring this view. The click of 

 rupture is not, in these cases, attended by the development 

 of a single bubble becoming surrounded by a group of 

 small visible bubbles, but, at the moment of rupture, a 

 milky, semi-opaque region develops in the tube. This 

 slowly rises, and clears away, as it turns into a mass of 

 excessively minute bubbles. Here, apparently at the de- 



