Notes . 
469 
experiments lead the authors to believe that it alone is quite adequate 
to effect the elevation by direct tension of the sap in tall trees. 
Explanations of the lifting of the sap from other causes prove 
inadequate. 
A reconsideration of the principal experiments of previous observers 
and some new experiments of the authors lead to the view that the 
ascent is principally in the lumen and not in the wall. 
The explanation of how the tensile stress is transmitted in the 
ascending sap without rupture of the column of liquid is found in the 
stable condition of this liquid. The state of stability arises from two 
circumstances : — the internal stability of a liquid when mechanically 
stretched, whether containing dissolved gases or not, and the addi- 
tional stability conferred by the minutely subdivided structure of the 
conducting tissue, which renders the stressed liquid stable even in 
the presence of free gas. 
By direct experiments upon water containing large quantities of 
dissolved air, the state of internal stability is investigated. And, 
further, by sealing up in the vessels, in which the water to be put 
under tension is contained, chips of the wood of Taxus dacca/a, the 
authors find that their presence in no case gives rise to rupture of 
the stressed liquid, but that this occurs preferably anywhere else, and 
usually on the glass walls. The establishment of tensile stress is 
effected in the usual way, by cooling the completely filled vessel. 
A measurement possessing considerable accuracy afforded 7| atmo- 
spheres as being attained in some of the experiments. 
The second condition of stability arises directly from the property 
of the pit-membranes to oppose the passage of free gas, while they are 
freely permeable to the motion of a liquid. Hence a chance develop- 
ment of free gas is confined in effect to the minute dimensions of the 
compartment in which it is evolved, and this one lumen alone is 
rendered for the time being non-conducting. On the other hand, in 
the water- filled portion of the tracheal tissue, the closing membranes, 
occupying the median and least obstructive position, the motion of 
the stressed sap is freely allowed. The structure of the conducting 
tissue is, in fact, a configuration conferring stability on a stressed 
liquid in the presence (from various causes) of free gas. As neither 
free gas nor unwetted dust particles can ascend with the sap, the 
authors contend that the state of tensile stress necessary to their 
hypothesis is inevitably induced. 
