4 On the Ascent of Sap. [Nov. 15, 



tion 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 

 onducting 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 baccata, 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^ atmos- 

 pheres 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-condncting. 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. 



The energy relations of the leaf with its surroundings, on the 

 assumption that evaporation at capillary water-surfaces is mainly 

 responsible for the elevation of sap, may be illustrated by the well- 

 known power of the water-filled porous pot to draw up mercury in a 

 tube to which it is sealed. The authors describe an engine in which 

 the energy entering in the form of heat at the capillary surfaces may 

 be in part utilised to do mechanical work : a battery of twelve small 

 porous pots, freely exposed to the air, keeping up the continuous 



