34 Scientific Proceedings, Royal Dublin Society. 



this method that the cohesive force or tenacity of water must be at least 

 42' 5 atmospheres. 



To raise water in one of the tallest trees, say, 100 m. higli, would require 

 a force equal to the hydrostatic head, viz., 10 atm., plus the force necessary to 

 overcome the resistance experienced by the transpiration stream. Ewart' 

 estimates this at about 50 atm. in all. This figure he arrives at by assuming 

 the same transpiration velocity and the same resistance as he found in lower 

 trees. As has been pointed out, his determinations of the resistance are 

 excessive ; and further, there is every reason to believe that, if the supply is 

 inadequate, the transpiration velocity will be reduced.^ This reduction in 

 velocity has been shown to occur in lower trees. Using my own determina- 

 tion for the resistance,^ we obtain a very much lower figure for the tension, 

 which must be supported by the transpiration current of the tallest trees. 



In the case of a stem with exceptionally high resistance, I found that a 

 head of water equal to the length of the stem traversed was able to move 

 water at the transpiration rate. If this figure holds for the highest trees — 

 and it is likely to be excessive, both because the transpiration rate in the high 

 tree will be slowed down, and because its wood will have a smaller resistance 

 than that of the tree in which the determination was made — we should 

 add to the 10 atm. necessary to support the head another 10 atm. to over- 

 come the resistance of the conducting tubes. The water-columns then in the 

 tree would have to support a tension of only 20 atm., which is well within 

 the minor limit obtained for the tenacity of air-saturated water. 



With regard to the osmotic pressures in the leaf -cells necessary to keep 

 them turgid against the pull on their sap, we have no information applying 

 to the higli trees under consideration. I have found that the cells of the 

 leaves of several low trees, e.g.. Laburnum and Tilia, are able to support an 

 external pressure of 26-30 atm. without showing signs indicating that their 

 turgescence was overcome.* It is certainly not unreasonable, in the absence 

 of direct observation, to assume similar pressures in the leaves of liiglier 

 trees. 



' Ewart, Roy. Soc. Phil. Trans., B, 1907, p. 391. 



2 Dixon, Eoy. Soc. Proc, B, 1907. 



3 It may be noted that the resistance of the wood of the yew in which my determination wa^ 

 made is exceptionally high, as its wood is exclusively formed of tracheides only ahout 2 mm. long, 

 and so the number of cross-partitions which must be traversed is exceptionally high. 



^ "On the Physics of the Transpiration Current." Notes from the Botanical School, Trinity 

 College, Dublin, No. 2. 1897. 



