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XVI. 



ON THE TENSILE STRENGTH OF SAP. 



By HENRY H. DIXON. Sc.D., F.E.S., 

 University Professor of Botany, Trinity College, Dublin. 



[Read January 27. Published March 10, 1914.] 



In 1893 Bohm (2) recorded an experiment in which a transpiring branch 

 drew np a column of mercury above the contemporary barometric height. 

 This experiment, in the light of our present knowledge, demonstrated clearly 

 that the sap in the branch was in a state of tension. 



Experiments made by Dr. Joly and the writer (5) in 1894, in which pieces 

 cut from the wood of trees were enclosed in large sealed glass tubes without 

 introducing weakness in the cohesion of the water, demonstrated the tensile 

 strength of sap up to 7| atmospheres. Further researches carried out by myself 

 in 1903 (3), in which tension was applied by means of a column of mercury, 

 showed that comparatively large pieces of living tissue, containing conducting 

 tracts and their sap, together with parenchymatous cells, when introduced into 

 water, did not spoil its cohesive properties. These tensile properties of sap 

 were also demonstrated up to tensions of about 150 atm. in capillary tubes in 

 later experiments (4). 



It may be noticed that in none of these experiments was dissolved air 

 intentionally excluded, but, rather, must have been present in the tensile liquid, 

 owing to the procedure adopted in each experiment. In some, air-saturated 

 water was in contact with the sap, which itself must have contained air. 



A priori there appeared no reason to suspect that the tensile properties of 

 the sap would be different from those of water. The passage of the sap 

 through cell-walls must preclude the possibility of the entry of undissolved 

 air, and the presence of traces of colloids in the sap could scarcely be 

 supposed to introduce a weakness.' 



In face of this evidence it is not a little surprising, then, to find that, 

 basing his opinion apparently on four negative experiments, A. Ursprung (7) 



^ In this connection it is worth recording that a soap film, made up with sap from tlie wood of 

 Populus sp., even when sufficiently thin to show the ' black spot ' was found to be wonderfully 

 stable — apparently more stable than a similar soap film made up with distilled water. This demon- 

 strates that a film of sap not more than 12 nix thick can sustain at least twice the surface tension of 

 the soap solution, and hence sets a minor limit to its cohesion of 42 atm. (cf. Proc. Eoy. Dublin Soc, 

 vol. xu (N. S.) No. 3, 1909, and Notes from the Bot. School, Trinity College, vol. ii. No. 1, p. 17). 

 SCIENT. PROC. R.D.S., VOL. XIV., NO. XVI. 2 N 



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