iv] VARIOUS THEORIES, &c. 107 



tion here. The principle involved was to oppose 

 pressure, by means of weights, to the resistance due 

 to turgidity, and he came to the conclusion that the 

 hydrostatic pressure concerned may easily reach three 

 and four atmospheres. 



The year 1884 saw the publication of several papers 

 on our subject, and one of these seems to have practi- 

 cally closed the discussion except as regards details. 

 I propose to take first one by Elfving, 1 because it in- 

 troduces some excellent criticisms on the foregoing 

 papers, as well as offering a survey of several physical 

 principles which had been either neglected or not 

 properly understood by previous writers. 



Elfving first gives a short summary of the older 

 views, with especial reference to the capillary theory 

 and the bearing of his own previous experiments, and 

 then proceeds to point out that the two sets of forces, 

 capillarity which is effective in the lumina of the tra- 

 cheides and vessels, and imbibition in the solid sub- 

 stance of their walls, together with the osmotic forces 

 concerned in root-pressure, and the expansion and 

 contraction of the air bubbles, must all be active in 

 causing the movement of the water up the stem of the 

 tree. 



1 " Ueber den Transpirationsstrom in den Pflanzen." Acta Societatis 

 Scientiarum Fennica, t. xiv. 1884. 



