ABSORPTION AND TRANSMISSION OP WATER 111 



a mechanical tension upon the minute films and columns 

 reaching to the foot of the tree. Strasburger's experiments 

 seem to show that atmospheric pressure cannot play a part 

 here ; for his tallest tree was as high as a column of water which 

 would balance a pressure of two atmospheres. However, it 

 is to be remembered that the water in a tree trunk is not in 

 continuous columns, but that the columns are divided by 

 air bubbles. 



That the leaves play the part just ascribed to them is 

 practically proved. Besides the experiments of Strasburger 

 may be cited that of Dixon, 1 wherein he showed that when 

 the leaves at the top of a tall shoot were killed, the upward 

 passage of water was checked, even though the stem were 

 still uninjured. 



The hypothesis that evaporation is the source of the 

 energy required in raising water dates back, in its general 

 form, to Dutrochet. 2 The main points of this idea are the 

 entrance of the water below and its evaporation above. The 

 point which has caused most trouble lies in the lack of proof 

 that water columns such as are found in the tree have 

 cohesion enough to be drawn up by evaporation to a height 

 far exceeding that to which this liquid would be supported by 

 one atmosphere. That it will cohere somewhat beyond the 

 height to balance a pressure of one atmosphere has been 

 shown by Bohm, 3 Askenasy, 4 and Copeland. 3 The former 

 of these showed that evaporation from the surface of a twig 

 of Thuya attached to an upright tube of water, the lower end 



1 H. H. DIXON, " Note on the R6le of Osmosis in Transpiration," Proceed. Roy. 

 Irish Acad., Sec. Ill, Vol. Ill (1898), pp. 767-75. 



2 M. H. DUTROCHET, M6moires pour servir d, Vhistoire anatomique et physiolo- 

 gique des v6g&taux et des animaux, Brussels, 1837. 



3J. BOHM, " Capillaritat und Saftsteigen," Ber. d. deutsch. bot. Ges., Vol. XI 

 (1893), pp. 203-12. 



*E. ASKANASY, "Beitrage zur Erklarung des Saftsteigens," Verhandl. d. naturl.- 

 med. Verein zu Heidelberg, N. F., Vol. V (1896), pp. 429-48. 



& E. B. COPELAND, "The Rise of the Transpiration Stream," Bot. Gaz., Vol. 

 XXXIV (1902), pp. 161-93, 260-83. 



