30 Henry H. Dixon. 



the cohesion of water these observations would be readily explained. 

 B ohm's words are such that we might, on first reading, think 

 that he gives this explanation were it not that he records the fact 

 that ,.Im Torricelli'schen Eaume fand sich stets ein winziges 

 Luftbläschen". The presence of this bubble would of course prevent 

 the cohesion of water coming into play to transmit the tension 

 generated by the transpiring branch. 



As a further annotation on the experiment Böhm quotes Jaeger 

 for the statement that the internal pressure (Binnendruck) of water 

 corresponds to about 2700 atm. and adds that "the length which a 

 water filament might theoretically attain in consequence of cohesion 

 is enormous. In comparison with it the highest tree on earth would 

 be a vanishingly small dwarf. Such water filaments, hanging from 

 the evaporating leaf-cells, with their lower ends in connection with 

 the water of the soil, are undoubtedly present in plants". 



These words would stand for a description of the cohesion theory 

 of the ascent of sap, and, were it not for other statements, we might 

 suppose Böhm held that view; but throughout this paper, as in all 

 those since 1889, he is arguing in favour of capillarity as supplying 

 the key to the situation, and all through the paper containing the 

 reference to cohesion he shows that he refers the phenomena in the 

 wood to capillarity e. g. „Das saftleitende Holz ist .... ein sehr 

 complicirtes Capillarsystem und die Capillarität ein noch dunkles 

 Gebiet der Physik", and in summing up he states „Wir können mit 

 mathematischer Bestimmtheit behaupten, daß die capillare Saugung 

 im saftleitenden Holze gross genug ist, um das Wasser in die Krone 



der höchsten Bäume zu heben Die Ansicht, dass der von 



den transpirirenden Blattzellen aus den Leitungsbahnen gesaugte 

 Saft in diesen „unter einem stark negativen Drucke stehe", wird 

 Strasburger wohl nicht weiter vertreten".^) Here there can be 

 no doubt that Böhm considers that the capillarity of the tracheae 

 raises the water to the top of the conducting tissues and that from 

 these it is convej^ed by atmospheric pressure to the leaf cells. 



I have made these quotations and given literal translations from 

 Böhm's papers to enable the reader to gain some idea of the relation 

 of his work to the views of other investigators. But owing to his 

 contradictory expressions it will, I am afraid, be always impossible to 

 clearly follow him. His views, so far as we can get at them, seem 

 to be: — that the capillary forces of the tracheae draw up the water 

 in the tree, atmospheric pressure drives water from the topmost 

 tracheae into the evaporating cells; Böhm saw no difficulty in the 



') J. Böhm, Ursache der Wasserbewegung m transpirirenden Pflanzen. Verb, 

 d. k. k. zool.-bot. Gesell, in Wien, 1890, pp. 151 and 156. 



