CHAPTER IV 



COHESION THEORY OF THE ASCENT OF SAP IN STEMS 



Origin of the theory.- In 1894 Dr. J. Joly 

 and the author published the first account of their 

 cohesion theory of the ascent of sap. Our attention 

 had been directed to the problem in the year 1892, when 

 the late Professor Strasburger had been good enough to 

 show us some of his experiments on high trees. After 

 more than a year's experimental work and discussion, we 

 were able to give an outline of our theory to the Dublin 

 University Experimental Science Association in March, 

 1894, and all the essentials were communicated to the 

 Royal Society in October of the same year. 



Other hypotheses examined. In the work lead- 

 ing up to our theory we of course submitted the 

 theories of previous investigators, so far as we were 

 acquainted with them, to full consideration and ex- 

 perimental examination. In addition to these we sub- 

 jected to investigation various other hypotheses formed 

 by ourselves. As these investigations naturally lead us up 

 to the cohesion theory it may be permissible to outline 

 them here briefly. 



Gravitational theory. In the first place it 

 seemed possible that gravitation itself might furnish 

 the force for lifting the upward moving water. This 

 at first seems paradoxical. Suppose the dilute sap 

 in the leaves to be concentrated by evaporation and 



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