Transpiration and the Ascent of Sap. 31 



height of the water columns owing to the cohesion of water; but 

 without doubt he was quite astray as to the conditions under which 

 cohesion could act, as a glance at his diagram and his statements 

 regarding the capillary forces of the wood will immediately show. 



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 called to the problem in the year 1892 when Prof. Stras- 

 burger 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 Univ. Experimental Science Association in March 1894, and 

 all the essentials were communicated to the Eoyal Society in October 

 of the same year.^) 



In the work leading up to our theory we naturally submitted 

 the theories of previous investigators, so far as we were acquainted 

 with them, to full consideration and experimental examination. In 

 addition to these we subjected various other hypotheses formed by 

 ourselves to investigation.-) As these investigations naturally lead 

 us up to the cohesion theory it may be permissible to briefly out- 

 line them here. 



In the first place it seemed possible that perhaps gravitation 

 itself might furnish the lifting force of the upward moving water. 

 This at first seems paradoxical. Suppose the dilute sap in the leaves 

 to be concentrated by evaporation and by the addition of carbo- 

 hydrates. The denser fluid thus produced and passed into the tracheids 

 would settle downwards. As it passed down it would displace up- 

 wards the less concentrated solutions entering at the root. An accu- 

 mulation of the denser material in the lower part of the tree may 

 be supposed to be prevented by the abstraction of materials from 

 the concentrated sap all the way down. In this way it is secured 

 that the ascending 'raw' sap is just overbalanced by the denser 

 descending column and the very dilute solutions brought into the 

 root might in this way be raised to any height. A model illustrating 

 the hypothesis is easily set up. A tube say 1 mm bore and closed 

 at the lower end is filled with a solution of a dye, e. g. Fuchsin, and 

 set upright. A small funnel containing a denser salt-solution is attached 

 to its upper end. The heavy solution immediately begins to gravitate 

 downwards and in doing so displaces an equal volume of the lighter 



') H. H. Dixon and J. Joly, On the Ascent of Sap (abstract). Proc. Roy. 

 Soc. Lond., Vol. 57, (1894), B, p. 3 and On the Ascent of Sap. Phil. Trans. Roy. Soc. 

 Lend , Vol. 186, (1895), B, pp. 56.3 et seq. 



^) H. H. Dixon, On the Physics of the Transpiration Current. Notes from 

 the Botanical School, Trinity College Dublin, 2, 1897, p. 4. 



