TRANSPIRATION AND ASCENT OF SAP DIXON. 411 



parts of the stem, namely, in the cells of the wood and of the medul- 

 lary rays, actmg to raise the water, and (2) in the energy trans- 

 mitted and applied by means of the physical properties of the con- 

 ducting tracts and of the water stream itself, not necessarily involv- 

 ing any special vital activity on the part of the cells of the stem. 

 Those hypotheses which belong to the first category may be dis- 

 tinguished as the vital and those of the second as the physical theories 

 of the ascent of sap. 



PHYSICAL THEORIES. 



The vital hypotheses of the ascent of the transpiration current 

 take no direct account of the inflow of energy at the leaves. The 

 entire sap-lifting force is applied in the stem. This appears to hold 

 good for all the vital hypotheses with the exception of that of Ewart, 

 who admits that possibly some of the energy needed to raise the 

 water ma}^ be directly transmitted downward from the leaves to the 

 stream in the stem. Of course ultimately the energy assumed by 

 the vital hypotheses to be expended in the stem is derivable from the 

 energetic substances formed in the leaves during photosynthesis, 

 and afterwards distributed to the cells of the stem. * * * 



In 1894 Dr. J. Joly and the author published the first account of 

 their cohesion theory of the ascent of sap. 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 con- 

 sideration and experimental examination. In addition to these we 

 subjected various other hypotheses formed by ourselves to investi- 

 gation.^ As these investigations naturally lead us up to the cohesion 

 theory, it may be permissible to briefly outline 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 

 carbohydrates. The denser fluid thus produced and passed into the 

 tracheids would settle downward. As it passed down it would dis- 

 place upward the less concentrated solutions entering at the root. 

 An accumulation 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 illus- 

 trating the hypothesis is easily set up. A tube, say, 1 millimeter 



1 H. H. Dixon : On the physics of the transpiration current. Notes from the Botanical 

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



