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XXIX. 



A QUANTITATIVE EXAMINATION OP THE ELEMENTS OF 

 THE WOOD OF TREES IN RELATION TO THE SUPPOSED 

 FUNCTION OP THE CELLS IN THE ASCENT OF SAP. 



By henry H. DIXON, Sc.D., F.R.S., 

 University Professor of Botany, Trinity College ; 



AND 



MISS E. S. MARSHALL, B.A. 



[Read December 15, 1914. Published January 7, 1915.] 



The wood of trees, which forms the conducting system for the ascending sap, 

 is composed of lifeless and living elements — tlie tracheae and cells. Some of 

 the living elements form vertical sheets or laminae in tlie wood, especially 

 surrounding the vessels. These cells constitute the wood parenchyma ; the 

 cells of the medullary rays form radiating bands of tissue penetrating between 

 the tracheae, and connecting together the tracts of wood parenchyma in a 

 radial direction, and putting them in communication with the bark on one 

 side, and often with the pith on the other. In no place do the cells of the 

 wood parenchyma or the medullary rays interrupt the vertical continuity of 

 the tracheae, which form continuous series running from the roots to the 

 topmost twigs. 



It was suggested by one of us (2) that this distribution precluded the 

 intervention of the cells in raising the transpiration stream. It seemed 

 certain that the action of these cells in secreting water laterally into the 

 tracheae, where it would be free to percolate downwards, oould be of no 

 assistance in raising the sap. 



Janse (9) took exception to this suggestion, and urged that it is based on 

 the neglect of the resistance to the flow of water offered by the tracheae. His 

 idea appears to be that if the cells secrete water fast enough into the tracheae 

 the upward current will only be diminished by the percolation downwards — 

 in fact, that the cells must not only raise the transpiration stream, but must 

 also overcome the leakage backwards due to the permeability of the wood. 

 This view seems to contemplate an amazingly inefficient mechanism for 

 raising the sap. 



