41 



On the Transpiration Curremt in Plants. 



By Henry H. Dixon, ScJX, Professor of Botany in Dublin University, and 

 Director of Trinity College Botanic Gardens. 



(Communicated by Professor J. Joly, F.B.S. Eeceived November 7, — Eead 



December 6, 1906.) 



In a paper* published in 1905, Professor Ewart investigates the question 

 •as to what force is required to move water through the waterways of plants 

 •at the same velocity as the transpiration current. His general conclusion 

 is that the resistance is so great that neither are sufficient forces generated 

 in the leaves to raise the water at the required velocity, nor is the tensile 

 strength of water adequate to transmit these forces downwards, if such 

 existed. In a subsequent paper he seems to have modified this latter view 

 and quotes Berthelot as stating that air-free water may support a tension of 

 over 200 atmospheres.^ 



It is needless to criticise Ewart's calculations of the resistance based on 

 Poiseuille's formula, which he himself admits is quite inapplicable to the case, 

 owing to the presence of cross-partitions and irregularities in the cross- 

 sections of the tracheal tubes. It may be noticed, however, that when care 

 was taken that discontinuities were not present in the water columns of the 

 wood experimented upon, the flow observed approximated to the flow 

 calculated by the formula. In an experiment on a piece of yew wood the 

 approximation was very remarkable. The actual amount transmitted^ through 

 a length of 15 cm. was 4*2 c.c. per hour, while the calculated amount was 

 9*8 c.c. The average distance from one another of the cross-partitions in the 

 fine tubes composing the wood is 0*25 cm. Therefore about 60 partitions 

 must be traversed in passing through a length of 15 cm. This indicates that 

 the resistance offered by the walls, or rather by the pits in the walls, to the 

 passage of water is very slight. 



Ewart also endeavours to find experimentally the resistance offered to the 

 transpiration current. His first method, that of Janse and Strasburger,§ was 

 as follows : " Leafy branches 4 to 8 feet in length were cut under water and 

 kept in darkness for half an hour. Clean ends were then cut under water, 

 placed in freshly filtered eosin solution, and at once exposed in the open on 



* "Ascent of Water in Trees," 'Phil. Trans.,' 1905. 



t " The Resistance to Flow in Wood-vessels," ' Ann. of Botany,' vol. 19, No. 75, July, 

 1905. 

 t " Ascent of Water in Trees," ' Phil. Trans., 5 1905, p. 50. 

 § Strasburger, ' Histologische Beitrage, 1901,' Part III, p. 778. 



