[Pkoc. Koy. Soc. A'lCTOKiA, 25 (N.S.), Pt. I,, l'J12.] 



Ar'I". Vill. -The A>icf'iil <in<l l)i'f>ccni of Water in Trers. 



By ALFRED J. EWAHT, l).8c., Ph.D, F.L.S. 

 (Professor of Botany aiul I'lant Physiolo<^y in the MellnKxrni' University). 



(Witli Platp VII.). 

 [Rea-l 9th May, ];)12. . 



The question us to whether the aid nf livinp: wood tissue is neoes- 

 sarv for the continuous conduction of water up loft)' trees is still an 

 unsettled one, bein<i" answered by some investigators in the affirmative. 

 and hx others in the negative. According to the theory put forward 

 by Dixon and Joly, as well as by Askensay, the ascent is wholly, or 

 almost wholly, due to the suction exercised by the transpiring leaves 

 upon the cohering columns of water suspended from the:ii in the wood 

 vessels ; so that the water is drawn up from the roots in umch the 

 same way that a rope might be hauled up bv hand. In a previous 

 paperl it was pointed out that an explanation was required, not merely 

 as to how the water was held suspended in the vessels, but also as to 

 how the kinetic resistance to How was overcome, and it was shown by 

 calculation and experiment that in actively transpiring trees the total 

 kinetic resistance to flow might be several times greater than the 

 statical resistance due to the height of the tree. In other words, 

 the suspended water columns in the vessels might at their highest 

 points in the tallest trees, be under a tension equivalent not merely 

 to a head of .300 feet of water, but to one of one or two thousand feet, 

 or even more. Water columns are capable of standing such tensions, 

 but only under conditions which are not presented in the wood vessels 

 of trees — namely, the water cohimns must be entirely free from air or 

 dissolved gases, they must be enclosed in rigid walls inqjermeable to 

 water and to dissolved gases, and apparently, also, to judge from some 

 of the experiments performed, the water must be as free as possible 

 from suspended solid particles. Further, so far as I am aware, all the 

 physical experiments which have been successful in demonstrating a 

 high tensile strength for columns of water, have l)een carried out with 

 ">t:or.a'y ?o!'v.>:ins. It "s cpiite nn open miestion ns to wliethcr a 

 column of water flowing with fair rapidity through a tube would 

 exhibit the same tensile strength as a stationary one, particularly if 

 its flow were interrupted by roughness and occasional transverse par- 

 titions, producing eddy currents or irregular flow instead of steady 

 stream line flow. This question is, of course, one for physicists, liut 



1 Phil. Trans. Hoy. Soc. London, B., vol. 19S, 1905, p. 41 



