20 Henry H. Dixon. 



velocity of the transpiration current, pressures equivalent to a head 

 of 6 to 33 times the height of the plant are required. To overconie 

 this resistance, which in the case of the highest trees would amount 

 to 50 — 100 atm., Ewart feels constrained to fall back on the vital 

 activities of the wood parenchyma. He supposes them to lend a helping 

 hand in some way and to overcome the resistance of the stem all 

 along its length. Fully realizing the absurdity of the earlier views 

 which suppose the cells of the wood to exert a propulsive force on 

 the rising stream, E w a r t ^) proceeds to devise ingenious suggestions 

 whereby the alteration of surface tension in the water surrounding 

 the bubbles may lead to a kind of peristaltic passage of the water, 

 or to the bodily transference of the intervening water columns upwards 

 in the stem. According to one suggestion substances are supposed 

 to be exuded by the cells into the tracheae which cause a local change 

 in the surface tension of the film enclosing a bubble. An annular 

 bulge on the bubble is supposed to ensue, which progressive exudation 

 chases up along the bubble, driving with it the film of water between 

 the bubble and the tracheal wall. Another suggestion is that the 

 water columns of the Jamin's chains in the wood are moved bodily 

 upwards under a difference of surface tension at their upper and 

 under surfaces. According to Ewart this difference may be accounted 

 for by supposing that the cells exude a substance into the wall of 

 the conducting tube at the level of the lower surface of the column 

 which prevents the wall from being thoroughly wetted by water. 

 The consequence is that a convex meniscus is formed below while 

 the concave one remains above. Under the action of the two the 

 water column is urged upwards. As another alternative Ewart 

 suggests that the living cells may maintain a difference of con- 

 centration at the two ends of each column and so keep up a constant 

 difference of surface tension under which the column will move 

 upwards. 



With regard to the first suggestion ; even if it were granted 

 that the living cells round a tube actually secrete substances which 

 would change the surface tension of water, it may be doubted if a 

 bulge such as Ewart postulates could be formed, as the part of 

 the film possessing the lowest surface tension would be instantaneously 

 spread out over the whole film. Further, it is hard to see how 

 diffusion from the cells could secure such a local action as is necessary. 

 The second hypothesis is scarcely more happy. If such a substance 

 as is postulated were introduced on the inner surface of a tube 

 containing a Jamin's chain it is evident that it would depress the 



') A. J. Ewart, loc. cit. and The Ascent of Water in Trees (Second Paper) 

 Phil. Trans, of Roy. Soc. Lond., Series B Vol. 199 pp. 341—392. 1908. 



