58 Henry H. Dixon. 



utilizes the energy entering at the evaporating surfaces in the 

 leaves.^) 



Direct measurements give the weight of the water transpired 

 and the height through which it is raised. There is considerably- 

 more uncertainty with regard to the work done in order to overcome 

 the resistance opposed to the transpiration current. But several attempts 

 have been made to approximate to it, 



Janse's^) method was to find the head of water required to 

 drive water through a branch at the same rate as the transpiration 

 current. He measured the transpiration curi-ent by determining the 

 loss of weight in a given time of the same branch when severed 

 from the tree, or else by determining the amount of water abstracted 

 by the branch from a weighed vessel in a given time. 



He gives as his results that the head of water required to produce 

 a flow equal to the transpiration stream equals 2 — 21 times the 

 length of the wood traversed. 



Strasburger^) used the second method of estimating the 

 transpiration rate. He states that in the case of the conifers, Taxus 

 and Tsuga, a head equal to several times the length traversed was 

 required. In the case of plants with vessels a much smaller resistance 

 was obtained, and he found that a head of 12 cm was able to drive 

 water at the transpiration rate through 10 cm of Acacia florihunda. 



Ewart^) has also made experiments on this subject. Using the 

 same method as Strasburger he found that the head required to 

 urge water through the conducting tracts amounted to 6—33 times 

 the length of stem traversed. 



A second method for determining the maximal transpiration "was 

 used by the last investigator. A branch bearing a small number of 



') It should be noticed that, while the point of support for the tensile water, 

 in the case of dead organs which continue to transpire, is always tue ;nenisci in the 

 cell-walls, yet it is quite possible that these menisci may never or seldom be called 

 into play in living leaves. The presence of soluble substances on the outer surface 

 of the cell-wall would perform the same function in supporting the evaporating 

 surface as the sub microscopic capillaries, or, again, the excretion of clean water on 

 the outside of the cells might replace these menisci Some process of this nature is 

 rendered probable by the fact that the water surface retreats from the outer cell- 

 walls after death, as is evidenced by the drying of leaves, although watersupplies 

 are available below. Cf. J. Joly in a Report of a Discussion on the Ascent of Water 

 in Trees. Ann. of Bet, Dec. 1896, p. 651 and H. K. Dixon, Physics of the 

 Transpiration Current. Loc. cit., pp. 35 et seq. 



^) J. M. Janse, Die Mitwirkung der Markstrahlen bei der Wasserbewegung 

 im Holz. Jahrb. f. Wiss. Bot., XVIII, 1887, pp. 1 et seq. 



^j E. Strasburg er, Ueber den Bau und Verrichtungen der Leitungsbahnen 

 in den Pflanzen. Jena 1901. 



*) A. J. Ewart, On the Ascent of Water in Trees (First Paper). Phil. Trans. 

 Roy. Soc. Lond. 



