42 Prof. H. H. Dixon. [Nov. 7, 



bright cloudless breezy June days between 10 a.m. and 1 p.m., with a shade 

 temperature averaging 18° to 22° C. The conditions for transpiration were r 

 therefore, optimal. 



" After a timed period the stem was removed and rapidly sectionised from 

 apex downwards until the eosin solution was visible in the wood, the length 

 of the remaining portion of the stem giving the rate of flow during the period 

 of observation. Portions of the same stems and also similar ones from the 

 same plants were then subjected to varying water pressures until closely 

 corresponding rates of flow were reached."* 



By this method, and by assuming that the velocities shown in these 

 branches are maintained throughout the stem, Ewart obtains results which 

 indicate that, in order to move water in the stems of plants at the velocity of 

 the transpiration current, pressures equivalent to a head of from 6 to 33 times 

 the height of the plant are required. 



There are several reasons why this unexpected result of Ewart's must be 

 regarded as incorrect. (1) The velocity of flow given in Ewart's experiment is 

 probably far in excess of even the maximum velocity of the transpiration 

 current in the intact plant. (2) The velocity cannot be assumed to be 

 uniform throughout high trees ; but may fall off from below upwards. 

 (3) Ewart's results for the resistance to flow in stems are not in agreement 

 with a large body of experiment to be quoted presently, but appear to be 

 excessive. 



First, with regard to the velocities in Ewart's experiment and in intact 

 trees. In intact trees the lifting forces generated in the leaves must do work 

 against the resistance to flow all along the path of the current, and, if the supply 

 is inadequate from the roots, against other opposing forces in addition, and 

 against the whole hydrostatic head. In Ewart's experiment not only are the 

 resistance of the lower part of the conducting system, the other opposing 

 forces and the hydrostatic head removed, but they are replaced by the 

 atmospheric pressure acting as a vis a tergo, urging the water upwards. 

 Naturally, then, a much greater velocity is attained in the latter case than 

 when the branch is still attached to the tree.f 



The subsequent wilting of the leaves of cut branches shows that clogging 

 afterwards greatly reduces the flow ; but, of course, in the experiment quoted 

 it is only the initial stage which is recorded. 



* Ewart, loc. ctt., p. 24. 



+ It may be mentioned that Janse's method of observing the amount of transpiration 

 by successive weighings of a severed branch is not falsified by this error, but probably 

 gives too small an amount after transpiration has proceeded for some time. Strasburger 

 considers that this is corrected more or less accurately by his over-estimate of the resistance 

 (Strasburger, loc. cit., p. 779). 



