n8 TRANSPIRATION AND ASCENT OF SAP ch. 



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 water from 6 to 33 times the height of the plant 

 are required. 



Objections. There are several reasons why this unex- 

 pected result of Ewart's must be regarded as incorrect. 

 (1) The velocity of flow given in E wart'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) E wart'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 experi- 

 ment and in intact trees. In the latter the lifting forces 

 generated in the leaves must do work against the resist- 

 ance 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. 1 



1 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 



