7o TRANSPIRATION AND ASCENT OF SAP ch. 



experiments have not been able to demonstrate a sensible 

 difference in either case. Furthermore, Ewart, working 

 very carefully by a different method, has failed to detect 

 the existence of these pumping actions in stems. Con- 

 sequently it is quite impossible to admit that any large 

 amount of work falls on the cells of the stem in the raising 

 of the sap. 



While these considerations show that forces of any great 



magnitude are not exerted by the cells 

 in the wood on the transpiration-cur- 

 rent, it seemed desirable by some more 

 careful method to test the matter, and 

 see if some much smaller force were 

 not assisting the upward flow of water. 

 In the ordinary methods of testing 

 this question, uncertainties arise from 

 the fact that conditions are not the 

 same before and after the reversal of 

 the current, or before and after the 

 death of the branches. These differ- 

 ences are principally due to changes 

 in temperature, which, as Ewart has 

 pointed out, entail large differences 

 in viscosity, and to clogging in the 

 experimental stem. 



In order to eliminate these sources 



T" of error, and so be in a position to 



C //)X' ]'[" detect the effect of even a very small 



force exerted by the stem-cells in 

 lifting water, I carried out some ex- 

 periments in the following manner : 

 Two straight branches (A and B, Fig. 13), about 80 cm. 

 long, without lateral shoots, and as similar to one another 

 as possible, 1 were passed through tubulures (a and b) in 



Fig. 13. 



1 I used Syringa vulgaris, as similar and straight branches of this shrub 

 are readily obtained. 



