vi TENSION REOUIRED TO RAISE THE SAP 12c 



Advantage of using small pressures. To obtain 

 the resistance, Ewart forced water through lengths 

 of branches under various pressures. The pressure 

 which gave the same velocity in the branch as that 

 estimated by his methods for the transpiration current 

 he took to be equivalent to the resistance experienced by 

 the transpiration current. Ewart does not mention how 

 he prepared the pieces, or how he cleansed the water for 

 the experiment. The introduction of air-bubbles or of 

 any clogging substance at the cut surface would materially 

 exaggerate the pressure needed to obtain the observed 

 velocity. Indeed, if the branch were transpiring actively 

 when the experimental pieces were removed, it would not 

 be sufficient to cut it under water ; for the liquid in the 

 branch being in tension, bubbles would be formed at the 

 surface of the cutting knife. These bubbles would require 

 some time to dissolve and disappear. The slimy materials 

 exuding from the injured cells also clog the branch and 

 raise the apparent resistance of its conduits. The com- 

 paratively high pressures with which Ewart worked would 

 render the clogging from this source and from any 

 impurity in the water more marked. The curves 

 reproduced in Fig. 19 illustrate this point. The 

 ordinates of the curves there shown indicate weights 

 of filtered tap-water transmitted through 3 cm. of 

 wood of Abies 'pectinata per second. The abscissae in- 

 dicate time in minutes. The curves show the diminution 

 in the rate of transmission for each pressure, the fall-off 

 in the amount being much more rapid for the higher pres- 

 sures. From these it is seen that it is desirable (especially 

 when using colour solutions, which, from the nature of 

 the case, cannot be distilled) to employ low pressures in 

 order to determine the resistance of the conduits apart 

 from the surface resistance. For this reason, then, when the 

 resistance of the tracheidal tubes was being determined it 

 seemed preferable to experiment with very low pressures, 



