1906.] 



On the Transpiration Current in Plants. 



47 



In many ways, then, we see it is established that supply largely controls 

 transpiration, and in neglecting this factor Ewart considerably exaggerated 

 the maximal velocities of the transpiration through the stems of trees. 



We come now to consider the amount of resistance experienced by the 

 transpiration current in its passage through the water conduits. 



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 comparatively high pressures with which 

 Ewart worked would render the clogging from this source and from any 

 impurity in the water more marked. Fig. 1 illustrates this point. The 



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