132 TRANSPIRATION AND ASCENT OF SAP ch. 



transmitting branch ; when reduced to velocity under unit 

 head the result is 2*17 cm. per hour. 



(3) Again, on p. 55 an experiment is recorded which is 

 suitable for comparison. A velocity of 19 cm. per hour 

 was observed in a branch 25 cm. long under a head of 

 4 metres. This becomes 1*19 cm. per hour under unit head. 



The mean of these three observations gives T57 cm. 

 per hour as the maximum velocity of flow in the yew wood 

 under unit head. 



The results of my own numerous observations, on the 

 other hand, made under very various conditions of pressure 

 and by different methods, point to a velocity exceeding 

 7 cm. per hour with the same head. 



The only explanation of this discrepancy which appears 

 possible is that in Ewart's experiments sufficient care was 

 not taken to prevent bubbles forming in the opened con- 

 duits, and to obviate clogging at the surface. This last 

 effect would be exaggerated in his experiments, as he 

 worked apparently in every case at such high pressures. 

 Reference to Fig. 20 illustrates this point. There it appears 

 that the erratic observations are those made at high 

 pressures, although at both high and low pressures similar 

 precautions were taken. It is in only the first of Ewart's 

 experiments quoted above that it is mentioned that the 

 experimental branch was cut under water. Other pre- 

 cautions are not mentioned. 



Whatever is the cause of the discrepancy, it is certain 

 that if Ewart had obtained my results, the difficulty of 

 resistance, which he finds to be fatal to the cohesion- 

 theory of the ascent of sap, would not have presented 

 itself to him, for the velocity of 7 cm. per hour, which he 

 demands in the stem of the yew, would not require a 

 pressure equivalent to a head of 65 metres of water, as 

 he supposes, but only to 11 metres. The recognition of 

 the fact that unit head produces a velocity of about 7 cm. 

 per hour invalidates his whole calculation, intended to 



