52 Prof. H. H. Dixon. [Nov. 7, 



It may be noticed that the most divergent observations are those made 

 at the higher pressures. 



When we compare these results with E wart's* a very wide discrepancy 

 is apparent. The results of three of his experiments allow themselves to be 

 readily compared with my figures. 



(1) He found that water travelled in a piece of yew stem, 35 cm. long, 

 at the rate of 11*7 cm. per hour under a head of 3 metres. The head here 

 is nearly 8*6 times the length of the transmitting wood. Assuming the 

 velocity proportional to the pressure, at unit head the water would travel 

 at 1*36 cm. per hour. This rate is stated to be maximal. 



(2) On pp. 51 and 52 it is stated that the rate of flow in a piece of 

 yew wood, 25 cm. long, under a head of 3 metres is 26 cm. per hour. 

 In this experiment the head is equal to a column 12 times the length of the 

 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 1*57 cm. per hour as the 

 maximal 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 conduits 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. Eeference to fig. 2 

 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 precautions 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 demandsf in the stem 



* Loc. cit., p. 49. 



i Loc. cit., pp. 56 and 57. 



