vi TENSION REQUIRED TO RAISE THE SAP 133 



show that to raise the sap in trees 150 metres high would 

 require a pressure approaching 100 atmospheres. The 

 facts of the case would be more correctly stated by saying 

 that if the amounts transpired by isolated branches, 

 under exceptionally favourable conditions for transpira- 

 tion, were transpired by the remaining branches of the 

 yew, and if we further assume that the velocity in the 

 trunk is maintained out into the finest branches of high 

 trees, then the resistance to flow in the conducting tracts 

 would be about equal to a head of water the same height 

 as the tree. 



My results for the resistance, which were obtained as 

 described already by directly measuring the velocity of 

 flow under a given head, were fully confirmed by other 

 experiments in which the amount transmitted under a 

 given head was observed. An estimate of the cross- 

 section effective in transmitting the current then gives 

 the velocity. 



A piece of a branch of Taxus baccata, 4 cm. long and 

 having a woody cylinder T35 cm. in diameter, was placed 

 with its long axis vertical. Water was supplied at 

 its upper surface just as quickly as it percolated through 

 the wood, so that the upper surface of the wood was con- 

 tinually wet, but the water was never appreciably piled 

 upon it. The amount of water transmitted in this manner 

 under unit head was 1 '356 grammes per hour. In order to 

 find the effective cross-section, after this observation was 

 made, a solution of eosin was supplied under similar con- 

 ditions. By this means the transmitting portions were 

 coloured, and the area of their cross-section easily estimated. 

 A mean of three such estimations gave the effective cross- 

 section as 0"7 sq. cm. From this it appears that under 

 unit head 1*93 grammes is transmitted per square centi- 

 metre per hour. Assuming with Ewart that the lumina 

 occupy about 0"25 of the cross-section, the velocity to 

 secure this rate must have been 75 cm. per hour. This 



