1906.] On the Transpiration Current in Plants. 53 



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 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 transpiration, 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 above 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 1*35 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 continually 

 wet, but the water was never appreciably piled upon it. The amount of 

 water transmitted in this manner under unit head was 1*356 gramme per 

 hour. In order to find the effective cross-section, after this observation was 

 made, a solution of eosin was supplied under similar conditions. 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 T93 grammes is transmitted per square centimetre 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 7'5 cm. per hour. This agrees 

 well with the results obtained with the other method. 



In order to determine the amount of flow without danger from the error 

 of clogging at the surface of supply, the flow was measured in a lateral 

 branch springing from a stout stem. Water under pressure was supplied at 

 the two cut ends of the stem. With this arrangement, owing to the 

 relatively large surfaces of supply, the clogging taking place at these does nob 

 encroach upon the amount necessary for the supply of the small lateral branch 

 for several hours, and consequently the rate of transmission in the lateral 

 branch remains constant for several hours. To quote one of these 

 experiments : A piece of yew stem 7'3 cm. long, having a straight lateral 



