128 TRANSPIRATION AND ASCENT OF SAP ch. 



fresh surface uppermost, and a little vaseline smeared 

 round the bark of that end. A drop of filtered concen- 

 trated eosin solution was placed on the upper surface, and 

 as the drops formed at the lower end they were drawn 

 away by lightly touching them with bibulous paper. As 

 the eosin sank through the wood it was kept constantly 

 replenished drop by drop above. After a definite time 

 the experiment was stopped, and longitudinal cleavage of 

 the piece of wood showed the distance travelled by the eosin 

 solution during the duration of the observation at unit head. 

 The following are the results of some of these observations : 



Table 13. 



In the last experiment the fluid transmitted was a solu- 

 tion of ferrocyanide of potassium. Its presence below was 

 detected by ferric chloride. The mean of these experiments 

 gives a velocity of 6*9 cm. per hour under a pressure equiva- 

 lent to a head of water equal in length to the experimental 

 piece of wood. 



Experiments were also made at lower pressures. For 

 these the water-pressure was applied at the lower end of 

 a vertical piece of wood, through a rubber tube bent into 

 a U -shape. This tube was filled with eosin solution, and 

 the surface of the solution was raised to the desired height 

 above the upper surface of the wood. 



The mean of these observations is 8 \5 cm. per hour, 

 calculated to unit head. If we exclude the two extreme 



