1906.] 



On the Transpiration Current in Plants. 



49 



to bubbles being generated in the conduits while preparing the wood for his 

 resistance experiments. He certainly does not mention having taken these 

 or similar precautions in this connection, and his results are three to four 

 times as high as those given when these precautions are taken. 



When determining the velocity of flow with colour solutions, I usually 

 worked with a head of water equal to, or less than, the length of the piece 

 of wood experimented with. When using unit head, a fresh surface was 

 cut with a razor on one end of a piece of a branch prepared as just described. 

 The piece was supported vertically with the fresh surface uppermost, and 

 a little vaseline smeared round the bark of that end. A drop of filtered 

 concentrated eosin solution was placed on the upper surface, and as the 

 drops formed at the lower end they were drawn away by lighly 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 time of 

 observation at unit head. The following are the results of some of these 

 observations : — 



Table II. 



Length of piece. 



Time of observation. 



Distance travelled. 



Velocity per hour. 



centimetres. 



minutes. 



centimetres. 



centimetres. 



7 2 



15-0 



1-7 



6-8 



6'0 



10-0 



1-2 



7-2 



4-0 



15-0 



1-7 



6'8 



3-5 



15-0 



1-8 



7 2 



10-0 



20-0 



2-2 



6-6 



3-0 



25-5 



3-0 



6-9 



In the last experiment the fluid transmitted was a solution 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 equivalent 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 observations we get 7*6 cm. per hour 



VOL. LXXIX. — B. E 



