vi TENSION REQUIRED TO RAISE THE SAP 129 



observations we get 7*6 cm. per hour as the velocity under 

 a head equal in length to the stem. The last observation 

 of the series was made on a piece of a narrow branch 

 about 05 cm. in diameter, the others on pieces about 1 cm. 

 in diameter. The thin distal portions of the wood in 

 almost every case offer a greater resistance to flow than 

 the thicker parts. The high estimates of resistance are 



Table 14. 



almost always obtained with the former. This difference 

 appeared almost constantly in my experiments. This fact 

 is probably of importance in determining the total resist- 

 ance in the intact plant. I have included the third obser- 

 vation in the table, although it diverges so markedly from 

 the mean, because I could see no error in the experiment, 

 and it is quite possible that a maximum result like this 

 is the nearest to the actual velocity in the uninjured tree. 



The higher mean in the second series of experiments for 

 the velocity of transmission is probably due to the fact 

 that clogging substances are less likely to accumulate 

 owing to the actually slower flow and to the position of 

 the surface of application. 



With care, good results may be obtained with higher 

 pressures, if the supply is from below. In the following 

 experiments (Table 15) the cylinder of wood was fixed 

 in the short arm of a vertical J -tube filled with a repeatedly 

 filtered solution of ferrocyanide of potassium. The moment 

 of penetration through the wood, which was 3 cm. long in 



K 



