350 Mr F. Darwin and Mr R. W. Phillips, [Nov. 23, 



tight. If a flaw were to exist in the india-rubber tubing it is 

 evident that the air would be drawn in through it instead of water 

 being drawn up through the thermometer tube. It may be sup- 

 posed therefore that if an incision is made into the vessels of the 

 plant the result would be the same, and no readings would be 

 obtainable since the apparatus would no longer be air-tight. The 

 following experiment shows that this result may actually occur. 

 A branch of Acacia (Robinia) was cut under water and fitted into 

 the potometer. The branch was cut half through, and after a 

 minute or two a chain of bubbles was seen rushing into the poto- 

 meter from the cut end of the branch, and no reading could be 

 taken since no water was being drawn through the thermometer 

 tube. On another occasion the same thing was observed, as shown 

 in the following table : 



Exp. 22. 



p.m. 



1.16 



18 



Rate 



1-47 

 1-43 



(i) A nick was cut down the pith : 



19 | 



The cut was rubbed with lard : 



21 | 12-7 



Branch sawed ^ through opposite (i) and 2 cm. lower 

 down • the place being covered with lard : 



30 | 4-9 



Exp. 22 also shows that a great depression follows the second 

 incision, even when leakage of air is prevented by using lard. 



Robinia was the only plant met with in which a single cut 

 reduced the rate of absorption to zero. And it is certain that in 

 the other plants experimented on, the great diminution observable 

 after the second cut was not due to leakage of air into the appa- 

 ratus. If this had been the case it is clear that the great fall in 

 rate of absorption would have occurred when the first incision was 

 made ; or at any rate the falls in rate caused by the first and 

 second cuts would have been equal. But this was not the case. 

 The fact that recovery, i.e. an increase in the rate of absorption, 

 usually follows the depression caused by double sawing, is a further 

 proof that leakage is not the cause of the depression. In the case 

 of Portugal laurel the following results show clearly that the kind 

 of leakage which occurs in Robinia has no share in depressing the 

 rate of absorption ; and this might have been inferred from the 

 fact that the wood consists chiefly of tracheids. 



