CH. IV] INCISION. 93 



ivy {Hedera helix) answer feirly well. Do not attempt to 

 compress the whole stem but cut away half of it before 

 applying the vice. The exposed surftice may for greater 

 security be rubbed with lard to prevent air leaking into 

 the vessels exposed ; in any case the part selected for 

 compression must be as far as convenient from the cut 

 end, so as to avoid the chance of air being sucked back 

 into the apparatus. The vice should be a light one, e.g. a 

 watchmaker's vice, so that it may support itself when it is 

 screwed on to the stem. 



When the rate of absorption is steady, compression 

 may be applied : it will be found necessary to screw the 

 vice with great force so that the compressed tissues are 

 squeezed to a mere plate. If the compression has been 

 continued for some time, and the vice is then unscrewed, 

 it will be noted that the absorption is very rapid and 

 that it soon slows down. This shows that negative 

 pressure rises during compression and falls when water 

 is- allowed freely to enter the vessels. 



(108) Incisions. 



In some trees it is obvious that the amount of wood 



in the transverse section is far greater than is absolutely 



needed to carry the transpiration current. Fit a branch 



of yew ^ {Taxus) to the potometer, and take a few readings, 



then saw it half through and read again. The rate of 



absorption will be unaltered, and the branch may indeed 



be almost severed before the rate of absorption is seriously 



1 In the case of yew it is better to remove the bark (at any rate in the 

 spring) because it is easily detached from the wood, and this makes it 

 difi&cult to slip on the rubber tube. 



