CH. IV] INCISION. 85 



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 

 depressed. The branch must be firmly supported in two 

 places and the incision made between them, otherwise 

 the weight of the branch will break the thin bridge of 

 wood which is ultimately left. 



When a slowing of the current has been clearly pro- 

 duced, cut the bridge through and compare its area with 

 that of the rest of the splint wood of the branch. 



(109) Gross-cuts. 



Take a branch of Portugal laurel (or of ordinary laurel) 

 which has stood some hours in water; fit it up in the 

 potometer and, as in experiment 108, support the branch 

 in two places, so that it may not break when incisions are 

 made. Having taken a few readings, saw the branch 



' In the ease 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 

 difficult to slip on the rubber tube. 



