86 EOSIN ABSORBED. [CH. IV 



half through at a spot between the two supported points, 

 and 10 — 15 cm. from the cut end. To see clearly how 

 far the saw-cut penetrates, and on which side of the 

 branch it lies, it is advisable to push a square piece of 

 cardboard (for instance a post-card) into the cut. This 

 serves as a guide in making the next cut, which must be 

 exactly opposite incision (i), and 2 cm. above it. It must 

 be slightly deeper than incision (i), so as to overlap it ; it 

 is easy to make sure of this if a second card is placed in 

 the second incision: the edges of the cards should be 

 parallel and should slightly overlap each o^her. The 

 points to note are that cut (ii) depresses the rate of 

 absorption very much more than cut (i), and that after 

 the fall, a rise in absorption -rate comes on. 



(110) Course shown by eosm solution. 



Remove from the potometer the branch used in ex- 

 periment 109, and place the cut end in strong watery 

 solution of eosin, taking off the bark of the part in which 

 the saw cuts were made, leaving, however, 2 or 3 inches 

 at the base unpeeled. The course of the fluid as it passes 

 up the stem is now traced by the eosin, the manner in 

 which the colour spreads at the doubly-cut region being of 

 course the chief point to be noticed. The reason for 

 leaving the bark on the terminal two inches (which is 

 not an essential precaution) is simply to ensure that any 

 superficial rise of fluid shall take place on the bark instead 

 of on the wood. 



(111) Air-pump. 



Cut a branch of Portugal laurel of the same size as 



