94 CROSS-CUTS. [CH. IV 



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, divide the bridge 1 and compare its area with that of 

 the rest of the splint wood of the branch. 



(109) Cross-cuts 2 . 



Take a branch of Primus lusitanica or of laurocerasus 

 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 

 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 other. 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. 



1 The bridge should be cut with a knife, the smooth surface so 

 produced makes the area of the bridge easily perceptible. 



2 Dufour, Sachs' Arbeiten, in. ; also F. Darwin and E. Phillips, loc. cit. 



