134 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



that when branches are sharply bent, the vessels, if 

 large and few, are compressed like suddenly bent 

 caoutchouc tubes, but although the sectional area is 

 enormously reduced, the lumen of the vessel is not 

 necessarily closed. Moreover, in the notching experi- 

 ments, with branches sawn half through, it is quite a 

 mistake to suppose that the continuity of the water 

 current is broken ; and by squeezing branches in a 

 vice it can be shown that the rapidity of the 

 diminished water-flow may be lessened or increased 

 as the sectional area of the vessels is reduced or en- 

 larged by screwing up or loosening the grip. 



Kohl used branches cut under water, as well as 

 completely rooted plants. He measured the rate of 

 transpiration in the normal condition, and then 

 squeezed the stem in the vice : then, taking numer- 

 ous readings, he compared the time it took to trans- 

 pire so much water. It was found possible to screw 

 the vice up tight enough to stop the flow altogether. 



Meanwhile a large series of very careful measure- 

 ments had been and were being made by Fr. Darwin 

 and W. Phillips of Cambridge, 1 of the rate of the 

 transpiration flow under various conditions. Using 



1 " On the Transpiration-stream in Cut Branches," Proc. Camb. 

 Phil. Soc. vol. v. pt. v., Nov., 1885, pp. 330-367. See also Nature, 

 May i, 1884. 



