LECT. VII. ] ANATOMY OF STEMS. 347 



responding to the number of the vascular 

 bundles. The fact, indeed, that the leaf is es- 

 sential for the formation of wood, had been ob- 

 served by Dr. Hales, who took off circles of 

 bark half an inch in breadth, at several places, 

 from two thriving shoots of a dwarf Pear tree, 

 leaving on all the remaining intervening ringlets 

 of the bark, except one, a leaf-bearing bud, which 

 produced leaves the following summer. Each 

 ringlet, on which a bud was left, grew and swelled 

 below the bud, or at its lower edge ; but that one 

 on which no bud was left " did not increase at 

 " all * :" but he drew a very different and less pro- 

 bable conclusion from his experiment, which it is 

 unnecessary to mention in this place. 



The above-mentioned experiments of Mr. 

 Knight readily explain why trees and shrubs, 

 the leaves of which are destroyed by caterpil- 

 lars, form scarcely any new wood in that sea- 

 son ; and, indeed, every one who has ever pruned 

 a tree, or shortened a growing twig, must have 

 observed that the part above the last leaf al- 

 ways shrivels and dies, while all below it con- 

 tinues to live and increase in diameter: and we 

 observe the same thing in the lower part of the 

 stem of a young tree, when a portion of the bark 

 has been gnawed off by sheep, or accidentally de- 

 stroyed. The part above the wound continues to 



* Vegetable Staticks> p. 145, fig. 28, 29, 30. 



