348 CONSERVATIVE ORGANS. [LKCT. VII 



augment in diameter, as represented at a in the 

 marginal cut, because it is supplied 

 by the alburnous matter furnished 

 from the leaves ; but that below it, b, 

 ceases to grow, and continues in the 

 same state, the communication by the 

 interior bark between it and the foli- 

 age being completely destroyed. From 

 the same cause, also, the pith of a 

 stem is thrown apparently out of its 

 centre, for some distance below the 

 point where a branch is given off; a circumstance 

 which I shall very soon have occasion more par- 

 ticularly to notice *. 



The only objection of any weight which has 

 been advanced against Mr. Knight's theory, is 

 founded on the fact, that when a stem is wholly, 

 or in the greater part stripped of its bark, and 

 the denuded surface excluded from the action of 

 the air, a glary fluid is exuded from the albur- 

 num, or soft wood, which gradually becomes or- 

 ganized and cellular. Detached spots of bark 

 are thus reproduced, and, gradually extending, 

 coalesce, until the stem is again clothed with 



* Vide Plate 6, fig. 6. which displays, moderately mag- 

 nified, a longitudinal section of an Elder (Sambucus nigra) of 

 two years growth, with a luxuriant branch of the same age : 

 a. the trunk has the pith c. in the centre, except below the 

 branch b. where the additional wood e. [augments the diameter 

 of the stem on that side, 



