84 



THE planter's GUIDE. 



parts the innermost is denominated tlie liher, from its 

 having been anciently used to write upon before the 

 invention of paper/" If the cortical layers be injured or 

 destroyed by accident, the part is again regenerated, and 

 the wound healed up without a scar. If the wound have 

 penetrated beyond the liber, the part is incapable of being 

 regenerated ; because, when the surface of the alburnum 

 is exposed to the air for any length of time, there will be 

 no further vegetation in that part. But if the wound be 

 not very large, it will close up, first by the production of 

 new bark issuing from the edges, and gradually narrowing 

 the wound, and then by the production of new layers of 

 wood formed under the bark as before. If a portion of 

 the stem only be decorticated, and covered with a piece 

 of bark from another tree, the two different barks will 

 readily unite. Hence we are enabled to ascertain how 

 far the liber extends ; and hence also the origin of graft- 

 ing, which is always effected by a union of the liber of 

 the graft with the stock.f 



"There is no fixed or definite period/' as Keith 

 observes, " that can positively be assigned as necessary to 

 the complete induration of the wood or bark, although it 

 seems to require a good many years before any particular 

 layer is converted from the state of alburnum to that of 

 perfect wood.^J In respect to bark, there is not any 

 circumstance which hastens this period so much as the 

 exposure of trees to the elements, even at an early period 

 of their age. In the same way as the action of the air 



* Keith's Physiological Botany, vol. i. p. 295. Du Hamel, Phys. des Arbres, 

 lib. i. pp. 3, 5. DeSaussure, Encyclop. Method, torn. i. p. 67. Also, Observations 

 sur I'Ecorce, &e. 



+ Senebier, Phys. Veget. torn. i. pp. 177, 178. Keith, vol. ii. p. 229. Knight, 

 Philosoph. Trans. 1803. Ellis, Anat. Veget. in Suppl. Encyclop. Britan. 



t Physiolog. Bot. vol. ii. p. 231. See also Kieser, Organ, des Plantes, chap, 

 ii. pp. 95, 96, et seq; also p. 153, &c. 



