118 



relative nature and importance of tliese prerequisites, in re- 

 ference to the art, and to one anotlier, and draw such con- 

 chisions as may be useful, in throwing light upon the principle 

 just now laid down. 



First : As to superior Thickness and Induration of Bark. 

 The bark of trees is accounted by phytologists, as among 

 the composite organs. It consists, first, of the epidermis, or 

 external cuticle or integument of the plant ; secondly, of 

 the cellular tissue or parenchyma, that is, the soft pulpy sub- 

 stance, situated immediately under the epidermis, and con- 

 stituting a sort of secondary integument ; and thirdly, of a 

 number of thin cortical and concentric layers, composing 

 the mass of the bark ; of which parts the innermost is de- 

 nominated the liber, from its having been anciently vised 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 in- 

 capable 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 pro- 

 duction 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 por- 

 tion of the stem only be decorticated, and covered with a 

 piece of bark from another tree, the two diflferent barks will 

 readily unite. Hence, we are enabled to ascertain how far 

 the liber extends ; and hence also, the origin of grafting, 



* Keith's Physiological Botany, Vol. I. p. 295. — Du Hamel, Phys. 

 des Arbres, L. I. 3. 5. — De Saussurc, Encyclop. Method. T. I. p. 

 67. — Also, Observations sur L'Ecorce, &c. 



