128 PRINCIPLES OF GARDENING. [CH. III. 



it is composed of as many layers as tliey are years of 

 age. It is in the innermost of these, which is called 

 the liber, that the vital returning circulation and se- 

 cretions are carried on for the time being almost ex- 

 clusively. These layers are concentric, or, as they 

 are usually termed, cortical layers; they are thicker 

 in feeble plants than in more vigorous plants of the 

 same species ; they are formed of wa\dng longitudi- 

 nal fibres, the meshes of the net- work they thus con- 

 stitute being filled with pulp. If the outer bark is 

 destroyed, but the wound does not penetrate below 

 the liber, the wound is healed up, otherwise the re- 

 moved part is unregenerated. In some roots, al- 

 though only annuals, the bark is composed entirely 

 of liber, and is \ery thick, as in the carrot and pars- 

 nep, in which it is remarkably separated by a light- 

 coloui'ed annular mark, from the central or woody 

 part. The liber is composed of various longitudinal 

 tubes, m which the true sap of the individual de- 

 scends after elaboration in the leaves : consequently 

 here are found in the most concentrated state the sub- 

 stances that are the peculiar products of each plant, 

 as the resin of the fir, the bitter principle of the cin- 

 chona, or Peruvian bark, &c. 



I will here pause, to remark upon some of the 

 remedies which have been recommended for the re- 

 moval of insects from the bark of trees. Oil has 

 been directed to be smeared over them, for the de- 



