INTRODUCTION. 



XV 



defined. The external coating of the tree, the 

 bark, is composed of two distinct parts. The 

 outer, called the cuticle or skin, consists entirely 

 of cellular tissue ; in young steins it is more or 

 less succulent, but in old stems becomes withered 

 and dry. Underneath the cuticle is a layer of 

 woody fibre, called the liber, or iniier hark. This 

 is succeeded by another layer of cuticle and liber, 

 there being as many rings of bark as there are of 

 wood. As each of these rings is deposited annu- 

 ally inside the old bark, the outer one of all was 

 formed during the same year with that circle of 

 wood which is next to the pith ; but since the 

 circumference of the tree when the first layer was 

 formed was much smaller than at any subsequent 

 period, and its power of expanding was very 

 limited after a year or two, the increased size of 

 the trunk compelled it to split into irregular 

 pieces ; and hence arises the rugged appearance 

 of the exterior of most trees. The Plane, and 

 some other trees which do not present this rough 

 appearance, annually throw off* the outer coat of 

 liber and cuticle in large plates, so that their bark 

 furnishes no criterion for discovering the age of 

 the trees. The Currant-tree throws off* its outer 

 bark in horny rings ; the Birch, in long thin 

 ribands. The cuticle is not confined to the trunk 

 of a tree, but invests the branches, leaves, and 

 even the most delicate parts of the flow^er, being 

 modified into hairs, down, prickles, &c., and 

 being frequently perforated for the transmission 

 of fluids and gases. Between the outer circle of 

 wood and the inner layer of bark is interposed 

 (while the tree is in a growing state) a mucila- 

 ginous fluid, called cambium^ which, as it exhibits 



