ORGANIC STRUCTURE. 99 



cuous in the first concentric layer as they are in 

 those which are afterwards imposed ; and are the 

 glossy waves of the grain of timber exposed hy the 

 plane, and so conspicuous in oak when cut into 

 panels. This description of the growth of the cylinder 

 of wood of the first year, applies to those of every 

 shoot afterwards made by the tree, and also of every 

 layer of wood which annually enlarges an old stem. 



The JBark. The next visible member of the 

 seedling stem is the outer covering or epidermis, con- 

 sisting of a thin colourless cuticle, inclosing a coat of 

 parenchyma. It covers the plumula before its expul- 

 sion from the seed, and continues for ever after on 

 the exterior of the stem ; although, on the generality 

 of shrubs and trees, it becomes so much distended 

 by the internal growth, that its identity disappears. 

 Another member of the bark starts into visible 

 existence at the end of the first summer, namely, 

 the liber. This is at first a part of that member 

 described as the wood ; but is discharged therefrom 

 at the end of summer, and then being distinct 

 receives the name of inner bark or liber. 



The bark is an excrementitious part of the plant. 

 It is increased in thickness every year, during the life 

 of the tree, by new layers of liber added to its interior 

 surface ; so that the numbers of layers of bark, like 

 those of the wood, always indicate the age of the 

 tree. This, though a general rule, has some excep- 

 tions ; as exemplified in the Arlutus andrdcline and 

 the Vitis vinifera, which plants discharge their outer 

 bark every second or third year. 

 H 2 



