PHYSIOLOGY OF TREES. 245 



Of the hark. — The bark is a distinct member, 

 thrown off from the cambium as an excremental 

 cloak or covering, over the whole surface of the 

 plant, root as well as stem. In the first year it 

 is a thin transparent film, called the cuticle, 

 which for ever afterwards remains on the exterior. 

 If permanent, the bark is increased in thickness 

 every following year by a layer discharged from 

 the cambium to its interior surface ; the outer 

 layers are called the outer bark, and the inner 

 layers are called the liber. The age of a tree 

 may be accurately ascertained by counting the 

 number of layers of which the bark is composed ; 

 its laminated structure is very conspicuous in 

 the common lime, and many other trees ; and is 

 attached to the stem by the medullary rays of 

 the wood, by the buds, shoots, and sometimes by 

 radicles which force their way through it. 



These three parts compose the seedling stem. 

 They previously existed in the seed, and are 

 developed by elongation and expansion ; no ad- 

 ditional membrane being added either at top or 

 bottom. The leaves are seated on the bark, to 



