THE LIGNEOUS SYSTEM. 



135 



411. The bark succeeds and replaces the epidermis, cover- 

 ing and protecting the wood. It is readily distinguished into 

 three parts, viz. : 



The inner, white bark (liber) ; 

 The middle, green bark (cellular) ; 

 The outer, brown bark (cortical). 



The substance of all these is parenchyma; and arranged, like 

 the wood, in layers. 



412. The liber ', or white bark, contains scattered bundles of 

 pleurenchyma and cienchyma with its cellular tissue. Its wood- 

 cells are very long ( 389), called bast-cells, and are strength- 

 ened with secondary deposits until quite filled up. Hence the 

 strength and toughness of flax and hemp. The strong material 

 of " Russian matting" is from the liber of the Linden-tree, and 

 the " lace" of the South Seas, from the Lace-bark tree. The 

 liber of other trees is not remarkable for strength. 



413. The cellular, or green bark, succeeds to the liber. Its 

 tissue resembles that of the leaf being filled with sap and 

 chlorophyl. It grows laterally, to accommodate itself to the 

 enlarging circumference of the tree, but 



does not increase in thickness after the 

 first few years. 



414. The cortical, or brown bark. Its 

 color is not always brown, being rarely 

 white (Canoe Birch), or straw-color (Yel- 

 low Birch), or greenish (Striped Maple), 

 or grayish (Beech, Magnolia). Its sub- 

 stance is always cellular tissue, but dif- 

 fering widely in consistency in different 

 species. Its new layers come from with- 

 in, formed from the green bark, while 

 its older are sooner or later cast off. 



115. The cortical layers sometimes accumulate to a 511< Wood of Oak _ 8ection longl . 

 considerable thickness (Maple, Hickory, Oak), but are tmlinal, showing, , medullary rays; 

 Anally rent and furrowed by the expanding wood. In *' wooc ells; c - porous d ' 

 the Cork Oak (Quercus suber) they attain an excessive growth, furnishing that useful 

 substance, cork. In Birch (Betula papyracea) these layers resemble paper, long abiding 

 by their elasticity the expansion of the trunk. 



416. The medullary rays (medulla, pith) are those fine 



