THE LIGNEOUS SYSTEM. 



141 



Thus the duramen (durus, hard) is formed of a firm and durable text 

 ure, the only part valued as timber. Its varying colors in cherry, wal- 

 nut, rose-wood, are well-known. 



595 



595, Cross-sections of an exogenous stem (Elm), of 2 years' growth ; 1, pith, 2, 3, annual layers 

 of wood, next the cambium, 4, bark; 596, and endogenous stem (Sorghum or Millet), whe»» 

 there is no distinction of layers. 



G99. The duramen is op no account in vegetation, and is in this respect 

 dead. Hence it often decays, leaving the trunk hollow, and the tree at the same 

 time as flourishing as ever. 



TOO. The bark succeeds and replaces the epidermis, covering 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. 



701. The liber or white bark contains scattered bundles of pleuren- 

 chyma and cienchyma with its cellular tissue. Its wood-cells are very 

 long (§ 666), called bast-cells, and are strengthened with secondary de- 

 posits 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. 



702. The cellular or green bark succeeds to the liber. Its tissue 

 resembles that of the leaf, being filled with sap and chlorophylle. It 

 grows laterally to accommodate itself to the enlarging circumference 

 of the tree, but does not increase in thickness after the first few years. 



703. The cortical or brown bark. Its color is not always brown, 

 being rarely white (canoe birch), or straw-color (yellow birch), or green- 

 ish (striped maple), or grayish (beech, magnolia).* Its substance is al- 

 ways cellular tissue, but differing widely in consistency in different 

 species. Its new layers come from within, formed from the green bark, 

 while its older are sooner or later cast off. 



