MISUTE AS ATOMY OF FLOWEiUNG PLANTS 2J5 



522. The Bark o[ a year-old stem consists of tlu-ee parts, 

 more or less distinct, namely, — beginning next the 

 wood, — 



1. The liber, or fibrous bark, the inner bark (Fig. 380, 

 1). This contains the bast hbers, the walls of which are 

 connnonly ligniiied, and other ele- 

 ments, as already briefly described. 

 In \\ ood}- stems, whenever a new 

 layer of wood is formed, some new 

 liber or inner bark is also formed 

 outside of it. 



2. The green or middle Bark (Fig. 

 380, 2). This consists mainly of 

 rounded parenchyma cells, contain- 

 ing chlorophyll granules like the 

 cells of the leaf. The green bark 

 of twigs firnctions as assimilating 

 tissue in the same way as the leaf 

 parencliyma. 



3. The corky layer or outer bark 

 (Fig. 380, 3), consisting of empty, 

 angular cells, closely coherent, the 

 walls of which are suherized, or 

 chemically altered in such a man- 

 ner as to be impermeable to water. 

 It is this which gives to the stems 

 or twigs of shrubs and trees the 

 aspect and the color peculiar to 

 each, — light gray in the Ash, pur- 

 ple in the Red Ma^^le, red in several 

 Dogwoods, etc. 



Sometimes the corky layer grows 

 and forms new layers inside the old for years, as in the 

 Cork Oak, which produces the cork of commerce, the 

 Sweet Gum Tree, and the White and the Paper Birch. 

 This growth proceeds from a formative layer, called the 

 cin-k cambium, lying on the inner boundary of tlic corlc. 

 The old cork, liriug dead and therefore incapaljlc of 



OUT. OF J;t-)T, — 15 



380. Cross section through 

 bark into the wood of 

 a Lilac twig: e, ejji- 

 dermis ; c, cork ; p, 

 collenchyma; g, green 

 rounded cells ; /, bast 

 fiber.s ; cr;, cambimn ; 

 IV, wood ; 1, 2, ;->, in- 

 ner, middle, and outer 

 bark. 



