MINUTE ANATOMY OF FLOWERING PLANTS 225 



522. The Bark of a year-old stem consists of three 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 fibers, the walls of which are 

 commonly lignified, and other ele- 

 ments, as already briefly described. 

 In woody 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 functions as assimilating 

 tissue in the same way as the leaf 

 parenchyma. 



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 Maple, 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 

 cork cambium, lying on the inner boundary of the cork. 

 The old cork, being dead and therefore incapable of 



OUT. OF BOX. 15 



380. Cross section through 

 bark into the wood of 

 a Lilac twig: e, epi- 

 dermis ; c, cork ; p, 

 collencbyma; g, green 

 rounded cells ; /, bast 

 fibers ; ca, cambium ; 

 IV, wood; 1, 2, 3, in- 

 ner, middle, and outer 

 bark. 



