140 



VEGETABLE LIP^E AND WORK. [SECTION IG. 



layer of wood outside the preceding one, between tliat and the bark. This 

 is exogenous growth, or outside-growing, as tlie name denotes. 



4(30. Some new bark is formed every year, as well as new wood, the 



former iniiide, as the latter 

 is outside of that of the 

 year preceding. Tlie ring 

 or zone of tender forming 

 tissue between the bark 

 and tiie wood lias been 

 called the Cambium Layer. 

 Cambium is an old name 

 of the physiologists for 

 nutritive juice. And this 

 thin layer is so gorged 

 with rich nutrifive sap 

 when spring growth is re- 

 newed, that the bark then 

 seems to be loose from 

 the wood and a layer of 

 viscid sap (or cambium) to 

 be poured out between the 

 two. Rut there is all 

 the while a connection of 

 the bark and the wood by 

 delicate cells, rapidly mul- 

 tiplying and growing. 

 431. 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 hiner Bark. This contains some 

 wood-cells, or their equivalent, commonly in the form of bast or bast-cells 

 (411, Fig. 444), such as those of Basswood or Linden, and among heibs 

 those of flax and hemp, wliicii are sjjuu and woven or made into cordage. 

 It also contains cells which are named sieve-Q,^'&, on account of numerous 

 slits and pores in tiieir walls, by which the protoplasm of contiguous cells 

 communicates. Li woody stems, whenever a new layer of wood is formed, 

 some new liber or inner bark is also formed outside of it. 



FiQ. 478. Piece of a stem of Soft Maple, of a year old, cut crosswise and length- 

 wise. 



Fio. 479. A portion of the same, magnified. 



Fig. 480. A small piece of the same, taken from one side, reaching from the hark 

 to the pith, and highly magnified: a, a small hit of the pith ; h, spiral ducts of what 

 is called the medullary sheath ; c, the wood ; d, d, dotted ducts in the wood ; 

 e,e, annular ducts;/, the liher or inner bark; g, the green bark; h, the corky 

 layer; t, the skin, or epidermis; /, one of the medullary rays, or plates of silver 

 grain, seen on the cross-section. 



