140 



VEGETABLE LIFE AXD WORK. [SECTION IG, 



layer of ■wood outside the preceding one, between that and the barii. This 

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



430. Some new i)ark is formed every year, as well as new wood, the 



former inside, as the latter 

 is outside of that of the 

 year preceding. The ring 

 or zone of tender forming . 

 tissue between the hark 

 and the wood has 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 nutritive 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. But there is all 

 the while a connection of 

 the bark and the wood by 

 delicate cc\h, rapidly mul- 

 tiplying and growing. 



431. The Bark of a 



year-old stem consists of 



beginning next the wood, — 



This contains some 



480 



three parts, more or less distinct, namely, 



1. The Liber or Fibrous Bark, the Luier Bark. 

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

 (111, Fig. 44'4), such as those of Basswood or Linden, and among herbs 

 those of flax and hemp, which are spun and woven or made into cordage. 

 It also contains cells which are named sieve-ceWsi, on account of numerous 

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

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

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



Fig. 473. Piucu of a .stem of Soft Maple, of a year old, cut crosswise and length- 

 wise. 



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



Fig. 4S0. A snnll piece of the same, taken froni one side, reaching from tlie bark 

 to the pith, and highly magnified: a, a.<mall hit of the jiith; b, .spiral ducts of what 

 is called the mrdiillary shrrith ; c, the wood; d, d, dotted ducts in the wood; 

 e,e, annular ducts;/, the liber or inner bark; (j, the green bark; //, the corky 

 layer; i, the skin, or epidermis; j, one of the medullary rays, or plates of silver 

 grain, seen on the cross-sectiuu. 



