140 



VEGETABLE LIFE AND WORK. [SECTION 16. 



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

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

 430. Some new bark 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 bark 

 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 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 Inner 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 herbs 

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

 It also contains cells which are named sieve-cells, 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 ot wood is formed, 

 some new liber or inner bark is also formed 01 t^ide of it. 



480 



Fio. 478. Piece of a stem of Soft Maple, of a yeai old, cut crosswise and length- 

 wise. 



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



FIG. 480. A small piece of the same, taken from one side, reaching from the bark 

 to the pith, and highly magnified: a, a small bit of the pith; b, spiral ducts of what 

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

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

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

 grain, seen on the cross-section. 



