INTERNAL STRUCTURE. 



77 



stem, and of the whole plant. Lastly, the inner bark, accord- 

 ingly named ENDOPHLCEUM, takes the special name of LIBER, and 

 is the most important portion of the bark in the stems of trees 

 and shrubs. Complete and well-developed liber, like that of 

 Linden or Basswood, contains two peculiar kinds of cells in 



addition to common parenchyma, both of the fibrous or vascular 

 class: viz., 1. CRIBRIFORM or SIEVE-CELLS, a sort of ducts the 

 walls of which have open slits, through which they communicate 

 with each other ; 2. BAST or BAST-CELLS, the fibre-like cells 

 which give to the kinds of inner bark that largely contain them 



FIG. 135. Portion of a transverse section (above), and a corresponding vertical sec- 

 tion (below), magnified, reaching from the pith (p) to the epidermis (e) of a stem of 

 Negundo, a year old : B. the bark ; W. the wood ; and C. the cambium-layer, as found 

 in February. The parts referred to by small letters are : p. a portion of the pith : 

 mr. small portion of a medullary ray where it runs into the pith; four complete med- 

 ullary rays as seen on a transverse section, appear in the upper figure, running from 

 pith to bark : ms, medullary sheath, a circle of spiral unreliable ducts, one seen length- 

 wise with uncoiling extremity in the lower figure : w, w. woody tissue : dd. one of the 

 dotted ducts interspersed in the wood : cl. cambium-layer or zone of new growth of 

 wood and inner bark : l-b. liber or inner bark, the inner portion of which is here cellular, 

 the outer (6) composed of slender and thick-walled bast-cells or true liber-cells: 

 ge. green envelope or inner cellular bark : ce. corky envelope or outer cellular bark : 

 e. epidermis. 



