LESSON 25.] 



AND EXOGENOUS STEMS. 



151 



331 



outer part of which is also cellular. This structure is very familiar 

 in common wood. It is really just the same in the stem of an herb, 

 only the wood is much less in quantity. Compare, for 

 instance, a cross-section of the stem of Flax (Fig. 352) 

 with that of a shoot of Maple or Horsechestnut of 

 the same age. In an herb, the wood at the beginning 

 consists of separate threads or little wedges of wood; 

 but these, however few and scattered they may be, are 

 all so placed in the 

 stem as to mark out 

 a zone (or in the 

 cross-section a ring) 

 of wood, dividing the 

 pith within from the 

 bark without. 



426. The accompa- 

 ny ing figures (which 

 are diagrams rather 

 than exact delinea- 

 tions) may serve to 

 illustrate the anat- 

 omy of a woody 

 exogenous stem, of 

 one year old. The 

 parts are explained 

 in the references be- 

 low. In the centre is 

 the Pith. Surround- 

 ing this is the layer J. 



of Wood, consisting both of wood-cells and of ducts or vessels. From 

 the pith to the bark on all sides run a set of narrow plates of cellular 

 tissue, called Medullary Rays : these make the silver-grain of wood. 

 On the cross-section they appear merely as narrow lines; but in 

 wood cut lengthwise parallel to them, their faces show as glimmer- 



FIG. 359. Cross-section of the Ptrm of Flax, showing its bark, wood, and pith. 



FIG. 353. Piece of a stem of Soft Maple, of a year old, cut crosswise and lengthwise. 



FIG. 354. A portion of the same, magnified. 



FIG. 355. 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, dotted ducts in the wood j e, e, annular ducts ; /, the libel 

 or inner bark ; g, the green bark ; A, the corky layer ; i, the skin, or epidermis ; /, one of the 

 medullary rays, or plates of -silver-grain, seen on the cross-section. 



