122 THE STEM. 



the Sweet Gum (Li qui (lumbar), and on some of our Elms (Ulmus 

 alata and racemosa). It also forms the paper-like, exfoliating layers 

 of Birch-bark. It is composed of laterally flattened parenchymatous 

 cells, much like those of the Epidermis (G9, Fig. 191, i), which 

 directly overlies it, and forms the skin or surface of the stem. 



218. The elements of an exogenous stem of a year old, especially 

 in a woody plant, accordingly are these, proceeding from the centre 

 towards the circumference : — 



I. In the Wood : 



1. The Pith, belonging to the cellular system (Fig. 194, 195, p). 



2. The Medullary Sheath, ms, ) which belong to the woody or 



3. The Layer of Wood, W, w, ) longitudinal system. 



4. The Medullary Rays, mr, a part of the cellular system. 



II. In the Bark : 



5. The Liber, I ; its bast-tissue, h, belongs to the woody system. 

 G. The Outer Bark, belonging wholly to the cellular system, and 



composed of two parts, viz. : 1st, the Green or Cellular En- 

 velope, ge, and 2d, the Corlcy Envelope, ce. 

 7. The Epidermis, e, or skin, Avhich invests the whole. 



219. An herbaceous stem does not essentially differ from a woody 

 one of this age, except that the wood forms a less compact or thinner 

 zone ; and the whole perishes, at least down to the ground, at the 

 close of the season. But a woody stem makes provision for contin- 

 uing its growth the second year. As the layer of wood continues to 

 increase in thickness throughout the season, by the multiplication of 

 cells on its outer surface, between it and the bark, and when growth 

 ceases this process of cell-multiplication is merely suspended, so 

 there is always a zone of delicate young cells interposed between the 

 wood and the bark. This is called the 



220. Cambium-Layer, (Fig. 194, 195, C). It is charged with or- 

 ganizable matter (protoplasm, dextrine, &c.) in the form of a mu- 

 cilage, which is particularly abundant in the spring when growth 

 recommences. This mucilaginous matter was named Cambium by 

 the older botanists; who supposed — as is still generally thought — 

 that the bark, then so readily separable, really separated from the 

 wood in spring, that, a quantity of rich mucilaginous sap was poured 

 out between them, and that this sap, or cambium, was organized 

 into a tissue, the inner part becoming new Avood, the outer, new 

 bark. But delicate slices will show that there is then no more inter- 

 ruption of the wood and inner bark than at any other season ; that 



