ORGANS OF NUTRITION. 



85 



to the rays, that is tangential to the circumference of the 

 stem {Jig. 175). In some stems such as those of Aristolochia, 

 the medullary rays are very conspicuous, forming large plates 

 between the wedges of wood. In other plants, such as the 

 Yew and Birch, they are comparatively small. 



4. The Bark or Cortical System — The bark is situated on 

 the outside of the stem, surrounding the wood, to which it is 

 organically connected by means of the medullary rays and 

 cambium-layer. At those periods of the year when the cells of 

 the cambium-layer are charged with sap it may be separated by 

 a slight force from the wood beneath, but such separation can 

 only be effected, as we have seen, by the rupture of the cells of 

 which it is composed. 



When the stem is first formed the bark is composed of 

 parenchyma, like the pith, but as soon as the wood begins to be 

 developed on the outside of the pith another layer of Av^oody 

 tissue, consisting of 

 liber-cells, is deposited 

 on the inside of the 

 bark, so that the latter 

 ultimately consists of 

 tw^o distinct systems ; 

 namely, an internal or 

 fibro -vascular, and an 

 external or parenchy- 

 matous. The paren- 

 chymatous system also 

 exhibits in all plants 

 destined to live for any 

 period a separation into 

 two portions, and the 

 whole is covered exter- 

 nally by the epidermis 

 already described. The 

 bark accordingly pre- 

 sents three distinct -^'S'- 1"6. Transverse section of a portion of the bark 

 Ici-orc inrlpi-icnrlontli' of a Dicotyledonous stem. a. Epidermis. 6. Epi- 

 la}eiS, mdependentl^ phloeum. c. Mesophloeum. rf. Eudophloeum. After 



of the epidermis, which Lindiey. 



is common to it with other external parts. Tliese three 

 layers are called, proceeding from within outwards, 1. Liber 

 or Inner Bark, or Endnphloeum (figs. 1 76, rf, and 171,/); 2. Cel- 

 lular Envelope or Green Layer, or Mesophloeum (figs. 176, 

 c, and 171, g)-, and 3. Suberous or Corky flayer, or Epi- 

 phloEum (figs. 176, 6, and 171, h). We shall first describe the 

 liber. 



a. The Liber or Inner Bark, or Endophloeum. (Figs. 176, rf, and 

 171,/.) — This is composed of that variety of woodv tissue 

 G 3 





