ORGANS OF NUTBITION, 83 



jb, Camhium-Layer, consisting of vitally active cells, from which 

 f new layers of wood and liber are formed. 



6. The Bark, composed of two systems. 



1. Imier Bark, Endophlmum, or Liber, formed essentially of 

 / liber-cells, and thus- belonging to the fibro-vascular sys- 



""N^ tem ; increasing by the annual addition of new layers on 



' its inner surface. 



2. Outer Bark, composed of parenchyma, and hence belonging 

 / to the parenchymatous system, and consisting of 



I a. Cellular Envelope, or Mesophloeum, composed of more or 



s./ less angular cells, with interspaces ; and giving the 



/* green colour to bark. 



/ b. Suberous Layer, or Epiphlceum, composed of flattened 



/— cells, forming a compact tissue, and giving the peculiar 



/ hues to the young bark. 



. 7. The Epidermis, which invests the epiphloeum in yoimg stems, 



— ^ and which is replaced after a certain age by the epiphloeum. 



B. Endogenous or Monocotyledonous Stem. — In this coun- 

 try we have no indigenous trees or large shrubs which exhibit 

 this mode of growth, although we have numerous herbaceous 

 plants, such as Grasses, Eushes, and Sedges, which have this 

 \y structure. In our gardens again we have various kinds of Lilies, 

 Yuccas, Tulips, and other bulbous plants, which are also endo- 

 genous in their growth. But it is in the warmer regions of the 

 globe, an^ especially in the tropics, where we find the most striking 

 and characteristic illustrations of such stems, and of all such the 

 Palms are by far the most remarkable. The appearance of 

 such plants, even externally, is very different from those of exo- 

 genous trees, for the stems of Palms are commonly of the same 

 diameter throughout, being uniformly cylindrical from below 

 upwards, instead of conical, as is the case with those of exo- 

 genous stems, and frequently rise to the height of 150 feet or 

 more, commonly without branching, but crowned at their sum- 

 mit by an enormous tuft of leaves {Jig. 170, 1). 



When we make a transverse section of a Palm stem, it pre- 

 sents, as we have seen (page 65), no such separation of parts 

 into pith, wood, medullary rays, and bark, as we have described 

 as existing in an exogenous stem; but the parenchymatous system 

 is diffused more or less over the entire surface of the section 

 (Jiffs. 156, m, and 171, a, a), while the fibro-vascular system 

 is arranged vertically in this, in the form of separate bundles, 

 which have no tendency to collect together so as to form zones 

 of wood, as in exogenous stems {Jiffs. 156, /, and 171, b, c, d). 

 * The whole is covered externally by a fibrous and parenchymatous 

 n layer, which is called the false bark or rind {Jig. 156, b) ; because 

 \ it is not a distinct and parallel formation to the wood, as is the 



g2 



