ORGANS or NUTRITION. 89 



Pith or Medulla, belonging to the parenchymatous system. 

 Medullary Sheath, composed chiefly of 1 ^j^^^^^ 

 spiral vessels. Ic, , * 



Woody Layers, one of which is formed | fibro-vascular sys- 



annually on the outside of the pre- f ^;°^' ^^''^ ^°f ^^^^ 

 1 •' , • ^ r TT7- 1 lorm the wood pro- 



vious layers, and consists of Wood- , n i 



cells and Pitted Vessels. J P^^^^ '^ ''^^^^' 



4. Medullary Bays, consisting of cellular plates connecting the 



pith and the bark. 



5. Cambium-Layer, composed of vitally active cells, from 



which new layers of wood and liber are formed. 



6. The Bark, composed of two systems. 



1. Inner Bark or Endophloeum, or Liber, formed of liber-cells 



chiefly, and belonging to the fibro-vascular system, 

 increasing by the annual addition of new layers on its 

 inner surface. 



2. Outer Bark, composed of parenchyma, and hence belong- 



ing to the parenchymatous system, and consisting of 



a. Cellular Envelope or Mesophloeum, giving the green 



colour to bark. 



b. Corky Layer or Epiphlceum. 



7. The Epidermis, which invests the epiphlceum in young 



stems; it is replaced after a certain age by the epi- 

 phlceum. 



B. Endogenous or Monocottledonous Stem. — In our coun- 

 try we have no trees or shrubs which exhibit this mode of growth, 

 although we have numerous herbaceous plants, such as Grasses, 

 Eushes, Sedges, &c. In our gardens again we have various kinds 

 of Lilies, Yuccas, Tulips and other bulbous plants, which are also 

 endogenous in their structure. It is in the warmer regions of 

 the globe, and especially in the tropics, where we find the most 

 striking and characteristic illustrations of Endogens; and of all 

 these the Palms are by far the most remarkable. The appearance 

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

 of exogenous trees, for endogenous plants have commonly no 

 branches, but their stems are almost uniformly cylindrical from 

 below upwards, frequently rising to the height of 1 50 feet or 

 more, and crowned at their summits by a magnificent tuft of 

 leaves {fig. 178, 1). 



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

 sents, as we have seen (page 71), no distinction of pith, wood, 

 medullary rays, and bark, but the cellular system is scattered 

 more or less over the entire surface {figs. 164, m, and 179, A, a), 

 the fibro-vascular being dispersed vertically in this, in the form of 

 separate bundles, which have no tendency to collect together so 



