ORGANS OF NTTEITIOX. 



66 



Upon making a transverse section of an Oak {fig. 155) we 

 observe that the two systems of which the stem is composed are 

 so arranged as to exhibit a distinct separation of parts. Thus 

 we have a central one, m, called the pith ; an external one, c e, or 

 hark; an intermediate wood, r, dispersed in concentric layers; 

 and little rays, h, connecting the pith and the bark, termed 

 medullary rays. Such a stem grows essentially in diameter by 

 constant additions of new matter on the outside of its wood, and 

 hence it is called Exogenous (from two Greek words signifying 

 outside growers). In a Palm stem no such distinction of parts 



Fig. 156. 



Mg. 155. Transverse section of an Oak-branch six years old. m. The me- 

 dulla or pith, c e. The hark. r. The wood. b. Medullary rays. Fig. 



156. Transverse section of the stem of a Palm. m. The cellular sul> 

 stance. /. The flbro-vascular bundles. The whole being invested by a 

 rind or false bark, h. 



can be noticed {fig. 156), but upon making a transverse section 

 we observe a mass of cellular substance, m, distributed through- 

 out it, and the fibro-vascular system arranged vertically in this 

 in the form of separate bundles, /, which have no tendency to 

 form layers of wood; and the whole covered externally by a 

 fibrous and cellular layer, h, which, as will be hereafter seen, 

 is formed essentially by the ends of the vascular bundles, and 

 which is termed the false bark or rind. This structure is called 

 Endogenous (from two Greek words signifying insidj grotvirs), 

 as such stems grow by the addition of new fibro-vascular bundles 

 towards their interior. These two structures, the Exogenous 

 and Endogenous, are characteristic of Flowering Plants. 



If we now turn our attention to Flowerless Plants, and make 

 a transverse section of a Tree-fern (fig. 157), we observe the 

 centre, m, to be either hollow or filled with parenchymatous 

 cells, the fibro-vascular system being arranged in irregular 

 sinuous plates around this, v, v, v, and forming a continuous or 

 interrupted circle near the circumference, which consists of a 

 rind e, inseparable from the wood beneath. This structure is 



F 



