EXOGENOUS OK DICOTYLEDONOUS STEM. 



37 



have no cotyledons, and are called Acotyledonmis (*, privative). The 

 terms connected with the embryo will be afterwards fully explained. 



Exogenous or Dicotyledonous Stem. 



72. The Exogenous or Dicotyledonous stem characterizes the trees 

 of this country. It consists of a cellular and vascular system : the former 

 including the outer bark, medullary rays, and pith ; the latter, the 

 inner bark, woody layers, and medullary sheath. In the early stage 

 of growth, the young dicotyledonous stem is entirely cellular; but 

 ere long fusiform tubes appear, forming bundles, having the appear- 

 ance of wedges (fig. 94 w w) 



arranged in a circle round 

 a central cellular mass of 

 pith (fig. 94 p\ which is 

 connected to the outer part 

 or bark, by means of cellu- r\ 

 lar processes called medul- 

 lary rays (fig. 94 r r r). At 

 first, the cellular portion is 

 large, the pith, bark, and 

 rays occupying a large por- 

 tion of the stem ; but by degrees new vascular bundles are formed, which 

 are deposited between the previous ones (fig. 95 n n n). By this means 

 the pith is more circumscribed, the medullary rays become narrow, and 

 the bark more defined. Such is the structure presented by an annual 

 herbaceous dicotyledonous stem, consisting of pith, a circle of fibro- 

 vascular and woody tissue, medullary rays, bark, and epidermis. 



73. The stems of trees and shrubs in their young state exhibit an 

 arrangement similar to that represented as occurring in the herbaceous 

 stem (fig. 95), with this difference, that the vascular circle is more 

 firm and solid. As ligneous stems continue to grow, further changes 

 take place by which their diameter is increased, and they are rendered 

 more dense. The shoots or young branches given out annually, how- 

 ever, are similar in structure to annual herbaceous stems ; and in 

 making successive sections from the apex of a branch, which is succu- 

 lent and green, to the base of a trunk, which is comparatively dry 

 and hard, the various changes which take place can be easily traced. 

 Fig. 96 represents a thin horizontal or transverse section of the upper 

 part of a young branch of Acer campestre. In the centre, TO, is the 

 pith, very large at this period of growth, and occupying at least one- 

 half of the whole diameter, its cells diminishing in size as they approach 



Fig, 94. Young Dicotyledonous or Exogenous stem, w w, Vascular bundles in the form of 

 wedges, p, Pith, rrr, Medullary rays. 



Fig. 95. -Same stem further advanced ; the letters as in fig. 94, n n n, new vascular wedges 

 interposed between those first formed. 



