50 



EXOGENOUS OE DICOTYLEDONOUS STEM. 



Fig. 112. 



appearance of wedges (fig. Ill w w) arranged in a circle round a cen- 

 tral cellular mass oi pith (fig 112 p), ■which is connected to the outer 

 part or hark by means of cellular processes called medullary rays (fig. 



1 1 1 r r r). At first the cellu- 

 lar portion is large, — the 

 pith, bark, and rays occu- 

 pying a large portion of the 

 stem ; but by degrees new 

 vascular bundles are formed, 

 which are deposited be- 

 tween the previous ones 

 (fig. 112 nnn). By this 

 means the pith is more cir- 

 cumscribed, 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. 



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

 arrangement similar to that represented as occurring in the herbaceous 

 stem (fig. 112), 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 suc- 

 culent 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. 113 represents a horizon- 

 tal or transverse section of the 

 upper part of a young branch of 

 Acer campestre. In the centre, 

 m, is the pith, very large at this 

 period of growth, and occupying 



Fig. 111. Young Dicotyledonous or Exogenous stem, w w, Vascular bundles in the 

 form of wedges, jj, Kth. r- r r, Medullary rays. Fig. 112. Same stem further advanced; 

 the letters as in flg. Ill, nnn. New vascular wedges interposed between those first 

 formed. Pig. 113. Horizontal section of young stem of Acer campestre, magnified twenty- 

 six diameters, m, Pith, e m, e m, Medullaiy sheath. fb,fb, woody bundles. « p. Pitted 

 vessels, r m, Medullaiy rays, c, Cambium or zone of tissue between the xylem or wood 

 portion, and phloem or bark portion, fc, Fibres of Endophloeum. v I, Latioiferous vessels, 

 e c. Cellular envelope, Mesophtaum. p, Corky envelope, Epiphlceum. e p. Epidermis. 



Fig. 113. 



