SECONDARY THICKENING. NORMAL DICOTYLEDONS. 



455 



duced from it, which constitute the secondary thickening, are in the main outlines the 

 same in the very great majority of stems of Dicotyledons and Gymnosperms. The 

 phenomena belonging to this category, to be treated of in the present Chapter, may 

 therefore be termed those of the normal growth in thickness., and the stems in question- 

 be termed the normal* stems, while those showing a different behaviour may be 

 contrasted with them, as anomalous forms (Chap. XVI). 



I. The origin of the Cambium takes place in cases without simultaneous 

 or previous formation of intermediate bundles, in the following manner. The delicate 

 cells arranged in radial rows, lying on the internal border of the phloem of ■. he leaf- 

 trace bundles (p. 325), remain meristematic ; divisions by means of tangential walls 



Fig. 194. — Cross-section tliro'jgli the fully elongated liypocotyledonary stem of Ricinus communis, t gy leaf- 

 trace bundle; c,cb cambial zone; cb interfascicular segments of the latter, arising by tangential division of the 

 parenchyma- of the medullary ray. For further explanation compare p. 332, From Sachs' TextbSok. 



go on in them in the succession to be described below, and the vascular bundle thus 

 grows in the radial direction. Sooner or later the tangential divisions are continued 

 from the lateral edges of the phlo'em-groups, through a band in each medullary ray 

 connecting these groups with one another, so as to give rise to a meristematic zone 

 in this band also. (Comp. Fig. 194.) The meristematic annular zone, which is thus 

 formed from the bundles, and from the segments belonging to the medullary rays,, is 

 the cambium. If the leaf-trace bundles in any cross-section are of unequal thickness, 

 the interfascicular completion of the cambium begins at the edges of the thickest, 



