ii 4 



PLANT LIFE. 



one. In stems of the few monocotyledonous plants which 

 undergo secondary increase in diameter, the internal generat- 

 ing layer arises from the pericycle. Upon division the inner 

 segments, chiefly, differentiate, and from them arise new- 

 isolated bundle pairs (in which the xylem bundle surrounds 

 the phloem bundle) and new pith (fig. 132). 



Fig. 132. — Portion of a transverse se< lion of a stem ol Dracaena, in process of secondary 

 thickening. <■, epidermis; I:, periderm; > . cortex, in which a bundle-pair 4 is passing 

 out to a leaf : .1 . stelar i ambium ; ; . g, vas< ular bundles ; nt, primary pith ; tt, see on- 

 dary pith. The amount of secondary thickening is shown by the radial arrangement 

 of cells of secondary pith. Magnified about 50 diam. — After Sachs. 



In the many dicotyledons whose stems increase in diam- 

 eter, the stelar cambium arises between the xylem and the 

 phloem bundles of each pair, and extends across the pith 

 rays which intervene, thus forming a complete zone nearly 



