114 



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 section of a stem of Dracaena, in process of secondary 

 thickening, e, epidermis ; h , periderm ; r, cortex, in which a bundle-pair b is passing 

 out to a leaf ; x, stelar cambium ; g, g, vascular bundles ; ///, primary pith ; st, secon- 

 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 



