114 



PLANT LIFE. 



one. In stems of the few monocotyledonous plants which 

 undergo secondary increase in diameter, the internal generat- 

 ing layer arises fronr the i)ericycle. Upon division the inner 

 segments, chiefly, differentiate, and from them arise new 

 isolated Inmdle ]iairs ( 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 Drac.Tna. in process of secondary 

 tfiickening. c', epidermis ; h, periderm ; 7-, cortex, in wllicll a bmidle-pair h is passing 

 out to a leaf ; x^ stelar cambium ; v, A', vascular bundles ; ///, primary pitb ; st, secon- 

 dary pitb. Tlie amount of secondary thickening; is shown by the radial arrangement 

 of cells of secondary jjitli, IMagnified about 50 diam, — After Sachs. 



In the many dicot3ic(lons 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 /one nearly 



