THE STEM ITS GENERAL STRUCTURE 63 



why dicotyledonous bundles form rings of wood and mono- 

 cotyledonous cannot (Figs. 75 and 76). The dicotyledon- 

 ous bundle (Fig. 76) has, running across it, a layer of brick- 

 shaped cells called cambium, which cells are a specialized 

 form of the parenchyma cells and retain the power of 



FIG. 76. THE DICOTYLEDONOUS BUNDLE OR WOOD STRAND. Upper figure 

 is of moonseed : 



c, cambium ; d, ducts ; i, end of first year's growth ; 2, end of second year's growth ; bast 

 part at left and wood part at right. Lower figure (from Wettstein) is sunflower: h, wood- 

 cells; ^vessels; c, cambium; /.fundamental tissue or parenchyma; b, bast; bp, bast 

 parenchyma; s, sieve-tubes. 



growing and multiplying. The bundles containing cam- 

 bium are called open bundles. There is no cambium in 

 monocotyledonous bundles (Fig. 75) and the bundles are 

 called closed bundles. Monocotyledonous stems soon cease 

 to grow in diameter. The stem of a palm tree is almost 



