NATURE OF PLANTS 



97 



through the opening up of the vascular cylinder just as in the 

 case of the dicotyledones, page 76. The vascular bundles thus 

 formed, however, do not long pursue a uniform course through 

 the stem, some soon turning into the pith and others bending 

 into the cortex. It is also especially noteworthy that there is 

 usually no cambium separating the xylem and phloem. It will 

 be seen by examining Fig. 61, that the tissues of the bundles are 



Fig. 61. Cross-section of a single vascular bundle of corn stem: ph, 

 phloem; x, small cells of the xylem; v, vessels of xylem; si, stereome that 

 forms a sheath about the bundle; p, parenchyma of the stem. 



of a similar character and have the same arrangement as in the 

 dicotyledons (Fig. 42) save for the absence of the cambium which 

 prevents the addition of new cells to the xylem and phloem. 

 Consequently there can be no considerable increase in the dia- 

 meter of the stem and no necessity therefore for the protective 

 layer of cork cells. 



