NATURE OF PLANTS 



91 



The vascular bundles are more or less scattered throughout the 

 stem and it is especially noteworthy that there is no cambium 

 separating the xylem and phloem. It will be seen by examining 

 Fig. 61, that the tissues of the bundles are 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 diameter of the stem and no 

 necessity therefore for the protective layer of cork cells. 



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

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

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



42. Apical Growth of the Stem.— All stems are characterized 

 by a more or less extended apical growth. The changes pro- 

 duced in stems by the growth of the cambium only result in 

 increasing their diameters but the elongation of all stems is 



