98 



ELONGATION 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 

 effected by the formation of new cells at the apex of the stem. 

 Fig. 62 shows the general features to be noticed in the tip of 

 a stem as seen in longitudinal section. New cells are being 

 formed at the apex by division just as in the case of the root. 

 Below this formative region is the zone of elongation in which 

 the cells gradually change in character so that the cortical and 

 central regions are apparent and in certain elongated cells we 

 note the first indication of the vascular bundles (Fig. 62, x). 



e c X 



Fig. 62. Longitudinal section of the tip of a growing stem: e, epidermis 

 extending over surface of the entire tip; a, formative region; 6, upper portion 

 of the zone of elongation; c, cortex; x, cells of the central region that by 

 further growth form the vascular bundles; /, first appearance of the leaves. 



The leaves also originate in this region through the active divi- 

 sion of the superficial cells of the stem. The branches of the 

 stem develop somewhat later in the axils of the leaves, and more 

 deeply located cells in the cortex co-operate in their formation. 

 This relationship of the leaves and branches to the stem is shown 

 in Fig. 63, which is a diagram of an elongating stem, showing the 

 relation of the apical region of the stem {a in Fig. 62), to the 

 lower and older portion. The cells in these young leaves and 

 branches by rapid division'and growth soon form the character- 



