GROWTH AND CELL DIVISION 



71 



stem, represented in Fig. 36, e and d. In the latter cases the meri- 

 stem is practically identical in form and structure with that of the 

 root tip. It does not, however, produce a protective rootcap, since 

 it is here protected by the enveloping leaves of the terminal bud. 



Growth in length takes place in the stem, as in the root, by the 

 continuous transformation of cells produced by the division of tl o 

 meristem cells into cells of the elongat- 

 ing zone. Later these become permanent 

 tissue and form the mature nodal and 

 internodal tissue of the older portions of 

 the stem. At the same time a cylindrical 

 cambium layer, in the region indicated by 

 the dark lines in the figure, begins to 

 divide to form the new water-conducting 

 and food-conducting tissue 

 of the vascular, or woody, 

 cylinder of the plant. 



Fig. 40 represents the 

 elongation of a woody stem 

 by a large terminal bud. 

 In this instance the gen- 

 eral structure of the meri- 

 stem and the method of 

 elongation are the same as 

 in the herbaceous stem. 

 The differences between 

 the two arise from the 

 fact that the herbaceous 

 stem has a more continu- 

 ous growth throughout the 

 season than the woody 



stem, and from the fact that the buds of the herbaceous stems con- 

 tain only a small part of the season's growth in miniature, while 

 those of the woody stem contain all of the nodes, internodes, and 

 leaves for the next year, laid down in advance. When such a bud 

 (a) begins to grow in the spring, the tissues of the elongating zone, 

 laid down the year before, produce the terminal twig of the sea- 

 son (c), while the meristem produces a new bud, shown in c, resem- 

 bling the mother bud (a) in structure and function. The tissues of 

 the elongating zone in a are already partially differentiated, but 



FIG. 40. Three diagrams showing method 

 of elongation of a lilac bud 



a, bud in winter condition, similar to Fig. 37, b ; 



b, elongation stage in spring, similar to Fig. 37, d; 



c, permanent condition, similar to Fig. 38. Meri- 

 stem, elongating, and permanent zones shaded as 



in Figs. 33, 34, and 39 



