186 



STEMS 



dormant period, primary meristems are usually protected by bud 

 scales, while, during their active period, they receive considerable 

 protection from the young leaves, which, although developing 

 laterally and behind the tips, 

 project forward and are usually 

 so crowded and folded together 

 that they hide the stem tips. 



Behind the stem tips the cells 

 formed from the primary meri- 

 stems begin to elongate and 

 modify into tissues and con- 

 tinue to do so until transformed 

 into the mature tissues of the 

 older parts of the stem. Stem 

 tissues differ: (1) in some im- 



FIG. 163. Lengthwise section 

 through the apical region of a stem 

 with two leaf stalks and the buds in 

 their axils included, showing the con- 

 nections of the vascular bundles of 

 leaves and of axillary buds or branches 

 with the vascular cylinder of the stem. 

 The vascular cylinder is represented 

 by shaded strands on each side of the 

 pith, the light area in the center of 

 the stem. Redrawn from Sargent. 



FIG. 164. Diagram of the vascu- 

 lar cylinder of the young stem of 

 Clematis viticella, showing by means 

 of dark lines the branching of the 

 vascular bundles at the nodes to sup- 

 ply the leaves and branches with bun- 

 dles. Modified from Nageli. 



portant ways according to whether the stem is monocotyledonous 

 or dicotyledonous; and (2) in minor ways according to whether 

 the stem is herbaceous or woody. Thus in trees the tissues of 

 the herbaceous tips differ some from those in the older regions 

 where corky bark and other woody features are developed. 



