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- 
Fic. 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. 
Fic. 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. 
