|Figure 32-1 Stem tips, showing terminal buds
of Aesculus, the horse chednut, and
tripetala. The abundant leaf scars of Magnolia
seem to confirm that the leaves
into an umbrella-like circle..
A stem may be defined as something that bears leaves. But this
definition is too simplistic; it needs to be expanded so as to
answer some pertinent questions. From where does a stem
come? What part of the seed produces it? When one dissects a
seed, it is found to bear a seed coat or two and, within the seed coat, an
embryo. The embryo consists of cotyledons (the seed leaves, which are food
sources), a radicle (which grows downward upon germination to produce the
root), and a plumule (which grows upward to produce the stem and leaves).
The cotyledons are attached to the plumule. The seedling thus has two parts:
the epicotyl (that part above the cotyledons) and the hypocotyl (that part
below the cotyledons). Thus, a stem can be defined as that part of a plant
above the hypocotyl.
Gymnosperms are entirely woody, while both woody and herbaceous
forms occur in angiosperms. Angiosperms include both monocots and dicots.
These differences create a need for several descriptions of stem anatomy.