Plant stems function in various capacities, primarily in nutrient transport and
physical support. The vascular system of plants, complete with xylem
and phloem, fills both purposes. Stems, along with roots, also store
food for the plant. Pith, a tissue that lies in the center of the stem (or,
as it is called in trees, the trunk), is responsible for some nutrient
storage as well.
Transport
The vascular system in the stem is continuous with that of the roots, providing
an easy route for water and nutrients to flow throughout the plant body. The
organization of vascular tissues is different for various types of plants--from
the scattered vascular bundles (containing both xylem and phloem) of
monocots to the more orderly ring formation found in dicots. This
difference in the organization of the vascular tissues has implications for the
way the plant grows. (For a general discussion of monocots and dicots, see
Plant Classification,
Monocots
and Plant Classification,
Dicots.)
Monocot Stems
In monocots, xylem and phloem are organized in vascular bundles scattered
throughout the stem. As the plant grows, monocot stems generate new vascular
bundles for the new tissue. Monocot stems in general possess a simpler
arrangement than that found in dicots; the main elements of the stem are merely
the vascular bundles and the pith (used for nutrient storage) that surrounds
them.
Dicot Stems
The vascular system found in dicots is somewhat more complex than that found in
monocots. In the dicot stem, the vascular bundles are arranged in a ring, with
pith concentrated at the core of the stem, rather than being scattered
throughout the plant interior. In each vascular bundle, the xylem and phloem
are separated by a substance called vascular cambium. As the plant grows,
existing bundles grow larger (rather than new vascular bundles being generated,
as in monocots). The vascular cambium operates by producing new xylem and
phloem cells, which in turn pushes the old cells outward and forces the bundle
to grow.
Within the class of plants known as dicots, herbaceous dicots and woody
dicots have different arrangements of vascular tissues. In herbaceous dicots
(plants, mostly annuals, with soft, non-woody stems), vascular tissue remains in
discrete bundles even at maturity. In contrast, when mature woody dicots
(plants, mostly perennials, with woody stems) reach maturity, the vascular
bundles join together to form continuous rings around the interior of the stem.
In addition, woody dicots develop a lateral system of nutrient transport in
which vessels called rays extend horizontally from the phloem to the pith,
carrying nutrients to the pith for storage or to the phloem for dispersal.
Figure %: Stem Cross-Sections
Support
In addition to distributing nutrients, vascular tissues also provide structural
support. In fact, the material commonly known as "wood" is actually xylem.
After a time, the xylem at the center of older trees (woody dicots) ceases to
function in transport and takes on a purely supportive role; this nonfunctioning
xylem is called heartwood. The newer xylem closer to the surface remains
active in the vascular system, and is called sapwood.
