BOTANY: PLANT PARTS AND
Ch. 1, pp. 3 - 8
The parts of a plant can be divided into two groups,
sexual reproductive parts and vegetative parts. Sexual reproductive
parts are those involved in the production of seed. They include
flower buds, flowers, fruit, and seeds. The vegetative parts include
leaves, roots, leaf buds, and stems. Although the vegetative parts are
not directly involved in sexual reproduction, they are often used in
asexual or vegetative forms of reproduction, such as cuttings.
and Functions: stems | leaves |
buds | roots
| flowers |
fruit | seeds]
Cross-Section of a Stem
Stems are structures which support buds and leaves and
serve as conduits for carrying water, minerals, and sugars. The
three major internal parts of a stem are the xylem, phloem, and
cambium. The xylem and phloem are the major components of a plants
The vascular system transports food, water, and
minerals and offers support for the plant. Xylem vessels conduct
water and minerals, while phloem tubes conduct food. The vascular
systems of monocots and dicots differ. While both contain xylem
and phloem, they are arranged differently. In the stem of a
monocot, the xylem and phloem are paired into bundles; these
bundles are dispersed throughout the stem. But in the stem of a
dicot, the vascular system forms rings inside the stem. The ring
of phloem is near the bark or external cover of the stem and is a
component of the bark in mature stems. The xylem forms the inner
ring; it is the sapwood and heartwood in woody plants. The
difference in the vascular system of the two groups is of
practical interest to the horticulturist because certain
herbicides are specific to either monocots or dicots. An example
is 2, 4, -D, which only kills dicots.
The cambium is a meristem, which is a site of cell
division and active growth. It is located between the xylem and
phloem inside the bark of a stem and is the tissue responsible for
a stems increase in girth, as it produces both the xylem and
Stems may be long, with great distances between leaves
and buds (branches of trees, runners on strawberries), or
compressed, with short distances between buds or leaves (fruit
spurs, crowns of strawberry plants, dandelions). Stems can be
above the ground like most stems with which we are familiar, or
below the ground (potatoes, tulip bulbs). All stems must have buds
or leaves present to be classified as stem tissue.
An area of the stem where leaves are located is called
a node. Nodes are areas of great cellular activity and growth,
where auxiliary buds develop into leaves or flowers. The area
between nodes is called the internode.
|Parts of a Stem
The length of an internode may depend on many factors.
Decreasing fertility will decrease internode length. Internode
length varies with the season. Too little light will result in a
long internode causing a spindly stem. This situation is known as
stretch or etiolation. Growth produced early in the season has the
greatest internode length. Internode length decreases as the
growing season nears its end. Vigorously growing plants tend to
have greater internode lengths than less vigorous plants.
Internode length will vary with competition from surrounding stems
or developing fruit. If the energy for a stem has to be divided
between three or four stems, or if the energy is diverted into
fruit growth, internode length will be shortened.
Although typical stems are above-ground trunks and branches,
there are modified stems which can be found above ground and
below ground. The above-ground modified stems are crowns,
stolons, and spurs, and the below-ground stems are bulbs, corms,
rhizomes, and tubers.
A crown is a region of compressed stem tissue from
which new shoots are produced, generally found near the surface
of the soil. Crowns (strawberries, dandelions, African violets)
are compressed stems having leaves and flowers on short
A spur is a compressed fruiting branch. Spurs are
short, stubby, side stems that arise from the main stem and are
common on such fruit trees as pears, apples, and cherries, where
they may bear fruit. If severe pruning is done close to
fruit-bearing spurs, the spurs can revert to a long, nonfruiting
A stolon is a horizontal stem that is fleshy or
semi-woody and lies along the top of the ground. A runner is a
type of stolon. It is a specialized stem that grows on the soil
surface and forms a new plant at one or more of its nodes.
Strawberry runners are examples of stolons. Remember, all stems
have nodes and buds or leaves. The leaves on strawberry runners
are small but are located at the nodes which are easy to see. The
spider plant also has stolons.
A tuber is an enlarged portion of an
underground stem like potato tubers, tulip bulbs, and iris
rhizomes are underground stems that store food for the plant. The
tuber, like any other stem, has nodes that produce buds. The eyes
of a potato are actually the nodes on the stem. Each eye contains
a cluster of buds.
A rhizome is a specialized stem which grows
horizontally at or just below the soil surface. They act as a
storage organ and means of propagation in some plants and are
similar to stolons. Some rhizomes are compressed and fleshy such
as those of iris; they can also be slender with elongated
internodes such as bentgrass. Johnsongrass is a hated weed
principally because of the spreading capability of its rhizomes.
Tulips, lilies, daffodils, and onions are plants that
produce bulbs--shortened, compressed, underground stems
surrounded by fleshy scales (leaves) that envelop a central bud
located at the tip of the stem. If you cut through the center of a
tulip or daffodil bulb in November, you can see all the flower
parts in miniature within the bulb. Many bulbs require a period of
low-temperature exposure before they begin to send up the new
plant. Both the temperature and length of this treatment are of
critical importance to commercial growers who force bulbs for
Corms are not the same as bulbs. They have
shapes similar to bulbs, but do not contain fleshy scales. A corm
is a solid, swollen stem whose scales have been reduced to a dry,
Some plants produce a modified stem that is referred to
as a tuberous stem. Examples are tuberous begonia and cyclamen.
The stem is shortened, flattened, enlarged, and underground. Buds
and shoots arise from the crown and fibrous roots are found on the
bottom of the tuberous stem. In addition, some plants such as the
dahlia and the sweet potato produce an underground storage organ
called a tuberous root, which is often confused with bulbs and
tubers. However, these are roots, not stems, and have neither
nodes nor internodes. It may sometimes be difficult to distinguish
between roots and stems, but one sure way is to look for the
presence of nodes. Stems have nodes; roots do not.
Stems are commonly used for plant propagation.
Above-ground stems can be divided into sections that contain
internodes and nodes. They are utilized as cuttings and will
produce stems that are good propagative tissues. Rhizomes can be
divided into pieces. Bulbs form small bulblets at the base of the
parent bulb. Cormels are miniature corms that form under the
parent corm. Tubers can be cut into pieces containing eyes and
nodes. All of these will produce new plants.
Types of Stems
A shoot is a young stem with leaves present. A twig is a stem
which is less than one year old and has no leaves since it is
still in the winter-dormant stage. A branch is a stem which is
more than one year old and typically has lateral stems. A trunk is
a main stem of a woody plant. Most trees have a single trunk.
Trees are perennial woody plants, usually have one main
trunk, and are usually more than 12 feet tall at maturity. Shrubs
are perennial woody plants that may have one or several main
stems, and are usually less than 12 feet tall at maturity.
A vine is a plant which develops long, trailing stems
that grow along the ground unless they are supported by another
plant or structure. Some twining vines circle their support
clockwise while others circle counter clockwise. Climbing vines
are supported by aerial roots, slender tendrils which encircle the
supporting object, or tendrils with adhesive tips.
Texture and Growth of Stems
Woody stems contain relatively large amounts of hardened
xylem tissue in its core, and are typical of most tree fruits and
ornamental trees and shrubs.
A cane is a stem which has a relatively large
pith and usually lives only one or two years. Examples of plants
with canes include rose, grape, and blackberry.
Herbaceous or succulent stems contain only
small amounts of xylem tissue and usually live for only one
growing season. If the plant is perennial, it will develop new
shoots from the root.
Life Cycles of Plants
Plants are classified by the number of growing seasons required
to complete a life cycle. Annuals pass through their entire life
cycle from seed germination to seed production in one growing
season and then die.
Biennials are plants which start from seeds and produce
vegetative structures and food storage organs the first season.
During the first winter a hardy evergreen rosette of basal
leaves persists. During the second season flowers, fruit, and
seeds develop to complete the life cycle. The plant then dies.
Carrots, beets, cabbage, celery, and onions are biennial plants.
Hollyhock, Canterbury Bells, and Sweet William are biennials
which are commonly grown for their attractive flowers.
Plants which typically develop as biennials may, in some
cases, complete the cycle of growth from seed germination to
seed production in only one growing season. This situation
occurs when drought, variations in temperature or other climatic
conditions are experienced. These cause the plant to
physiologically pass through the equivalent of two growing
seasons, in a single growing season. This phenomenon is referred
to as bolting.
Perennial plants live for many years, and after reaching
maturity, typically produce flowers and seeds each year.
Perennials are classified as herbaceous if the top dies back to
the ground each winter and new stems grow from the roots each
spring. They are classified as woody if the top persists, as in
shrubs or trees.
Stems as Food
The edible portion of cultivated plants such as asparagus and
kohlrabi is an enlarged succulent stem. The edible parts of
broccoli are composed of stem tissue, flower buds, and a few small
leaves. The edible part of potato is a fleshy underground stem
called a tuber. Although the name suggests otherwise, the edible
part of the cauliflower is proliferated stem tissue.