VEGETATIVE MORPHOLOGY OF FLOWERING PLANTS
Stems, roots, and leaves are the vegetative parts of vascular plants. Stems are the
basic organs, as they give rise to other plant organs, such as roots, leaves, and flowers.
Both aerial and subterranean stems are divided into nodes (the places where leaves
arise) and internodes (the regions between adjacent nodes). Stems may be simple or
variously branched, either at the base or along the length of the stem, and either
herbaceous (leaflike in texture; non-woody) or woody. The soft tissues found in stems
of herbaceous plants result from primary, not secondary, growth. Woody plants,
however, exhibit primary growth in their first year followed in subsequent years by
secondary stem growth, which involves the addition of vascular tissue each year around
the previous year's vascular tissue. The yearly increment of vascular tissue can be
recognized in many plants by annual rings. When evident, the annual rings can be
counted to determine the age of the plant.
herbaceous stem
woody stem
The stem structure of a plant is usually responsible for its overall manner of growth or
habit, which may be described as herbaceous, shrubby, or arborescent. Herbaceous
plants may live only one year (annuals) or persist for two (biennials) or more years
(perennials). The annual condition is one that permits a plant to complete its life cycle
in one growing season, thereby avoiding extreme environmental conditions, particularly
high temperatures and low rainfall. Suffrutescent plants are herbaceous perennials
with herbaceous above-ground parts that die back annually to a woody base. Other
herbaceous perennials exist with persistent above-ground stems. A shrubby plant is
woody, considerably branched, and usually less than ten meters tall, and an
arborescent plant is one that is treelike.
Using the following terms and illustrations, identify the stem types on plants you see on
campus. Note that the term 'rootstock' frequently has been used to include a rhizome or
a caudex. A caudex is a short, woody, persistent stem at or just beneath the surface of
the ground; its anatomy merges stem with root tissue and is often difficult to define
precisely. Subterranean stems obviously must be dug up before they can be identified.
Make sure you can interpret and recognize the following stem types and conditions
since these frequently are of great diagnostic value and are used in plant identification.
ABOVE-GROUND STEM TYPES AND CONDITIONS
Ascendent
Decumbent
Erect and
Caulescent
aerial stem horizontal to the ground
only near the base, being mostly
erect
aerial stem horizontal to the ground,
ascending only near the stem apex
upright aerial stem with the leaves
arising from all or some of the nodes on
the above-ground stems
Erect and
Acaulescent
(Scapose)
upright aerial stem (=scape) with
the leaves arising directly from an
underground stem structure
Procumbent
aerial stem horizontal to the ground
but not rooting at the nodes
Repent
aerial stem horizontal to the ground
and rooting at the nodes
Stolon
elongate above-ground stem
horizontal to the ground (may be
procumbent or repent)
Thorn
stiff, woody, modified stem with a
sharp point
UNDERGROUND STEM TYPES
Bulb
short and narrow underground stem
surrounded by several to many thick,
fleshy, nutritive, protective leaves
(common example: onion)
Corm
short, thick, and nutritive
underground stem surrounded by two
to several pairs of membranous,
protective leaves (common example:
gladiolus)
Rhizome
common underground stem; usually
long, slender, and creeping
Tuber
short, thick and nutritive
underground stem; usually a
shortened part of a rhizome (common
example: potato)
Roots anchor plants in soil, often store carbohydrates,
and serve as the principal organs for absorption of
water and nutrients. Primary roots are those derived
from the primary root of a seedling. These include a
fibrous root system in which the branches are all
about the same size, and a taproot system in which
the central root is larger than the branches. When
water-absorbing structures arise from a stem, they
form an adventitious root system. Most monocots
and seedless vascular plants (ferns and their allies)
and some dicots have a fibrous adventitious root
system. Many dicots and most gymnosperms have a
root system that is either fibrous or taproot.
Leaves are the primary photosynthetic organs of
plants, and are borne only at the nodes of a stem.
Leaves usually consist of a blade (lamina) and a
petiole (except when sessile), and may be either
simple or compound. If compound, the individual
units of the leaf are called leaflets, which are either
pinnately or palmately arranged. On either side of the
petiole or leaf base, there are often s t i p u l e s
subtending the leaf; these may be leaflike, or modified
as bristles or sheaths. In the axil, or angle between
each simple or compound leaf and stem, there is an
axillary bud that will develop into a shoot system
(branch) if not inhibited by plant hormones (auxins).
Pruning or removing the terminal parts of branches of
shrubs or trees usually is a good way to release the
hormonal inhibition of axillary buds located lower on
the branches, thus permitting new branches to
develop and resulting in a diffuse branching system.
Leaves that persist on a plant for several years or
growing seasons are called evergreen, while those
that fall after one growing season are deciduous.
Leaves vary in shape, margin, apex, base, and
vestiture (refers to covering of glands or hairs; when
surface is smooth, without hairs or glands, the
condition is called glabrous). Leaves also vary by
their arrangement on a stem (phyllotaxy). The common leaf arrangements are
alternate, opposite, whorled, and spiral. On the following pages, you will find examples
of the common features of leaves. Become acquainted with these terms. Use plants in
your own surroundings to study and identify leaf characteristics: duration (evergreen or
deciduous), leaf arrangement, leaf type (simple or compound), pattern of leaf venation,
shape, nature of apex and base, and margin.
Vegetative morphology worksheet. Describe the stem and leaf morphology of the
unknown plants on display in the lab.
Unknown
1
2
3
4
5
Leaf
Arrangement
Venation
pattern
Leaf type
Shape
Apex/base Margin