the leafy liverworts and mosses, the aerial portion
of most vascular plants can be understood as being made up
of more or less branched axes (stems) and appendages
(leaves). Together, these axes and their appendages
are referred to as shoots. However, in the
vascular plants two novelties warrant mention:
of the rigid, exoskeleton-like cellulose walls of their cells,
plants typically increase in size and complexity only in certain
places and, in these regions, only at certain stages of development.
Cell division is more or less restricted spatially to apical
(stem and root tip), intercalary, and internal areas that
are called meristems. These are responsible, respectively,
for morphogenesis (production of new axial cells and new appendages),
elongation, and increase in girth. Elongation also occurs
as a result of changes in cell size and shape that result
from changes in cell wall plasticity. Each of these regions
is also associated with differentiation of cell types, e.g.
xylem and phloem.
- a second, axial system that enables the plant to aggressively
obtain water and dissolved nutrients from its surroundings.
Mycorrhizal associations between plants and fungi further
increase the effectiveness of this system for obtaining
water and nutrients. Roots typically develop from the
root axis of the embryo. However, in many groups also
regularly arise in other locations as well, as well as
adventitiously, e.g. a result of injury.
tissue - specialized tissues for conducting water
(xylem) and photosynthate (phloem) differentiate in stems,
leaves, and roots. As a result, vascular plants are able
to achieve much greater variation in size, and can occupy
many more habitats.
of the morphological variation exhibited by plants,
together with increased understanding plant development has
made it clear that the typological description of vascular
plant organization used here has its limitations. Alternative
ways of understanding plant organization exist, and are reviewed
by by R.
modifications, other features
below for explanation of some terms used)
- prostrate, underground shoot for perennation and vegetative
spread, often dimorphic with horizontal portions (more
or less elongate internodes, reduced scale-like, non-photosynthetic
leaves) and erect, aerial portions (larger, photosynthetic
leaves). Many monocots (e.g. Medeola; also grasses
and sedges, ginger, among others) exhibit rhizomes.
- like a rhizome, but above ground, growing along the
surface of the substrate (e.g. Phragmites, Fragaria,
- shoot modified for perennation, the internodes extremely
short and the leaf bases swollen as storage organs (e.g.
- shoot modified for perennation, the internodes short
and the stem swollen as a storage organ (e.g. Arisaema,
dimorphism - production of two kinds of shoots,
usually differing in internode length (short shoots, long
shoots) and leaf morphology (e.g. Larix), and
often correlated with production of reproductive structures
indeterminate growth - growth that is programmed in
some way to cease after a certain time, or that continues
indefinitely. Almost all leaves exhibit determinate growth,
that is, they reach maturity and cease to grow. Stems
(of perennials), on the other hand, often continue growing
until they die, are destroyed, or the entire plant dies.
and sympodial growth - In monopodial growth, an axis
develops due to the activity of a single apical meristem.
With sympodial growth, an single axis develops from the
activity of a succession of apical meristems, each of
which in turn either is converted into a (terminal) inflorescence
or for some other reason ceases to grow. A monopodium
may thus exhibits indeterminate growth, whereas the growth
of successive, individual sympodia is determinate.
above: rhizomes of Medeola virginiana (Liliaceae)
(Click to view enlargement)
Stolons of Phragmites australis (Poaceae)
(click to view enlargement)
Bulbs of Allium tricoccum (Alliaceae)
(click to view enlargement)
Corm of Arisaema triphyllum (Araceae)
(click to view enlargement)
and internodes - stems can be thought of as being
made up of nodes, the points at which leaves are attached,
and internodes, the intervening, larger or smaller extent
of stem between these points.
scars and bundle scars - when leaves fall off,
a scar remains showing the area of the stem to which the
leaf was attached. Within the leaf scar it may be possible
to discern bundle scars, where the xylem and phloem leading
to the leaf (vascular bundles) were localized.
- these are the apical meristems of shoots, and are usually
enclosed by immature leaves or (more commonly, in regions
with markedly seasonal climates) modified leaves (bud
buds represent the growing point (apical meristem)
at the tip of a stem, enclosed by immature or modified
(lateral) buds are apical meristems, similarly enclosed,
that are formed in the axils of leaves.
also occur in other positions, sometimes regularly,
and otherwise as a result of wounding.
of twig segments - in regions with markedly seasonal
climates the elongation of stems is discontinuous, and
annual increments may be demarcated by the scars where
bud scales were attached and enclosed the apical meristem
during the season unfavorable for growth.
- how long leaves remain attached to the stem
leaves are shed regularly, usually at the end of the
(evergreen) leaves remain attached to the stem more
or less indefinitely, in any case for more than a
single growing season.
- parts of the leaf, and how they're put together
(S1, S2); usually paired structures associated with
the base of the leaf where it attaches to the stem
the leaf stalk
leaves can be distinguished from a branch with many
simple leaves if axillary buds can be found associated
with the point at which the rhachis attaches to the
stem, but not with the points at which the
leaflets are attached to the rhachis. Even this rule
of thumb breaks down in a small number of unusual
cases, suggesting that the distinction between leaves
and shoots cannot always be made with certainty.
- of leaves on the stem (phyllotaxy)
- gymnosperm examples
- of the leaf blade; a rich terminology, some of which
can be defined quantitatively, is available
and margination - of the leaf blade