Structural axis of a vascular plant
This article is about the stem part of a plant. For the use of stem node in phylogenetics and cladistics, see
Phylogenetic nomenclature
.
A
stem
is on of two main structural axes of a
vascular plant
, the other being the
root
. It supports
leaves
,
flowers
and
fruits
, transports water and dissolved substances between the roots and the shoots in the
xylem
and
phloem
, photosynthesis takes place here, stores nutrients, and produces new living tissue.
[1]
The stem can also be called
halm
or
haulm
or
culms
.
The stem is normally divided into nodes and internodes:
[2]
- The nodes are the points of attachment for leaves and can hold one or more leaves. There are sometimes axillary
buds
between the stem and leaf which can grow into branches (with
leaves
,
conifer cones
, or
flowers
).
[2]
Adventitious roots
may also be produced from the nodes.
Vines
may produce
tendrils
from nodes.
- The internodes distance one node from another.
[2]
The term "
shoots
" is often confused with "stems"; "shoots" generally refers to new fresh plant growth, including both stems and other structures like leaves or flowers.
[2]
In most plants, stems are located above the soil surface, but some plants have
underground stems
.
Stems have several main functions:
[3]
- Support for and the elevation of leaves, flowers, and fruits. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits.
- Transport of fluids between the roots and the shoots in the xylem and phloem.
- Storage of nutrients.
- Production of new living tissue. The normal lifespan of plant cells is one to three years. Stems have cells called
meristems
that annually generate new living tissue.
- Photosynthesis.
Stems have two pipe-like tissues called
xylem
and
phloem
. The xylem tissue arises from the cell facing inside and transports water by the action of
transpiration pull
,
capillary action
, and
root pressure
. The phloem tissue arises from the cell facing outside and consists of sieve tubes and their companion cells. The function of phloem tissue is to distribute food from photosynthetic tissue to other tissues. The two tissues are separated by
cambium
, a tissue that divides to form xylem or phloem cells.
Specialized terms
[
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]
Stems are often specialized for storage, asexual reproduction, protection, or
photosynthesis
, including the following:
- Acaulescent
: Used to describe stems in plants that appear to be stemless. Actually these stems are just extremely short, the leaves appearing to rise directly out of the ground, e.g. some
Viola
species.
- Arborescent: Tree with woody stems normally with a single trunk.
- Axillary bud
: A bud which grows at the point of attachment of an older leaf with the stem. It potentially gives rise to a shoot.
- Branched
: Aerial stems are described as being branched or unbranched.
- Bud
: An embryonic shoot with immature stem tip.
- Bulb
: A short vertical underground stem with fleshy storage leaves attached, e.g.
onion
,
daffodil
, and
tulip
. Bulbs often function in reproduction by splitting to form new bulbs or producing small new bulbs termed bulblets. Bulbs are a combination of stem and leaves so may better be considered as leaves because the leaves make up the greater part.
- Caespitose: When stems grow in a tangled mass or clump or in low growing mats.
- Cladode (including
phylloclade
): A flattened stem that appears leaf-like and is specialized for photosynthesis,
[4]
e.g.
cactus
pads.
- Climbing
: Stems that cling or wrap around other plants or structures.
- Corm
: A short enlarged underground storage stem, e.g.
taro
,
crocus
,
gladiolus
.
- Decumbent: A stem that lies flat on the ground and turns upwards at the ends.
- Fruticose: Stems that grow shrublike with woody like habit.
- Herbaceous
: Non woody stems which die at the end of the growing season.
- Internode: An interval between two successive nodes. It possesses the ability to elongate, either from its base or from its extremity depending on the species.
- Node: A point of attachment of a leaf or a
twig
on the stem in seed plants. A node is a very small growth zone.
- Pedicel
: Stems that serve as the stalk of an individual flower in an
inflorescence
or
infrutescence
.
- Peduncle
: A stem that supports an
inflorescence
or a solitary flower.
- Prickle
: A sharpened extension of the stem's outer layers, e.g.
roses
.
- Pseudostem: A false stem made of the rolled bases of leaves, which may be 2 to 3 m (6 ft 7 in to 9 ft 10 in) tall, as in
banana
.
- Rhizome
: A horizontal underground stem that functions mainly in reproduction but also in storage, e.g. most
ferns
,
iris
.
- Runner
: A type of stolon, horizontally growing on top of the ground and rooting at the nodes, aids in reproduction. e.g.
garden strawberry
,
Chlorophytum comosum
.
- Scape
: A stem that holds flowers that comes out of the ground and has no normal leaves.
Hosta
,
lily
,
iris
,
garlic
.
- Stolon
: A horizontal stem that produces rooted plantlets at its nodes and ends, forming near the surface of the ground.
- Thorn
: A
modified stem
with a sharpened point.
- Tuber
: A swollen, underground storage stem adapted for storage and reproduction, e.g.
potato
.
- Woody
: Hard textured stems with secondary xylem.
- Sapwood: A woody stem, the layer of secondary phloem that surrounds the heartwood; usually active in fluid transport
Stem structure
[
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]
Stem usually consist of three tissues:
dermal tissue
,
ground tissue
, and
vascular tissue
.
[5]
Dermal tissue covers the outer surface of the stem and usually functions to protect the stem tissue, and control gas exchange.
[6]
The predominant cells of dermal tissue are epidermal cells.
[7]
The dermal tissue of aquatic plants stems may lack the waterproofing found in aerial stems.
Ground tissue usually consists mainly of parenchyma cells and fills in around the vascular tissue. It sometimes functions in photosynthesis. Most or all ground tissue may be lost in woody stems.
Vascular tissue provides long distance transport and structural support. The arrangement of the vascular tissues varies widely among plant species.
Dicot stems
[
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]
Dicot
stems with primary growth have
pith
in the center, with vascular bundles forming a distinct ring visible when the stem is viewed in cross section. The outside of the stem is covered with an epidermis, which is covered by a waterproof cuticle. The epidermis also may contain
stomata
for gas exchange and multicellular stem hairs called
trichomes
. A cortex consisting of
hypodermis
(collenchyma cells) and
endodermis
(starch containing cells) is present above the
pericycle
and vascular bundles.
Woody dicots and many nonwoody dicots have
secondary growth
originating from their lateral or secondary meristems: the
vascular cambium
and the
cork cambium
or phellogen. The vascular cambium forms between the xylem and phloem in the vascular bundles and connects to form a continuous cylinder. The vascular cambium cells divide to produce
secondary xylem
to the inside and
secondary phloem
to the outside. As the stem increases in diameter due to production of secondary xylem and secondary phloem, the cortex and epidermis are eventually destroyed. Before the cortex is destroyed, a cork cambium develops there. The cork cambium divides to produce waterproof cork cells externally and sometimes phelloderm cells internally. Those three tissues form the
periderm
, which replaces the epidermis in function. Areas of loosely packed cells in the periderm that function in gas exchange are called lenticels.
Secondary
xylem
is commercially important as wood. The seasonal variation in growth from the vascular cambium is what creates yearly tree rings in temperate climates. Tree rings are the basis of
dendrochronology
, which dates wooden objects and associated artifacts.
Dendroclimatology
is the use of tree rings as a record of past climates. The aerial stem of an adult
tree
is called a
trunk
. The dead, usually darker inner wood of a large diameter trunk is termed the
heartwood
and is the result of
tylosis
. The outer, living wood is termed the sapwood.
Monocot stems
[
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]
Vascular bundles are present throughout the
monocot
stem, although concentrated towards the outside. This differs from the dicot stem that has a ring of vascular bundles and often none in the center. The shoot apex in monocot stems is more elongated. Leaf sheathes grow up around it, protecting it. This is true to some extent of almost all monocots. Monocots rarely produce
secondary growth
and are therefore seldom woody, with
palms
and
bamboo
being notable exceptions. However, many monocot stems increase in diameter via anomalous secondary growth.
Gymnosperm stems
[
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]
All
gymnosperms
are woody plants. Their stems are similar in structure to woody dicots except that most gymnosperms produce only
tracheids
in their xylem, not the vessels found in dicots. Gymnosperm wood also often contains
resin
ducts. Woody dicots are called hardwoods, e.g.
oak
,
maple
and
walnut
. In contrast, softwoods are gymnosperms, such as
pine
,
spruce
and
fir
.
-
The trunk of this
redwood
tree is its stem.
Fern stems
[
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]
Most
ferns
have rhizomes with no vertical stem. The exception is
tree ferns
, which have vertical stems that can grow up to about 20 metres. The stem anatomy of ferns is more complicated than that of dicots because fern stems often have one or more
leaf gaps
in cross section. A leaf gap is where the vascular tissue branches off to a
frond
. In cross section, the vascular tissue does not form a complete cylinder where a leaf gap occurs. Fern stems may have
solenosteles
or
dictyosteles
or variations of them. Many fern stems have phloem tissue on both sides of the xylem in cross-section.
Relation to xenobiotics
[
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]
Foreign chemicals such as air pollutants,
[8]
herbicides and pesticides can damage stem structures.
Economic importance
[
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]
There are thousands of species whose stems have economic uses. Stems provide a few major staple crops such as potato and
taro
.
Sugarcane
stems are a major source of sugar.
Maple sugar
is obtained from trunks of
maple
trees.
Vegetables
from stems are
asparagus
,
bamboo shoots
, cactus pads or
nopalitos
,
kohlrabi
, and
water chestnut
. The spice,
cinnamon
is bark from a tree trunk.
Gum arabic
is an important food additive obtained from the trunks of
Acacia senegal
trees.
Chicle
, the main ingredient in
chewing gum
, is obtained from trunks of the chicle tree.
Medicines obtained from stems include
quinine
from the bark of
cinchona
trees,
camphor
distilled from wood of a tree in the same genus that provides
cinnamon
, and the muscle relaxant
curare
from the bark of tropical vines.
Wood is used in thousands of ways; it can be used to create
buildings
,
furniture
,
boats
,
airplanes
,
wagons
,
car
parts,
musical instruments
,
sports equipment
,
railroad ties
,
utility poles
, fence posts,
pilings
,
toothpicks
,
matches
,
plywood
,
coffins
,
shingles
,
barrel
staves,
toys
,
tool
handles,
picture frames
,
veneer
,
charcoal
and
firewood
.
Wood pulp
is widely used to make
paper
,
paperboard
,
cellulose
sponges,
cellophane
and some important
plastics
and
textiles
, such as
cellulose acetate
and
rayon
.
Bamboo
stems also have hundreds of uses, including in paper, buildings, furniture, boats, musical instruments,
fishing poles
,
water pipes
, plant stakes, and
scaffolding
. Trunks of palms and
tree ferns
are often used for building. Stems of
reed
are an important building material for use in
thatching
in some areas.
Tannins
used for tanning
leather
are obtained from the wood of certain trees, such as
quebracho
.
Cork
is obtained from the bark of the
cork oak
.
Rubber
is obtained from the trunks of
Hevea brasiliensis
.
Rattan
, used for furniture and baskets, is made from the stems of tropical vining palms.
Bast fibers
for textiles and rope are obtained from stems of plants like
flax
,
hemp
,
jute
and
ramie
. The earliest known paper was obtained from the stems of
papyrus
by the ancient Egyptians.
Amber
is
fossilized
sap from tree trunks; it is used for
jewelry
and may contain preserved animals. Resins from conifer wood are used to produce
turpentine
and
rosin
. Tree bark is often used as a
mulch
and in growing media for container plants. It also can become the natural
habitat
of
lichens
.
Some ornamental plants are grown mainly for their attractive stems, e.g.:
See also
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]
References
[
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]
Further reading
[
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]
External links
[
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]