Stems
are a part of the shoot system of a plant. They may range in length from a few
millimeters to hundreds of meters. They also vary in diameter, depending on the
plant type.
Stems
are usually above ground, although the stems of some plants, such as the potato,
also grow underground. Stems may be herbaceous (soft) or woody in nature.
Their
main function is to provide support to the plant, holding leaves, flowers, and
buds; in some cases, stems also store food for the plant. A stem may be
unbranched, like that of a palm tree, or it may be highly branched, like that
of a magnolia tree.
The
stem of the plant connects the roots to the leaves, helping to transport
absorbed water and minerals to different parts of the plant. The stem also
helps to transport the products of photosynthesis (sugars) from the leaves to
the rest of the plant.
Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes. Nodes are points of attachment for leaves, aerial roots, and flowers.
The stem region between two nodes is called an internode. The
stalk that extends from the stem to the base of the leaf is the petiole.
An
axillary bud is usually found in the axil (the area between the base of a leaf
and the stem) where it can give rise to a branch or a flower. The apex (tip) of
the shoot contains the apical meristem within the apical bud.
Parts of a stem: Leaves are attached to the plant stem at areas
called nodes. An internode is the stem region between two nodes. The petiole is
the stalk connecting the leaf to the stem. The leaves just above the nodes
arise from axillary buds.
Stem is also,
in botany, the plant axis that bears buds and
shoots with leaves and, at its basal end, roots.
The stem conducts water, minerals, and food to other parts of the plant; it may
also store food, and green stems them produce food.
In most plants the stem is
the major vertical shoot, in some it is inconspicuous, and in others it is
modified and resembles other plant parts (e.g., underground stems may look like
roots).
Read On: Flowering Plants: Structure and Characteristics
Functions of Stems
Stem,
in botany, the plant axis that bears buds and
shoots with leaves and, at its basal end, roots. The stem
conducts water, minerals, and food to other parts of the plant; it may also
store food, and green stems them produce food.
In most plants the stem is the major
vertical shoot, in some it is inconspicuous, and in others it is modified and
resembles other plant parts, for example underground stems may look like roots.
The primary functions of the stem are to support the leaves; to conduct water and minerals to the leaves, where they can be converted into usable products by photosynthesis; and to transport these products from the leaves to other parts of the plant, including the roots.
The stem conducts water and nutrient
minerals from their site of absorption in the roots to the leaves by means of
certain vascular tissues in the xylem.
The movement of synthesized foods
from the leaves to other plant organs occurs chiefly through other vascular
tissues in the stem called phloem.
Food and water are also frequently stored in
the stem. Examples of food-storing stems include such specialized forms as tubers, rhizomes,
and corms and the woody stems of trees and shrubs.
Water storage is developed to a high degree
in the stems of cacti,
and all green stems are capable of photosynthesis.
In simple term, the following
are the functions of stems:
I. They hold the leaves in the best position for light which is
used in photosynthesis
2. They hold the flowers and fruits in the best position for
pollination and dispersal respectively
3. They conduct water and mineral salts from the roots to the
leaves and manufactured food from the leaves to the roots, growing regions and
storage organs of the plant.
4. Some stems are modified for special functions such as climbing,
protection, food storage, water storage and vegetable reproduction
5. Young parts of stems contain chlorophyll; hence they take part
in photosynthesis.
Types of Stem
There is a variety of stem structures adapted to perform diverse
functions. They may be aerial or underground. Aerial stems may be erect, rigid
and strong, holding themselves in an upright position; while there are some too
weak to support themselves in such a position. They either trail the ground or
climb neighbouring plants and other objects.
Some stems remain permanently underground and from there
periodically give off aerial shoots under favourable conditions; such stems are
for food storage and perennation.
There are three types of
stem: Underground stem, Aerial stem and Sub- aerial stem.
1. Underground stem: Stems
of some plants remain in the ground and serve the function of perennation and
storage of food. They produce aerial shoots annually. They resemble roots
superficially but are distinguishable by the presence of scale leaves and buds
at nodes. Such stem also act as a means of vegetative propagation. The modified
underground stems are the following:
(ii) Tuber: The underground stem
becomes enlarged at the growing tips by the accumulation of stored food,
commonly starch, tubers are produced e.g. Potato. The eyes of potato are nodes
at each of which 1-3 buds are produced in the axils of small scaly like leaves.
(iii) Bulb: Bulb is a short
underground stem with fleshy leaf base called scales. Stem is very much reduced
and becomes disc like. The discoid stem in convex or conical in shape and bears
highly compressed internodes. These node bear fleshy scales.
On the upper side, disc bears terminal bud
surrounded by number of leaves. The axillary buds are present between the axis
of leaves. The adventitious roots are borne on the lower side of the disc. E
.g. Onion.
(iv) Corm:Corm is short, thick
and un-branched underground stem with stored food material. It grows vertically
and covered by thin sheathing leaf bases of dead leaves called scales. The corm
bears buds at their nodes. These buds are responsible for giving off
adventitious roots. Corm serves the functions of food storage, vegetative
propagation and perennation. Corm is more or less rounded in shape or often
somewhat flattened from top to bottom, e.g. Colocasia.
2. Sub – aerial stems: Lower buds of the stem in some plants grow out into short, lateral branches. These are named according to their origin, nature and mode of reproduction:
(i) Runner: It
grows prostrate in all directions above the soil level. Nodes bear scale
leaves. It has a creeping stem with long internodes. On the lower sides, nodes
bear adventitious roots. Runner develops from the axils of lower leaves of
aerial stem which sends slender horizontal branches in the form of runners.
When older parts of plant die, the branches separate from parent plant and form
independent plants e.g. Doob grass.
(ii) Stolon: It
is a slender lateral branch which appears from the lower part of main axis.
This lateral branch grows aerially for some distance and becomes arched and
finally touches the ground to give rise to new shoot with the help of its
terminal bud. It also bears roots to get fixed with the soil e.g. Jasmine.
(iii) Offset: Offset
is more shorter and thicker. It is usually found in aquatic plants like water
hyacinth and Pistia. It bears a cluster of leaves near the water or ground
level and gives adventitious roots inside water or ground from all nodes, e.g.
Pistia.
(iv) Sucker: Like
the stolon the sucker is also a lateral branch but it grows obliquely upwards
and gives rise to a new plants e.g. Mentha.
3. Aerial stems: These
modified aerial stems perform unusual functions. Different forms of these stems
are the following:
(i) Stem tendril: It
is a leafless, spirally coiled branch formed in some climbers and helps them in
climbing neighbouring objects; they may be modification of axillary bud, e.g.
Passiflora.
(ii) Stem thorn: Stem
thorn is a hard, straight and pointed structure; it is a defensive organ; also
helps in climbing; originates from axillary or terminal bud, e.g. Duranta.
(iii) Pylloclade: It
is a green, flattended or cylindrical stem which takes the form and function of
leaf. It contains chlorophyll and is responsible for carrying on
photosynthesis. It bears succession of nodes and intemodes at long or short
intervals. Phylloclades are found in xerophytic plants where the leaves either
grow feebly or fall off early or modified into spines e.g. Opuntia,
(iv) Cladode: Phylloclade
with one or two internodes is called cladode e.g. Asparagus. In Asparagus
cladodes are needle-like, slightly flattened green structures which appear in
cluster in the axil of a scaly leaf. Main stem bears leaf spines at its nodes.
A scale leaf is found just above the spine. Every branch on main stem bears
only scale leaves. In the axil of scale leaves cluster of cladodes appear
(v) Bulbil: Bulbil is the
modification of vegetative or floral bud. It is swollen due to storage of food.
It can function as an organ of vegetative propagation e.g. Dioscorea.
The Internal Anatomy of the Stem
A
transverse section (T.S) of a dicot stem under the low power or electronic
microscope shows three major regions- the epidermis (on the outside), the
cortex (the middle) and the endodermis (on the inside).
The cortex is further
sub-divided into collenchyma and parenchyma. A ring
of vascular bundles is arranged inside the endodermis or starch sheath.
The vascular bundle consists of
phloem (outside), a layer of cambium (middle) and xylem tissue (inside). In
monocots however, the T.S. of the stem shows that vascular bundles are
scattered without cambium, hence, no increase in thickness in monocots. Also there
is no central pith and the cortex is thin.
Structure of a Stem
The stem divides into
nodes and internodes. The nodes give rise to the leaves and hold the buds which
grow into branches. The internodes separate two nodes.
Internally, it
contains three basic types of tissues: Dermal tissue, Ground tissue, and Vascular tissue
all of which are made of simple cells.
1. Epidermis: The epidermis is a
single layer of cells that make up the external tissue of the stem called
dermal tissue. This tissue covers the stem and protects the underlying tissue.
Woody plants have an extra layer of protection on top of the epidermis known as
bark. In some cases, the bears’ multi-cellular hairs and a few stomata.
2. Ground
tissue divides into two- the central portion is known
as the pith and the cortex which lies between the vascular tissue and the
epidermis.
The cortex can be further divided into three
layers:
a.
Hypodermis: It is the outermost layer of
the cortex. It is formed of 4 to 5 cell thick layer of collenchymatous cells.
These cells are living and contain chloroplasts.
b.
General cortex: Lies below the
hypodermis. It consists of thin-walled parenchymatous cells with intercellular
spaces. Some of the cells have chloroplasts and are known as chlorenchyma.
c.
Endodermis: The innermost layer
of the cortex. It is made up of a single row of compact barrel-shaped cells
without intercellular spaces. The cells of endodermis store starch grains and
so they are known as the starch sheath. Casparian strips are distinctly visible
in endodermal cells.
d. The vascular tissue of the stem consists of the complex tissues xylem and phloem which carry water and nutrients up and down the length of the stem and are arranged in distinct strands called vascular bundles. Cambium is a strip of thin-walled cells that lie between the xylem and phloem in dicot plants. Cambium is made up of merismatic cells and is responsible for secondary growth. It is absent in monocots.
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