THE STEM
The stem is the ascending part of the axis bearing branches,
leaves, flowers and fruits. It develops from the plumule of
the embryo of a germinating seed.
It shows distinction into nodes and internodes, where node
is the region from where leaves are born and internodes are
the region between two nodes. It bears bud, which may be
terminal or axillary.
A bud can be defined as a condensed embryonic shoot that
has a growing point surrounded by closely packed
immature leaves. When bud grows, the internodes become
longer and the leaves spread out, resulting in the formation
of a young shoot.
• The largest axillary bud is cabbage.
• Bamboo is considered to the tallest herb, tallest shrub
arborescent grass. Bamboos are called culms, after the
nature of their stems.
Forms of Stem
Stem may be aerial, subaerial or underground. In most
plants, stems grow above the soil. These are aerial stems.
aerial stems of some plants trail or creep on the ground. They
are called subaerial stems. In some plants, the stems grow in
the soil and are called underground stems.
i. Aerial Stems
The aerial stems have two forms, i.e. reduced stem and
erect stem.
(a) Reduced Stems It is reduced to a small disc.
nodes and internodes are not distinguished’
e.g., carrot, radish, turnip, etc. In some aquatic
plants, the reduced discoid stem is green
flattened to float on the surface of water. It does not
bear leaves, e.g. Lemna, Wolffia, Spirodela.
underground structures also a reduced, non-green
stem is found, e.g., garlic, onion and lily.
(b) Erect Stems These stems are strong enough to renal
erect or upright without any external support.
ii. Subaerial Stems
In subaerial stems, some part lives underground,
the remaining part of the stem is aerial.
The subaerial stems are also divided into two forms
(a) Upright Weak Stems These stems are weak
climb up -a support to expose their foliage and
reproductive organs.
These are of two types; twiners and climbers
• Twiners These are long, slender and very
sensitive and coil around an upright support on
coming in contact, e.g., Dolichos lablab (bean)
Clitoria (butterfly pea), Cuscuta.
• Climbers These have weak and flexible stem
which climb up a support with the help of certain
clasping or clinging structures, e.g. Bougainvillea.
(b) Prostate Weak Stems These weak stems take support
of the ground for spreading as growth occurs.
They are of following four types
• Trailers (creepers) They trail along the surface
and do not climb up.
• Runners These are subaerial weak stems
grow horizontally along the soil surface,
Cynadon (lawn grass), Centella (brahmi
Oxalis, etc.
• Stolons These subaerial weak stems are
horizontal or branched runners with long
internodes which can pass over small obstacles.
Stolons also propagate vegetatively like runners,
e.g., Fragaria verica (strawberry), Jasminum
(jasmine), Mentha piperita (peppermint).
Offsets These weak stems are one internode
long, stout, slender and run horizontally and
terminate in a bud at a short distance that
develops into adventitious roots, e.g., Pistia
(water lettuce), Eichhornia (water hyacinth), etc.
iii. Underground Stems
The stems of some plants lie below the soil surface. They are
non-green, store food as means of perennation and
vegetative propagation.
They are of following types
(a) Rhizome It is a prostrate thick stem growing
horizontally beneath the soil surface. It has distinct
nodes and internodes. The nodes bear small scale
leaves with buds in their axils, e.g., Zingiber
officinale, (ginger) Curcuma domestica (turmeric) and
banana.
(b) Suckers These are non-green slender stems that grow
horizontally in the soil and ultimately come out to
form a new aerial shoot. Each sucker contains one or
more nodes with scale leaves and axillary buds,
e.g., Mentha (podina), Chrysanthemum (guldaudi).
(c) Corm It is a swollen condensed form of
underground stem which grows in the vertical
direction in the soil. It stores a large amount of food,
e.g., Amorphophallus, Colocasia (arabi).
(d) Tuber It is a swollen terminal end of underground
stem branches. Each tuber has many notches on the
surface called eyes or buds. These are actually
reduced form of internodes which can grow into new
plants, e.g., Solanum tuberosum (potato).
(e) Bulb It is a highly reduced disc-like stem. It bears a
large number of fibrous adventitious roots at its base.
Leaf bases form bulblets. The bulblets grow into new
plants, e.g., Allium cepa (onion), Allium sativum
(garlic).
Branching Pattern of Stem
The stems may be branched or unbranched. Branching in
stems may be dichotomous and lateral.
(i) The dichotomous branching occurs by the division
of the apical growing point or bud into two equal
parts in a forked manner. It occurs in lower
plants—cryptogams (non-flowering plants), higher
plants—Hyphaene (palm), Canscora, Pandanus
tectorius (screw pine), etc.
(ii) The lateral branching occurs from the axillary buds
of the nodes, e.g., Pinus, grapevine, etc.
Functions of Stem
Stems perform various primary and secondary functions.
Primary Functions
(i) It bears leaves, fruits, flowers and seeds in position.
(ii) It conducts water and minerals to roots, leaves,
flowers, fruits, etc.
(iii) It holds flower in suitable position, so that
pollination and fertilistaion can take place.
Secondary Functions
(i) Many stems store food as reserve food materials.
(ii) Some stems also help in photosynthesis and
vegetative propagation.
(iii) The underground stems help in perennation.
(iv) Stem branches provide support to its various parts.
Modifications of Stem
The various forms of aerial stem modification are following
i. Stem Tendrils
These are thin, long and sensitive structures which can coil
around a support.
Tendrils can be of following types on the basis of their
origin
(a) Axillary Arise from axiallary buds, e.g., Passiflora
(passion flower).
(b) Extra axillary Develop near the axillary bud,
e.g., Luffa, Cucurbita (pumpkin), etc.
(c) Apical bud These are modified to form tendrils,
e.g., Vitis vinifera.
(d) Floral bud These are modified to form tendrils,
e.g., Antigonon.
ii. Stem Thorns
The stem thorns are stiff, woody, sharp and pointed. They
develop from axillary bud. They protect the plants from
browsing animals, e.g., Citrus, Duranta, Bougainvillea,
pomegranate, etc.
iii. Prickles
These are modified stems and act as climbing organs. They
protect the plants from grazing animals and also help in
climbing in some cases, e.g., Argemone maxicana (prickly
poppy), Rosa indica (rose), Bombax malabarica (sembal),
etc.
iv. Phylloclade
These are green, flattened structures bearing several nodes
and internodes. The true leaves are reduced to spines or
scales. They show unlimited growth. Some phylloclades
also store food and water. The phylloclades are examples
of some xerophytic plants, e.g. Opuntia (nagaphani),
Casuarina, Euphorbia.
v. Cladodes (Cladophylls)
They are green photosynthetic stems generally one inter
node long. These develop by the modification of only
stem branches of limited growth and are green
(photosynthetic). The true leaves of the plant are reduced
to scales or spines, e.g., Ruscus, Asparagus.
vi. Bulbils
These are modified vegetative or floral buds arising in the
axil of scale or foliage leaves. The bulbil helps in vegetative
propagation, e.g., Lilium, Agave, Dioscorea (wild yam),
Oxalis, etc.