Transcript Modified stem

Modification of the stem
Some of the most important types of modifications of
stem are as follows:
• Underground modifications of Stem
• Sub aerial modifications of Stem
• Aerial modifications of Stem.
Underground modifications of the stem
Many plants produce underground stems for perennation
and food storage. They produce aerial shoots annually.
Such stems resemble roots superficially but can be
distinguishable from roots by presence of following
features:
• Presence of nodes and internodes
• Presence of leaf scales, buds and adventitious roots at
the nodes.
• Internal structure that resembles that of aerial stem
and not of root.
Rhizome
Rhizome
It is fleshy, non-green underground stem with distinct
nodes and internodes.
• Nodes bear dry leaf scales with axillary buds.
• Terminal buds are present.
• Adventitious roots arise from the lower side.
• A rhizome that grow obliquely is a root stock
rhizome (e.g. Banana etc.) and when it grows
horizontally it is a straggling rhizome (e.g. Ginger).
MODIFIED STEM
Rhizome
Horizontal stem,
usually thick, that
grows partially or
entirely underground
(iris, canna,
sansevieria)
MODIFIED STEM
Bulb
A specialized storage organ (usually
underground) composed of a
compressed stem enclosed by fleshy or
papery leaves or leaf bases.
Middle
A concentric or tunicate stem bulb
enclosed by concentric or sheathing
leaves or leaf bases (onion, garlic,
amaryllis, tulip)
Lower. A scale or non-tunicate bulb stem enclosed by separate, thick
(usually fleshy) scale-like leaf bases
(lily)
Underground stem: bulb
Bulb
A highly condensed, discoid stem with its upper surface a
terminal bud and with many, fleshy leaf scale.
Characterised by:
* A cluster of adventitious roots that arise from the base
• Tunicated or scaly (covered by a sheath of dry membranous
scale leaves, e.g. onion and garlic
• In case of garlic, the bulb consists of an aggregate of bulblets
or cloves, each covered by its individual tunic.
• A scaly bulb is without a tunic, e.g., Lily.
MODIFIED STEM
Corm
Short, thickened,
underground stem
with few nodes and
short internodes,
usually enclosed by
dry, scale-like leaves
(gladiolus, crocus,
taro)
Corm
A condensed form of rhizome growing in vertical
direction.
• More or less spherical with a flattered base
• Has distinct circular nodes and internodes
• Nodes bear leaf scales and axillary buds
• Adventitious roots arise either from its base or all
over the body (e.g. Colocasia, Corcus)
MODIFIED STEM
Tuber
Large, underground storage
organ formed by thickening
or swelling of the sub apical
portion of an underground
stem
(caladium, Irish potato,
Jerusalem artichoke)
Stem tuber
• A swollen tip of an underground lateral stem (stolon)
• Covered by a corry skin with a number of small
depressions called “eyes”
• Each eye represents a node, bearing one or more
buds subtended by a leaf scar (= scale leaf)
• A big scar at one end (heel end) of a potato marks its
attachment to the stolon
• Adventitious roots are usually absent (e.g. potato).
Sub aerial modifications of stem
• In sub aerial modifications, the stem is partly aerial
and partly underground.
• Short aerial branches and adventitious roots develop
at the nodes.
• Detachment of entire branch or a node can develop
into a new plant.
• The plants are commonly known as creepers and
their sub aerial stem modifications are meant for
vegetative propagation.
Runner
• A runner is a creeping stem with long internodes,
that run horizontally on soil surface.
• Nodes bear axillary buds, scale leaves and
adventitious roots.
• Runner arises from an axillary bud.
• Runners break off and grow into individual plants,
and thus help in vegetative propagation (e.g.
Cynodon, Oxalis)
Runner
• 2. Sucker:
• It arises from the basal, underground part of
the main stem. It grows horizontally for a
distance under the soil and then emerges
obliquely upwards. It develops a leafy shoot
and adventitious roots before separating from
the mother plant. The common suckerbearing plants are Chrysanthemum (Or.
Sebati), Musa (banana), Mentha (Or podina),
strawberry, pineapple etc.
Stolon
• A weak lateral branch that arises from the base of
main stem
• After growing aerially for some time, it bends
downwards to touch the ground, where its terminal
bud gives rise to a new shoot and adventitious roots
(e.g. Jasmine).
Stolon
Offset
• A short runner with one internodes long
• Originates from leaf axil, grows as a short
horizontal branch
• Produces a rosette of leaves above and
adventitious roots below (generally found in
aquatic plants like Pistia (water lettuce),
Eichhornia (water hyacinth)
Aerial modifications of stem
• Aerial stem or buds may get modified to
perform such special functions as climbing,
protection, food storage, vegetative
propagation, etc.
Stem tendrils
• Stem or its branches get modified into green, thread
like leafless structures called tendrils which are
meant for climbing.
• Tendrils may be branched or un-branched.
• A scale leaf is always present at the point of
branching of the tendril.
Stem tendril
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•
•
•
Stem tendrils are of four types:
Axillary tendrils, e.g. Passiflora
Extra-axillary tendrils, e.g. Cucurbita
Leaf-opposed or apical bud tendrils, e.g.
grapevine (Vitis)
• Floral bud or inflorescence tendrils, e.g.
balloon vine
Thorns
Straight, pointed, hard or woody structures
which sometimes bear leaves, flowers or even
may be branched (found in Citrus, Carrissa,
and are for defense or climbing as in
Bougainvillea).
Thorns
Phylloclades
• These are fleshy, green flattened or cylindrical
branches of unlimited growth.
• The leaves are modified into spines or scales to check
transpiration (Opuntia, Euphorbia, Casuarina)
Cladodes and Cladophylls
Cladodes or Cladophylls
• These are green cylindrical or flattened leaflike branch of limited growth (e.g. Asparagus,
the cladodes are one internode long and in
Ruscus the cladodes are two internode long).
• They assist in photosynthesis.
MODIFIED STEM
Thorn
A stem modified into
a short (usually),
sharp point (e.g.
Rosea)
MODIFIED STEM
Spur
A compressed stem
with short internodes,
usually bearing leaves,
flowers and/or fruit.
Many fruit trees bear
flowers and fruits on
spur shoots (e.g. apple,
pear )
MODIFIED STEM
Cladophyll
A flattened stem,
specialized for
photosynthesis (e.g.
Cactus)
MODIFIED STEM
Pseudobulb
Thickened bulb-like
leaf (or stem) base
that resembles a
bulb
(orchid)
MODIFIED STEM
Stolon
Horizontal stem,
usually thin, that
grows above ground,
and usually forms
roots and plantlets
at nodes that
contact the ground
(grasses)
MODIFIED STEM
Runner
Slender stem with
elongated internodes
(Strawberry, Spider
plant)
The illustration is
that of a spider plant
(Chlorophytum
comosum)
This stem does
photosynthesis, stores
water, but also
produces a defense
chemical: mescaline…a
hallucinogen.
Lophophora wiliamsiipeyote