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Modification of
Stems
The stem shows different modifications like root to perform
some special functions like synthesizing the food materials, conducting water
and mineral salts to the leaves, poliage to sunlight for photosynthesis etc., Depending
on their nature they are of 3 types
as follows
Ø Underground stem modification
Generally stems are present above the soil
(aerial) but in some plants they grow below the soil which are called as underground or subterranean or geophyllous
stems or stem modifications. They will store food
materials and the well protected from herbivorous animals, by performing these functions they are also known as multipurpose stem modifications.
These stems contains nodes, internodes, scale
leaves axillary and terminal buds. Based on their growth and the storage of
food materials they are of 4 types as follows:
·
Rhizome
ü It
is the thickest underground stem which grows horizontally below the soil
consisting of nodes and internodes.
ü It
is brown in colour and dorsiventral producing aerial branches and reproductive
organs on the dorsal side and adventitious roots on the ventral side.
ü The
nodes consists of scale leaves and branches arises from axillary buds (to store
the food materials and maintains the horizontal growth) and the aerial branches
of rhizome are called as scapes.
ü The
vertical rhizome of this is known as root stock.
Examples: Ginger, Turmeric
·
Corm
ü The
stem which grows vertically in the soil at a particular depth consisting of
nodes and internodes.
ü The
food materials are synthesized in aerial branches and stored by the stem. Hence
it becomes tuberous.
ü Leaves
are reduced to scale leaves and the axillary buds produced daughter corms and
it consists of some special roots are called as pull roots or contractile
roots.
Example:
Amorphophyllus, Colacasia
·
Stem
tuber
ü In
this, branches develops from the lower part of the stem and grows into the
soil, the apices of these branches stores food materials hence they will become
tuberous which are known as stem tubers and can grow at any depth.
ü This
is covered by a brown coloured layer periderm which bears many eyelike
structures represents the nodes.
ü Each
eye possesses a semi-lunar leaf scar which represents the position of scale
leaf and also helps in vegetative propagation.
ü The
vegetative propagation through eyes is called sprouting.
Example: Potato, Helianthus
·
Bulb
ü It
is a special type of underground stem which is reduce to biconvex dislike
structure with number of adventitious roots on its lower side and does not
stores the food.
ü The
leaves develop on the upper side of the disk and grow above the soil.
ü The
leaf bases stores the food and water hence they will be fleshy.
ü The
axillary buds produce daughter bulb and terminal bud is at the centre and
develops aerial shoots producing inflorescence.
Depending on arrangement of scale leaves they are of two
types as follows:
o Tunicated bulb: In this the leaf bases
of scale leaves overlap one above the other in a concentric circles and the
entire bulb is covered by dry, membranous scale leaves called as tunica.
Example: Onion
o Scaly bulb or naked bulb or imbricate bulb:
The scale leaves are fleshy which are loosely arranged are called as cloves. It
the leaf is covered by tunica and the bulb is not covered with that, called as
naked bulb. It the scale leaves are arranged in a group with shinning tunica,
called as imbricate bulb. If the stem and leaf bases stores the food materials
they appear in the form of solid bulbs.
Example:
Tuberose, cloves ,Lilium
Ø Sub-aerial stem modifications:
The stem which grows partly aerial and partly
below the soil are called as sub-aerial stems which are specialized for vegetative propagation. They are of four types as follows
·
Runners
ü In
this, the stem creeps on the soil and the rooted at every node.
ü When
the internodes break of nodes leads an independent life, such stems are known
as runners which help in vegetative propagation.
Examples: Hydrocotyle, Oxalis
·
Stolons
or walking stems
ü In
this the slender branches arises from the base of the stem which grows
obliquely downwards.
ü When
these branches touch the soil they produce adventitious roots, called as
stolons. It separated from the mother plant they leads an independent life.
Examples: Jasmine, Nerium
·
Suckers
ü In
this underground branches grows obliquely upwards from the axillary buds of
nodes present below the soil.
ü These
branches produce adventitious roots on the ventral surface, called as suckers.
If separated they leads an independent life.
Examples:Mentha,Chrysanthemum
·
Offsets
ü Stem
is reduced to a disc like structure and the leaves from this grows in rosette
manner.
ü The
axillary buds of these leave develop into short and the slender branches of one
Internodal length and grows horizontally above the water, called as offsets.
ü The
apex of each set bears number of leaves on the upper side and the adventitious
roots on its lower, if it breaks they leads an independent life.
Example:
Pistia, Lemna
Ø Aerial stem modifications
The stem which grows aerially consisting of
vegetative parts a floral parts of plants growing in different environmental
conditions and undergo modifications to perform various function are called as
aerial stem modifications. They are of 6
types are as follows
·
Tendrils
Some weak stemmed plants produce wiry, coiled
and sensitive structures for the purpose of climbing, called as tendrils. They
coil around the support and helps in climbing formed from the axillary or
terminal buds.
Examples: Passiflora, Vitis vinifera
·
Thorns
These
are hard, woody pointed structures which meant for protection develops either
from axillary bud or terminal bud.
Examples:
Duranta, Carissa
·
Hooks
These are woody, curved sensitive structure developed either
from axillary or terminal bud.
Examples: Hugonia, Artobotrys
·
Phylloclades
ü In
these plants leaves are modified into scale leaves or spines and man stem
branches modifies into green leaf like structures to perform photosynthesis
known as phylloclades or cladophylls.
ü Phylloclades
which contains only one internode known as cladode.
Examples: Asparagus, Opuntia, casuarina
·
Tuberous
stem
The aerial stem of the plants stores food
materials and become tuberous known as tuberous stem.
Examples: Brassica, Bulbophyllum
·
Bulbils
`The plants consisting of vegetative and
floral parts modified into condensed branches which stores food materials,
these modified buds are known as bulbils.
Examples: Diascorea, Oxalis
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