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Modifications of Stem
2013-04-05, 7:23 PM

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

Category: My files | Added by: anve
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