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Stem
2013-04-05, 7:22 PM

STEM

The system which grows aerially in a plant is known as shoot system which develops from the plumule of the embryo and the axis of this is known as stem.

The branches are developed on the stem either horizontally or obliquely and the appendages like leaves, bud  and flowers are developed on the stem as well as on the branches.

Ø  Characters

·         The stem grows always in +ve phototropic and the –ve geotropic manner.

·         The stem always grows vertically which is maintained by the apical bud and it is divided into nodes and internodes, leaves are developed on the stem at nodes.

·         The axillary buds are developed in the axils (angle between leafs and stem) which produces branches.

·         Colour – young stems (green), woody stems – (brown).

·         Generally stem is in the shape of cylindrical but it is triangular in bilberry, quadrangular in tulasi and ridged in pumpkin.

 

Ø  Functions

·         The stem helps in exposing the foliage to sunlight for efficient photosynthesis.

·          The stem conducts water and mineral salts absorbed by the roots to the leaves and also synthesis the food material.

Ø  Types

Based on the morphology these are classified into three types. They are as follows:

·         Herbs

The plants without having woody nature which can survive for about few years. According to the life span they are of four types as follows

ü  Annuals: survive for one year. Example: Rice

ü  Biennials: survive for two years. Example: radish

ü  Perennials: survive for more than two years. Example: Day lilies.

ü  Ephemerals: survive for few weeks. Example: spring beauties.

·         Shrubs

The plants which have woody nature but no trunk which develops strong branches at the ground level and gives bushy appearance.

Example: Rose

·         Trees

The plants that shows large wood grows away from the soil at a greater height on stem. Depending on development of branches they are 3 types as follows:

§  Deliquescent trees

After producing some branches stem stops its growth then axillary buds produce many branches and the main stem looks like umbrella.

Example: Banyan tree.

§  Excurrent trees 

Apical bud grows indefinetly and the main stem grows vertically in racemose manner, all the branches formed from the axillary buds are arranged aropetally to make the stem conical.

Example: Eucalyptus

§  Caudex trees

Main stem remains unbranched, long and short, leaves are arranged at the apex of the stem.

Example: Coconut tree.

·         Weak stemmed plants: the plants which grow with weak stem either horizontally or vertically with the help of support. They are of 3 types namely

*      Erect stemmed plants

*      Prostrate plants

*      Climbers

*      Erect stemmed plants or Culm

Stems are differentiated into nodes and internodes and the stem looks like a jointed one which is called as Culm.

ü  Internodes: Area of a plant stem between two nodes.

                      Example: sugarcane.

ü  Nodes: Area of a plant stem from which leaves grows.

*      Prostrate plants or trailors

Plants are fixed in soil at one place which is long, slender stems and branches present along the surface of the soil in many directions. They are of two types as follows

·         Decumbent plants: The plants which are fixed in soil at one place with the help of roots. They consist of vegetative parts which grow horizontally and reproductive parts which grow vertically.

Example: Tridax

·         Procumbent plants: The part of the stem consists of vegetative and reproductive parts which spread out on the ground.

Example: Tribulus

·         Climbers: In this plant grows verticvally with the help of support. They are of 5 types:

ü  Twinners

ü  Tendrils

ü  Root climbers

ü  Stragglers

ü  Lianes

§  Twinners: In some weak stemmed plants the stem twines around the support and grows up which are known as twinners. They coil around the support loosely at first and then tights it, depending on the coiling they are of 2 types as follows

ü  Dextrose: Coils in clockwise direction. Example: Diascora

ü  Sinistrose: Coils in anti-clockwise direction. Example: Dolichos

§  Tendrils: The weak stemmed plants produce sensitive and specialized climbing organs which are called as tendrils. They coil around the support and helps in climbing.

Example: Passifora

§  Root climbers: In some plants the weak stems produces climbing roots at nodes. They will climb up with the help of adventitious roots.

Example: Piper betel

§  Stragglers: The plants which are woody climbs up with the help of modified organs like hooks and thorns are called as stragglers.

Example: Bougainvillea

§  Lianes: These are woody twinners, in the young stages the stem around the support and grow up woody after maturity stage.

Example: Bauhinia


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