Stem and its modification

June 2, 2018

Introduction

It is the aerial part of the vascular plant.
In botany it means, the plant axis that bears buds and shoots with leaves and, at its basal end, roots.
It develops from the plumule and bears branches, flowers and leaves.
It is differentiated into nodes and internodes which may not be distinct in some cases.
It is normally positively phototrophic i.e. it grows or bends towards the light.
The branches and leaves arise from the nodes.
It also bears different kinds of buds such as terminal bud – present at the apex of the shoot, the axillary bud present at the axil of the leaf and floral bud, which may be terminal or axillary.
It has various functions. They are as follows:

Support for and the elevation of leaves, flowers and fruits. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits.
Transport of fluids between the roots and the shoots in the xylem and phloem.
Storage of nutrients.
Production of new living tissue. The normal lifespan of plant cells is one to three years. Stems have cells called meristems that annually generate new living tissue.

Modifications of stem

1) Underground modifications

These are usually organs of perennation.
The stem develops under the soil in a defoliated condition and give off aerial shoots annually under favourable conditions.
They are always thick and fleshy having a good deposition of food material in them.
They are of following types:

a) Rhizome

It is a thick and fleshy underground stem, which grows horizontally near the soil surface.
It contains a dry scaly leaves at distinct nodes.
It bears buds in axils of scaly leaves and a terminal bud.
The buds serve for vegetative propagation.
e.g., ginger, fern, mint, etc.

b) Tuber

Image source: quora

c) Corm

d) Bulb

It is a short underground shoot with many scaly leaves.
The food material is stored in the scaly leaves.
The stem is very much shortened or reduced enclosed by fleshy scale-like leaves.
The bulb has a terminal bud which gives off aerial shoot.
The base of stem contains numerous adventitious roots.
e.g., onion, garlic, lilies, etc.

2) Sub-aerial modifications

They are found in plants with weak stems in which branches lie horizontally on the ground.
A part of the stem grows below the soil and a part above the ground.
Hence, it is called the sub-aerial stem.
These mainly take part in vegetative propagation.
They are of following types:

a) Runner

It is a long, slender, prostrate stem with long or short internodes.
It creeps on the ground and gives off roots at the nodes.
The runner gives rise to new plants, either from axillary or terminal buds.
e.g., grass, Oxalis, Mint, Marsilea, etc.

b) Stolon

It is a long, slender, lateral branch that arises from the base of the stem.
The terminal bud produces a daughter plant when touches the soil.
A stolon may or may not grow for some distance giving out roots and a bud at each node.
e.g., black jasmine, silver weed, hawk weed etc.

c) Sucker

An obliquely upwards growing branch arises from the underground part of the stem or root.
It gives rise to an aerial shoot or anew plant.
It is much shorter and stouter than a runner.
e.g., Chrysanthemum, etc.

d) Offset

A horizontal, short, more or less thickened stem.
It originates from the axil or a leaf, extends for a short distance and then produces a cluster of leaves (rosette) above and adventitious roots below.
It is usually found in aquatic plants.
e.g., water lettuce (Pistia), water hyacinth, Eichhornia, etc.

3) Aerial modification

a) Phylloclade

b) Cladode

c) Stem tendril

d) Thorn

It is a hard, often straight, pointed and woody structure.
It may bears leaves, flowers in certain plants.
The thorns arise in spring as axillary shoot with normal leaves and with a special bud.
The apex soon stops growth and hardens into a woody point, from which the undeveloped leaves fall away, leaving it naked.
It helps in climbing.
e.g, lemon, Duranta, Crataegus, etc.

e) Twiner

Long slender and branched stem climbing by twisting its body around the support.
e.g., Cuscuta, etc.

f) Climbers

A weak stem climbing on support by means of special structures such as rootlets, hooks, leaf tendrils, stem tendrils, etc.

g) Bulbil

It is a special modification for reproduction of the plant.
It develops from the axillary bud, which becomes large and fleshy owing to the storage of food material in their leaves.
Falling on the ground, it produces a new plant.
e.g., Agave, Dioscorea, Lily, etc.