Morphology and functions of stem:

The aerial part of the plant body is collectively described as shoot system. Main axis of this shoot system is called the stem.

Stem can be defined as the ascending part of the plant axis which develops from plumule, grows by means of a terminal bud, bears leaves and reproductive structures, and is differentiated into nodes and internodes.

The stem exhibits various characteristics such as:

1. It is usually positively phototropic, negatively geotropic and negatively hydrotropic.

2. It bears a terminal bud and axillary buds in the axils of leaves, for growth.

3. It is differentiated into nodes and internodes.

4. At the nodes, it produces dissimilar organs such as leaves, and flowers, and similar organs, such as branches, exogenously, (i.e. originate from outer tissues).

5. The young stem is green and capable of performing photosynthesis.

Buds:

Bud is a compact young shoot consisting of a condensed stem, overlapped by young, immature leaves.

 

Types of Buds:

According to their functions, buds may be of following two types:

1. Vegetative buds,

2. Floral buds or reproductive buds

 

1) Vegetative buds:

The buds which develop into branches are called vegetative buds, or leaf buds.

Structure of leaf bud is clearly seen in longitudinal section of cabbage, which is the largest terminal bud.

 

On the·basis of their position, buds may be of following types:

i) Apical or Terminal buds:

These buds are located at the tip or apex of the stem.

ii) Axillary buds:

Buds which lie in the axils of leaves are called axillary buds.

iii) Accessory buds:

Sometimes additional buds develop either on the sides or above the axillary buds. These are called accessory buds.

iv) Adventitious buds:

Occasionally, buds appear at places other than nodes. Such buds are called adventitious buds.

Generally, these buds arise from roots and leaves.

 

2) Floral buds or Reproductive buds:

The buds which develop into flowers are called floral buds or reproductive buds.

 

Functions of stem:

The primary functions of the stem are,

1. To produce and support lateral appendages such as branches, leaves, flowers and fruits,
2. Conduction of water, and minerals to different parts of the shoot, and
3. Transportation of food to all plant parts.

Stem may, however, get modified to perform additional or accessory functions, such as

1. storage of food and water;

2. proliferation and propagation;

3. procuring support for climbing;

4. protection from browsing animals;

5. perennation i.e. to tide over unfavorable conditions and

6. synthesis of food.

 

Modifications of stem :

These modifications fall under three heads:

I) Underground modifications

II) Sub-aerial modifications

III) Aerial modifications.

I. Underground Modifications:

In many herbaceous plants, stem develops below the soil and is called underground stem.

Such stem remains dormant during unfavorable conditions and gives off aerial shoots under favorable conditions.

These underground stems often store food and are thick and fleshy.

 

Underground stem performs three functions -

1. storage of food,

2. perennation and

3. vegetative propagation.

 

They can be differentiated from roots by;

- stem like internal structure,

- exogenous branching,

- presence of nodes and internodes,

- occurrence of foliage or scale leaves at the nodes with axillary buds and

- absence of root cap.

 

Underground stems are of the following types:

 

1. Rhizome:

It is a prostrate, dorsiventral, thickened brownish stem, which grows horizontally under the surface of the soil.

It shows distinct nodes and internodes.

It possesses a terminal bud and axillary buds in the axil of each scale leaf present at node.

Rhizome remains dormant under the soil and at the onset of favorable conditions; the terminal bud grows into the aerial shoot which dies at the end of favorable season.

Growth of rhizome takes place with the help of lateral buds. This type of rhizome is known as sympodial rhizome e.g. Ginger (Zingiber officinale), Turmeric (Curcuma domestica), Canna, etc. .

In some plants, growth of rhizome occurs with the help of terminal bud. These are called monopodial rhizomes. e.g., Lotus, Pteris (a fern), etc.

2. Stem tuber :

Tubers are actually the swollen tips or ends of special underground branches swollen due to storage of food (carbohydrate like starch).

The tubers show nodes and internodes and nodes bear scale leaves with axillary buds, commonly called 'eyes'.

Under favorable conditions 'eyes' sprout and produce aerial shoots.

Thus tuber helps in vegetative propagation. Tubers do not produce adventitious roots, thus they differ from rhizomes. e.g., Potato (Solanum tuberosum).

3. Bulb:

It is a condensed, disc like underground stem, which itself does not store food material.

The upper surface of disc like stem is slightly conical and bears centrally placed apical bud and many concentrically arranged, overlapping scale leaves.

Inner scale leaves or leaf bases store food and are thick and fleshy, while outer few scaly leaves remain thin and dry and are protective in function.

Lower surface of disc like stem produces adventitious roots.

The discoid stem with compactly arranged fleshy leaves above and fibrous roots below is commonly called bulb.

Bulb is almost spherical When the fleshy scale leaves surround the apical bud in the form of concentric rings, it is called tunicated bulb e.g. onion.

Sometimes they may partially overlap each other by their margins only, such bulbs are called scaly bulbs e.g. garlic.

4. Corm :

Corm is a short, stout, fleshy, upright and thickened underground stem. It bears many buds in the axils of scale leaves which develop into daughter corms.

At the bases or even from sides of stem adventitious roots develop.

Corm is a condensed form of rhizome growing vertically.

e.g., arbi (Colocasia) , zamikand (Amorphophallus etc.)

 

II. Sub-aerial modifications:

In this type of modification, stem trails on the soil or is found partially beneath the soil in such a way that it remains in contact with both, the soil and air.

Stem is modified for perennation and vegetative reproduction.

Branches of stem are found on the surface of soil.

In the axils of scale leaves on such branches, axillary buds are developed and aerial shoot is formed.

The sub-aerial stem modification is of following four types.

 

1. Runner :

These are special, narrow, green, horizontal or prostrate branches which develop at the base of erect shoots called crowns.

A number of runners arise from each erect shoot.

They spread in different directions and bear new crowns above and tufts of adventitious roots below at certain intervals.

Each runner has one or more nodes. The nodes bear scale leaves and axillary buds.

e.g., Lawn grass (Cynodon dactylon), Hydrocotyl (Centella asiatica), Oxalis, etc.

2. Stolon :

Stolon is a slender lateral branch that arises from the base of the main axis.

Initially stolon grows upwards like an ordinary branch and then bends down and touches the soil where its terminal bud gives rise to a new shoot and adventitious roots.

e.g., jasmine, Mentha and strawberry

3. Offset :

It is commonly called the runner of aquatic plants.

It is shorter and thicker than runner.

It helps in the vegetative propagation in aquatic plants.

e.g., water hyacinth or jalkumbhi (Eichhornia). and Pistia.

4. Sucker:

Sucker is a runner like branch which develops from the axil of scale leaf in the underground part of stem.

It grows horizontally below the soil for some distance and comes above the soil obliquely and produces green leaves to form aerial shoots.

The sucker can, therefore, be called underground runner. e.g., Chrysanthemum.

 

 

III. Aerial Modifications

Sometimes aerial stem or its vegetative or floral buds undergo modifications to form specialized structures like tendrils, thorns, phylloclades or bulbils to perform

specific functions. These stems are called metamorphosed stems.

Following are the types of aerial modifications of stem :

 

1. Stem tendril:

It is a modification of stem in which axillary bud modifies to form a thin, wiry, and highly sensitive structure called tendril.

Tendrils help the plant to attach itself to the support and climb.

They are found in plants with weak stem. The tendrils are leafless, coiled structures with sensitive adhesive glands for fixation.

An example of axillary tendril is Passiflora (Passion flower).

In Vitis, apical bud is modified into tendril and further growth is resumed by axillary bud. Such growth is called sympodial growth.

In Cucurbita-extra-axillary bud is modified into tendril while in Antigonon, floral bud is tendrillar.

2. Thorn:

Thorn is a hard, pointed usually straight structure produced by modification of axillary bud.

Leaves, branches and flowers are developed on thorns at the nodes, indicating that it is a modified stem.

It provides protection against browsing animals.

e.g. Citrus, Bougainvillaea, Duranta etc.

In Carrisa, apical bud is modified into thorn.

3. Phylloclade:

The phylloclade or cladophyll is a stem which gets transformed into leaf like structure.

The phylloclade is green, flattened structure with distinct nodes and internodes.

- It is thick, fleshy and succulent, (i.e. it contains mucilage for retention of water) in Opuntia or Nagphani,

- It is cylindrical in Casuarina and Euphorbia tirucalli and

- It is ribbon like in Muehlenbeckia.

 In xerophytes, leaves get modified into spines or get reduced in size to check the loss of water due to transpiration and therefore stem takes up the function of leaf, i.e. photosynthesis.

 

4. Cladodes:

These are green branches of limited growth (usually one internode long) which have taken up the function of photosynthesis.

True leaves are reduced to scales or spines , e.g. Asparagus.

5. Bulbils:

When axillary bud becomes fleshy and rounded due to storage of food, it is called bulbil.

It gets detached from the plant, falls on ground and develops into a new plant. e.g. Dioscorea.