Stem in Biology: Definition, Types, and Key Roles

Plants are the biggest producer of oxygen on our planet. Plants take in carbon dioxide and return it through photosynthesis exhaling oxygen which is then inhaled by us humans while respiration. The plant has various organs and the central axis of the plant is the stem which bears all the organs. These plants have various parts which help have different functions. The six main parts are leaves, fruits, flowers, seeds, roots and stems.

What is a Stem?

In Botany, a stem meaning is an axis of vascular plants that appears from the roots, contains vascular tissues like the xylem and phloem, bears buds and shoots with leaves, and transports water, minerals and food to the other parts of the plants.

Structure of Plant

Definition of Stem

A Stem is defined as an ascending part or an axis of the vascular plant that bears branches, leaves, fruits and flowers.

Structure of Stem

• The stem is divided into two parts

• The Nodes - Give rise to the leaves and branches.

• The Internodes - Separates two nodes.

• The stem contains three basic types of tissues which are as follows:

• Ground tissue,

• Dermal tissue, and

• Vascular tissue.

• Epidermis: A single layer of cells making up the external tissue of the stem called the dermal tissue. Woody plants have an extra layer on top of the epidermis known as bark.

• Ground Tissue - It is divided into two - the pith and the cortex which lie between the vascular tissue and the epidermis.

• Pith - Central core of spongy tissue that is surrounded by strands and then by the cortex.

• Cortex - It is divided into three layers.

• Hypodermis - It is the outermost layer of the cortex.

• General Cortex - It lies below the hypodermis.

• Endodermis - Innermost layer of the cortex.

Growth in a Stem

Growth in a stem takes place in two different ways:

• Primary Growth - In primary growth, the growth is achieved at the apical tips of the stem through the quickly dividing meristematic tissue.

• Secondary Growth - It is the increase in the thickness of the stem through the lateral meristems.

Types of Stems along with Examples of Stems

Underground Stems - These stems produce aerial shoots and remain at the ground level. These stems store food and are capable of vegetative propagation.

Underground stems have many types which are listed below.

• Rhizome - A rhizome is an underground stem that has distinct nodes and internodes. For example, Ginger.

• Tuber - A horizontal swollen underground stem that expands at its growing tips due to the accumulation of stored food, commonly starch. For example, Potatoes.

• Bulb - A short disc-like underground stem with an overlapping fleshy base with leafy scales. For example, onions.

• Corm - A short, vertical, swollen unbranched stem of a plant that serves as a food storage organ. For example, Colocasia.

Types of Stem

Subaerial Stems

The stems that run parallel to the ground and do not rise above the ground are called subaerial stems. Subaerial stems have the following types:

• Runner - A Runner runs horizontally to the ground and has long internodes. For example, strawberries and grass.

• Offset - Offsets are short, less thickened branches with short internodes and arise from the axil of the lower leaves. These are often seen in aquatic plants. For example, Pistia.

• Stolon - A stolon is similar to a runner but arises from the base of the main stem and often bends downwards touching the ground. For example, Mint.

• Sucker - Suckers are similar to stolons. It grows obliquely underground before emerging upwards. For example, Chrysanthemum.

Sub-aerial Stem Modifications

Aerial Stems

Stems that are found above the ground are called aerial stems. Aerial stems have the following types:

• Thorns - Thorns are hard woody and pointed structures that appear on the axils of leaves. For example, Roses.

• Tendril - Tendrils are thin spiral coiled-like structures that are sensitive to touch. For example, Grapevines, pumpkins and cucumbers.

• Bulbils - Bulbils store food and detach from the mother plant to develop into a new plant. For example, Dioscorea.

• Cladode and Phylloclade - Phylloclade are flattened cylindrical stems whereas cladode is flattened stems that perform photosynthesis. Phylloclade can store water. For example, Ruscus.

Aerial Stem Modifications

Functions of Stems

The various functions of stems are listed below:

• Stems support and hold leaves, flowers and fruits.

• Stems play an important role in holding the plant upright.

• The water and minerals are transported across the stem through the xylem and phloem present in the vascular bundle.

• Some stems can store food and water.

• Few green stems contain chloroplasts and are capable of photosynthesis.

• The stems arrange the leaves in a way that they can receive sunlight directly to perform photosynthesis efficiently and also perform gas exchange.

• Some stems are modified to perform vegetative propagation, a form of asexual reproduction in plants.

• Pollination and fertilisation can be carried out with the help of stems which arrange the flowers and fruits in a position that makes it easier to perform.

Interesting Facts

• In most plants, stems are appeared to be on top of the ground, but there are some stems that are found underground called stolons or rhizomes.

• Except for the few plant stems, most stems are aerial that bear the other parts of the plant like flowers, leaves, fruits and seeds.

Key Features

• A Stem is developed from the plumule of an embryo.

• Stem fields play an important role in holding the plant upright in adverse conditions.

• Several Stem fields are capable of storing water and food in desert conditions.

• Some items are included in the daily diet and are edible such as rhubarb, celery and leaks.