The main functions of stems are to support and elevation of leaves, fruits, and flowers. Stem arranges leaves in a way that it gets direct sunlight to perform photosynthesis. Xylem and Phloem conduct water across the plant. Stems stores food, water, and nutrients. Cells of a stem, meristems, produce new living tissues. Underground stem, Aerial stem, and subaerial stem are three different types of Stem. A stem has many important functions it performs other than letting you climb a tree. Let us take an in-depth look at the stem of plants.
A plant stem is one of the two main structural axes of a vascular plant. It is the part of the plant that lies above the ground. Few stems are also found underground and are considered to be stem modifications.
Functions of Stem
- It supports and holds leaves, flowers, and fruits.
- The stem allows the leaves to arrange in a way that they are able to receive direct sunlight in order to efficiently perform photosynthesis. The arrangement and position of leaves also allow for gas exchange.
- The xylem and phloem present in the vascular bundles of stems conduct water and minerals across the plant.
- Stems bear flowers and fruits in a position that facilitates the processes of pollination, fertilization, and dispersion of seeds.
- Some stems undergo modification to store food and water. Example: succulents.
- Few green stems contain chloroplasts and are capable of carrying out photosynthesis as well.
- Some stems are modified to carry out vegetative propagation which is a form of asexual reproduction seen in plants.
Structure of a Stem
The stem divides into nodes and internodes. The nodes give rise to the leaves and hold the buds which grow into branches. The internodes separate two nodes.
Internally, it contains three basic types of tissues: Dermal tissue, Ground tissue, and Vascular tissue all of which are made of simple cells.
- Epidermis: The epidermis is a single layer of cells that make up the external tissue of the stem called dermal tissue. This tissue covers the stem and protects the underlying tissue. Woody plants have an extra layer of protection on top of the epidermis known as bark. In some cases, the bears’ multi-cellular hairs and a few stomata.
- Ground tissue divides into two- the central portion is known as the pith and the cortex which lies between the vascular tissue and the epidermis.
Learn more about the concept of the Tissue System here in detail.
The cortex can be further divided into three layers:
- Hypodermis: It is the outermost layer of the cortex. It is formed of 4 to 5 cell thick layer of collenchymatous cells. These cells are living and contain chloroplasts.
- General cortex: Lies below the hypodermis. It consists of thin-walled parenchymatous cells with intercellular spaces. Some of the cells have chloroplasts and are known as chlorenchyma.
- Endodermis: The innermost layer of the cortex. It is made up of a single row of compact barrel-shaped cells without intercellular spaces. The cells of endodermis store starch grains and so they are known as the starch sheath. Casparian strips are distinctly visible in endodermal cells.
- The vascular tissue of the stem consists of the complex tissues xylem and phloem which carry water and nutrients up and down the length of the stem and are arranged in distinct strands called vascular bundles. Cambium is a strip of thin-walled cells that lie between the xylem and phloem in dicot plants. Cambium is made up of merismatic cells and is responsible for secondary growth. It is absent in monocots.
What are Plant Tissues?
Growth in a Stem
Growth in stems occurs in two ways:
- Primary growth occurs at the apical tips of the stem by virtue of the rapidly dividing merismatic tissue in these regions of the stem.
- Secondary growth is actually the increase in the thickness of the stem by virtue of the lateral meristems. These are absent in the herbaceous plants as they lack cambium which is responsible for this type of growth.
Types of Stems
Based on their location with respect to the ground, there are three types of stems:
- Underground stem
- Aerial stem
- Subaerial stem.
Understand Tissue System
These stems remain at the ground level and produce aerial shoots that rise above the soil. Their roots are superficially present. These stems are meant for storage of food and perennation. These stems are also capable of vegetative propagation.
They are of different types as follows:
- Rhizome- is a thickened underground stem that has distinct nodes and internodes and scaly leaves at the nodes. Example: Ginger.
- Tuber- is a horizontal underground stem that becomes enlarged at its growing tips due to the accumulation of stored food, commonly starch. E.g. Potato.
- Bulb- It is a short underground stem with a fleshy base with leafy scales. The stem is actually reduced to form a disc-like structure. The nodes bear fleshy scales. On the upper side, the disc bears a terminal bud surrounded by a number of leaves. E .g. Onion.
- Corm- is a short, vertical, swollen underground stem of a plant that serves as a food storage organ to enable the plant to survive adverse conditions. E.g Colocasia
Know more about Secondary Growth
These stems run parallel to the ground and give off roots at certain intervals or nodes.
They are further divided into the following types:
- Runner- It grows parallel to the ground and has a creeping stem with long internodes. On the lower surface, the nodes give out adventitious roots at regular intervals. A runner develops from the axils of lower leaves of the aerial stem
- Offset- These are shorter and thicker than the runner and are often seen in aquatic plants
- Stolon- It is similar to a runner but arises from the lower part of the main axis.
- Sucker- These stems are similar to the stolon but it grows obliquely upwards and gives rise to a new plant
What is Inflorescence?
These stems are found above the ground and perform varied functions.
They are of the following types:
- Thorns- These stem modifications appear as hard, woody and sharp outgrowths that protect the plant. example: roses
- Tendril – These types of stems are slender, twining strands that enable a plant to seek support while climbing on other surfaces.
- Phylloclade- This type of stem is a green, flattened or cylindrical one that resembles a leaf. A phylloclade is capable of performing photosynthesis and we can find them in xerophytes or in other plants that have little or no leaves.
- Cladode- This is a modification of the phylloclade where it contains one or more internodes.
- Bulbil- These stems are actually modified axillary buds which become fleshy and rounded due to the storage of food. They become detached from the plant, fall o the ground and develop into a new plant, thus help in vegetative propagation.
Solved Example for You
Q: What kind of a stem is a potato?
Sol: The correct option is (a) Tuber
A potato is an underground modification of a stem and it stores food in the form of starch. This type of stem modification is called a tuber.