Plant Stem: Functions, Parts and Types
It is generally erect, strong, and usually grows away from the soil and towards sunlight.
Plant stems are negatively geotropic, i.e., they grow upwards or away from the force of gravity.
In contrast, roots are positively geotropic, meaning it grows downwards, towards the center of the Earth.
The stem performs many important functions that help plants to grow and survive. To provide mechanical support to the leaves and reproductive organs and transport of water and nutrients are the primary functions of the stem of the plant.
Different Parts of a Plant Stem:
Node: A node is a point on the stem from which leaves, buds, aerial roots, and branches arise.
Internode: The space between the two successive nodes is called an internode. It is also known as the “internodal” area. It helps the plant to grow and keeps it erect.
Apical Bud: Apical buds are found at the tip, or apex, of the stem, where primary growth of the plant or increasing the height occurs. It is also called the terminal bud.
Axillary Bud: An axillary bud is usually found in the leaf axil, the area between the base of a leaf and the stem. Axillary buds give rise to a shoot that may be vegetative (branch) or reproductive (flowers). It is also called the lateral bud.
Petiole: The thin stalk with which leaf is attached to the stem node is called a petiole. The petiole is commonly known as leaf stalk.
A leaf with a petiole is called a petiolate leaf, whereas leaves without them are called sessile or stalkless.
Types of the Stem
- Underground stem,
- Aerial stem,
- Sub- aerial stem.
1. Underground Stem
The stem that grows inside the soil is known as the underground stem. They produce aerial shoots annually.
Storage of food and perennation are the main functions of underground stems. These stems are also capable of vegetative propagation.
They are of different types as follows:
Rhizome: is a non-green underground stem with distinct nodes and internodes and dry, scaly leaves at the nodes. It grows horizontally or obliquely. Rhizomes store plant nutrition in the form of proteins and starches. Example: Ginger.
Tuber: is a short and thickened underground stem that grows horizontally below the ground. It stores plant nutrition in the form of starch. Example: Potatoes.
Bulb: is a short underground stem with internal buds surrounded by fleshy leaves or leaf bases that helps the plants survive adverse environmental conditions. Example: Onion.
Corm: is a short, vertical, swollen underground stem of a plant-covered by thin sheathing leaf bases of dead leaves called scales. These dry leaf bases help protect the stem of the corm from harsh weather conditions and lack of moisture.
Corm serves the functions of food storage, vegetative propagation, and perennation. E.g., Crocus
2. Sub-Aerial Stem
They are further divided into the following types:
Runner: It is a creeper that runs horizontally along the surface of the soil. Runners have long internodes. The nodes have scale leaves, adventitious roots, and auxiliary buds. An underground runner is known as sobole.
Example: Grass, Cynodon, Oxalis.
Offset: These are shorter and thicker than the runner with a single internode. It originates from the leaf axis and grows horizontally. Offsets are often found in aquatic plants like water lettuce, water hyacinth, etc.
Stolon: It grows above ground for some time and then bends towards the ground until it touches the ground. Stolon arises from the lower part of the main axis. Example: Jasmine, colocasia, etc.
Sucker: The sucker stem is very similar to the stolon, but it grows obliquely upwards and gives rise to a new plant. Example: Garden chrysanthemum, strawberry, pineapple, mentha, etc.
3. Aerial Stem
The aerial stem is further divided into the following types:
Tendril: These types of stems are slender, spirally coiled, which help a plant to climb. Example: Passiflora, Grapevine etc.
Bulbil: These are modified axillary buds which become fleshy and swollen due to the storage of food. They help in vegetative propagation to form a new plant. Example: Dioscorea.
Thorn: These are hard, woody, and pointed structures that protect plants from grazing animals. It originates from the axillary or terminal bud. Example: roses, citrus, bougainvillea, duranta etc.
Cladodes: These are non-fleshy and cylindrical that contain only one internode. Example: Asparagus, butcher’s broom.
Phylloclade: These are green, fleshy, and flattened or cylindrical branches containing chlorophyll and photosynthesis. This modification is found in xerophytic plants and stores water. Example: Opuntia, Casuarina, etc.
Functions of Stem
- It supports and holds the leaves, flowers, and fruits.
- The stem also helps in the arrangement of leaves to receive direct sunlight to perform photosynthesis efficiently. The proper arrangement and positioning of leaves are also helpful for gas exchange.
- It transports water, nutrients, and the products of photosynthesis from the root to the leaves and other parts of the plant with the help of xylem and phloem present in the vascular bundles of stems.
- Some plant stems are modified to perform functions like food storage, vegetative propagation, and perennation.
- Few green stems perform photosynthesis as well.
- The plant stem serves as a means of asexual reproduction in many plant species. Many plants, such as ginger, onion, and dahlia, grow from buds present on the stem’s surface.