The stem is the axis of the plant that bears roots at the base and leaves, flower buds, fruits, etc., at the axis of the stem. The general function of the stem is to transfer food, water and minerals to the other parts of plants. But in some plants, these stems get modified to perform some specific functions, such as providing structural support, and other functions, such as storage of food, vegetative propagation and protection. Before we start with aerial stems, we need to understand the three main types of modification.
Underground modifications
In this, stems get modified under the ground mainly to survive under unfavourable conditions, such as high temperature, humidity, etc. These stems generally store food. Some examples of underground modification are:
Bulb: onion, garlic
Rhizome: ginger
Tuber: potato
Sub-aerial modification of the stem
This kind of modification is in which half of the plant grows above the ground and the rest grows below the ground. This type of modification is only meant for vegetative propagation. The upper half of the stem grows into a fully developed plant.
Examples of sub aerial stems
Runner: lawn grass
Sucker: mint
Stolon: jasmine
Aerial stem modification
In this type of modification, the stem grows vertically or erect above the ground. Depending upon the different conditions, such as extreme dry or humid conditions, the stem gets modified into Tendrils, thorns, hooks, Phylloclade, etc., to perform specific functions such as climbing, protection, support synthesis, or synthesis storage of food and vegetative propagation.
There are five main types of aerial stem modifications, which are:
Tendrils
This is a type of aerial modification in which aerial stems are modified into leafless thread-like structures called tendrils—the main function of tendrils in climbing. As the stems of certain plants are weak, tendrils help by coiling themselves around the support and climb. These tendrils can be seen at the branching point and can be further branched or unbranched.
Examples of plants with tendrils:
Pumpkin, cucumber, grapevine
Thorns
This is a type of aerial stem modification in which axillary buds are modified into sharp, pointed, hard and woody structures called thorns. These thorns may or may or may not bear flowers. These thorns also protect grazing animals.
Example- lemon
Phylloclade
This type of modification is only meant for vegetative propagation. It is a type of modification in which green modified stems perform photosynthesis to produce and store food. This type of modification under unfavourable conditions where leaves are reduced into thorns to prevent transpiration, i.e., water loss. These types of plants are found in dry and desert areas and are mainly xerophytes.
Example: Asparagus, opuntia
Cladodes
Cladodes is an aerial stem modification similar to phylloclade, except it consists of only one node. Cladodes help in photosynthesis, but they cannot store food. Hence, they are non-fleshy and cylindrical.
Example: Ruscus, asparagus
Bulbil
Bulbil is an axillary vegetative or floral bud modification. Its main function is to store food for the plant body. Detachment of bulbil gives rise to new plants. Hence, it is meant for vegetative propagation.
Example: Dioscorea
Function of aerial stem modification
- Vegetative propagation- Vegetative buds or floral buds fall on the ground to propagate a new plant
- Climbing- Weak stems need support to climb. Tendrils are aerial stem modifications that coil around the support and help the plant grow upward
- Support- Weak green stems get supported by aerial stem modification to stand erect
- Photosynthesis- Some aerial green stems perform photosynthesis
- Storage of food- Some aerial stems get modified and become fleshy to store food
- Protection- Aerial stems get modified into thorns to protect grazing animals
- Prevent transpiration: Aerial stems get modified into phylloclades in order to prevent transpiration, i.e., loss of water.
Conclusion
Aerial stem modification occurs under unfavourable conditions and performs some crucial functions, such as providing support, protection, synthesis of food and storage of food. Some common aerial stem modifications are tendrils, phylloclade, cladodes, bulbil and thorns.