🌿 The Stem
The stem is the ascending part of the axis of a plant that develops from the plumule of the embryo. It bears branches, leaves, flowers, and fruits.It is typically aerial, growing away from the soil (negatively geotropic).
🧩 Characteristics of the Stem
- Grows upward (positive phototropism).
- Bears nodes and internodes.
- Bears axillary and terminal buds.
- Green and photosynthetic in young stage and later often becomes woody and dark brown.
- Conduction of water, minerals, and food through xylem and phloem.
🌱 Functions of Stem
- Support & Spreading: Spreads out branches, bearing leaves, flowers, and fruits.
- Conduction: Conducts water, minerals, and photosynthates (food) between roots and leaves.
- Other Functions: Some stems perform specialized functions like food storage, support, protection, and vegetative propagation.
🌾 Types of Stem Modifications
Stems undergo structural changes to perform special functions such as food storage, climbing, protection, or vegetative propagation.
1️⃣ Underground Stem Modifications
These serve as organs of perennation, storage, and vegetative propagation.
| Type | Example | Distinct Feature |
|---|---|---|
| Rhizome | Ginger, Turmeric | Horizontal stem with nodes, internodes, and scale leaves |
| Tuber | Potato | Swollen tip of underground stem with 'eyes' (buds) |
| Corm | Colocasia, Gladiolus | Solid, spherical stem with tunics and nodes |
| Bulb | Onion, Garlic | Condensed stem with fleshy scale leaves |
2️⃣ Subaerial Stem Modifications
Used for vegetative propagation and limited movement.
| Type | Example | Description |
|---|---|---|
| Runner | Grass, Oxalis | Creeping stem with nodes that root at intervals |
| Stolon | Strawberry | Arches out and roots after touching ground |
| Sucker | Mint, Chrysanthemum | Arises from base of stem and grows upward |
| Offset | Eichhornia, Pistia | Short, thickened lateral branch in aquatic plants |
3️⃣ Aerial Stem Modifications
Perform various functions like climbing, protection, and photosynthesis.
| Modification | Function | Example |
|---|---|---|
| Tendrils | Climbing | Cucumber, Gourd, Grapevine |
| Thorns | Protection | Bougainvillea, Citrus |
| Phylloclade | Photosynthesis | Opuntia, Euphorbia |
| Cladode | Modified green stem | Asparagus |
| Bulbil | Reproduction | Dioscorea, Agave |
🌼 Significance of Stem Modifications
- Perennation: Survive unfavorable conditions (Rhizome, Corm).
- Storage: Store food or water (Tuber, Bulb).
- Vegetative propagation: Develop new plants (Runner, Sucker).
- Climbing: Reach sunlight (Tendrils).
- Protection: Defend from grazing (Thorns).
💡 Questions and Answers
Q1. Name the underground stem used for propagation and food storage.
A1. Potato – stem tuber used for both functions.
Q2. Differentiate between corm and bulb.
A2. Corm is solid with nodes/internodes (Colocasia), bulb has fleshy scale leaves (Onion).
Q3. What are aerial modifications of the stem? Give examples.
A3. Tendrils (climbing – Gourd), Thorns (protection – Bougainvillea), Phylloclade (photosynthesis – Opuntia).
Q4. What are the primary functions of the stem?
A4. The main functions of the stem are:
- Support: It supports and spreads out branches, leaves, flowers, and fruits, positioning them to receive sunlight and facilitate pollination/dispersal.
- Conduction: It contains vascular tissues (xylem and phloem) that conduct water and minerals from the roots to the leaves and photosynthates (food) from the leaves to other parts of the plant.
Q5. Differentiate between a stem tendril and a stem thorn. Give an example of each.
A5.
- Stem Tendril: A modification for support. It is a slender, spirally coiled structure that arises from an axillary bud and helps the plant to climb. Example: Grapevines, Gourds.
- Stem Thorn: A modification for protection. It is a hard, woody, pointed structure, also arising from an axillary bud, that deters browsing animals. Example: Citrus, Bougainvillea.
Q6. Differentiate between a rhizome and a tuber, both of which are modified for storage.
A6.
- Rhizome: A horizontal, underground stem that grows beneath the soil surface. It is characterized by having distinct nodes, internodes, scale leaves, and buds (e.g., Ginger).
- Tuber: A swollen, fleshy terminal end of an underground stem branch. It does not have a clear node/internode structure like a rhizome, but possesses buds called 'eyes' (e.g., Potato).
Q7. What is an offset? Give an example.
A7. An offset is a specialized stem modification for vegetative propagation found in many aquatic plants. It is a short, thick, horizontal lateral branch (like a runner) with short internodes. Each node produces a rosette (cluster) of leaves at the top and a tuft of roots below. Example: Pistia (water lettuce), Eichhornia (water hyacinth).