Stem Characteristics and Modifications: Tendrils to Thorns

The stem is the main axis of a plant that grows upward, bears leaves, branches, flowers and fruits, and connects roots to leaves. In many plants the stem changes its form to climb, store food, protect the plant or make new plants. These changed stems are called stem modifications and include tendrils, thorns, underground stems and flat leaf-like stems.

Learning stem characters helps us tell a stem from a root or leaf and understand how plants like potato, ginger, grape and cactus survive, spread and protect themselves. Many foods we eat are not roots but modified stems.

Key Concepts

Stem characters: Presence of nodes where leaves arise and internodes between them, buds at leaf axils, growth usually towards light, and origin from plumule. Underground modifications: Rhizome like ginger grows horizontally with nodes and scale leaves. Tuber like potato is swollen tip of underground branch with eyes which are buds. Bulb like onion is highly reduced stem with fleshy scale leaves. Corm like Colocasia is vertical swollen stem. Aerial modifications: Stem tendrils in grape and cucumber coil to climb. Thorns in Bougainvillea and Citrus are hard pointed structures from axillary buds for protection. Phylloclade in Opuntia is flat green stem doing photosynthesis while leaves become spines. Cladode is a phylloclade of one internode like in Asparagus. Sub-aerial modifications: Runner in grass, stolon in strawberry, offset in Eichhornia and sucker in mint help in vegetative propagation.

How It Works

1. Underground stems stay below soil, store food in parenchyma and perennate through winter or dry season, then sprout from buds.
2. Tendrils are sensitive to touch. When they contact a support they coil around it due to faster growth on the opposite side, pulling the weak stem up towards light.
3. Thorns develop from buds, have vascular tissue inside and reduce grazing by animals and water loss.
4. Runners and stolons grow horizontally, root at nodes and form new independent plants when the connection breaks.

Worked Example

A sample has nodes, internodes and 6 eyes on a swollen oval body measuring 8 cm long. Another sample is flat, green, 5 cm wide with spines and no true leaves. A third is a thin coiled structure 15 cm long from a grape vine. Identify them. Step 1: Eyes are axillary buds, so first is a stem tuber, potato, not a root. Step 2: Flat green photosynthetic stem with spines is phylloclade of Opuntia. Step 3: Thin coiled climbing structure from stem is a stem tendril. Count: underground modification = 1, aerial modifications = 2.

Common Misconceptions

• Misconception: Potato and onion are roots. Correction: Both have nodes, buds and scale leaves, so they are modified stems.
• Misconception: Thorns and spines are the same. Correction: Thorns are modified stems or branches with vascular tissue, spines are modified leaves, prickles are outgrowths of epidermis like in rose.
• Misconception: Tendrils are always leaf modifications. Correction: In grape and cucumber tendrils arise from stem or axillary bud position.

Real-World Applications

Potato, ginger, turmeric and onion are important food and spice crops propagated through stem modifications. Farmers use runners of strawberry and suckers of banana to produce genetically identical plants quickly. Cactus phylloclades show how desert plants reduce leaves to save water. Tendril crops are trained on wires in vineyards to increase light exposure and fruit yield.

Thus stems are identified by nodes, buds and internodes, and they modify into storage organs, tendrils for climbing, thorns for protection and runners for spreading to new places.