Stem Modifications
தண்டு உருமாற்றங்கள்
A potato is a stem, not a root — it has nodes with buds (the 'eyes'). Sweet potato is the root. If you can't spot the node, you can't tell the organ, and NEET loves this trap.
The idea
Stems are modified for food storage, support, protection and vegetative propagation. Underground stems such as potato (tuber), ginger and turmeric (rhizome), Colocasia (corm) and onion (bulb) store food and help the plant survive bad conditions. Stem tendrils that develop from axillary buds help weak plants climb, as in cucumber, pumpkin, watermelon and grapevine. Axillary buds also form woody, straight, pointed thorns in Citrus and Bougainvillea. Opuntia has a flattened, green, fleshy stem (phylloclade) that photosynthesises, while Euphorbia has a fleshy cylindrical stem. Sub-aerial stems such as runners (grass, strawberry), stolons (mint, jasmine) and offsets (Pistia, Eichhornia) spread and propagate the plant.
Why it matters
A modified stem lets the plant store food, climb, defend itself or spread without seeds — while keeping the nodes and buds that prove it is a stem.
NEET lens
NEET asks 'which is NOT a stem modification' — pitcher of Nepenthes is a leaf, Citrus thorns are stem. Know the pairs: tendril (cucumber, pumpkin, grapes) and thorns (Citrus, Bougainvillea) arise from axillary buds; phylloclade = Opuntia; runner = grass, strawberry; offset = Pistia, Eichhornia. Edible underground stem = potato, not carrot or sweet potato.
Clinical significance
Food security rests on modified stems: potato, ginger, turmeric, Colocasia and onion are all stems. Farmers propagate them vegetatively from 'seed' tubers and rhizome pieces, which keeps the variety true to type but also spreads viral diseases from one crop to the next.
Introduction
The stem normally bears leaves, buds and flowers, but in many plants it takes on other jobs — storing food, climbing, protecting, photosynthesising or spreading. However changed it looks, a modified stem still has nodes and internodes.
Core concept
Underground (storage, perennation): tuber — potato (eyes = nodes with buds); rhizome — ginger, turmeric; corm — Colocasia; bulb — onion. Tendrils (from axillary buds) — cucumber, pumpkins, watermelon, grapevines. Thorns (from axillary buds, woody and pointed) — Citrus, Bougainvillea. Flattened green stem (phylloclade) — Opuntia; fleshy cylindrical stem — Euphorbia. Sub-aerial, for vegetative spread: runner — grass, strawberry; stolon — mint, jasmine (lateral branch arches down to the ground); offset — Pistia, Eichhornia (one internode long, each node gives rosette of leaves and roots).
Go deeper
How to prove an organ is a stem: look for nodes, internodes, buds and scale leaves. A potato 'eye' sprouts a shoot, so the potato is a stem; a sweet potato has no nodes and develops from adventitious roots, so it is a root. Spines of cactus-like plants can be leaves (Opuntia's spines are modified leaves) while its flat green pad is the stem.
From Class 10
You learnt that potatoes and ginger can grow new plants without seeds (vegetative propagation). That works because they are stems — every 'eye' or bud on them can grow into a new shoot.