🌱 Intermediate Practical Manual – Stem Modifications

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 🌱 Intermediate Practical Manual – Stem Modifications

1️⃣ Rhizome (Example: Ginger – Zingiber officinalis)

✨ Features of Rhizome:

  1. Stem is horizontal, dorsiventral, brown, and branched.

  2. It is non-green, thick, and fleshy (stores food).

  3. Distinct nodes and internodes are present.

  4. Brown scale leaves occur at each node.

  5. Both axillary buds and terminal buds are present.

  6. Adventitious roots arise from the ventral side.

👉 By these characters, the specimen is identified as a Rhizome of Ginger, an underground stem modification.

2️⃣ Corm (Example: Colocasia – Colocasia antiquorum)

✨ Features of Corm:

  1. Underground structure, grows vertically in soil.

  2. It is non-green, thick, and solid (stores food).

  3. Nodes and internodes appear as circular ridges.

  4. Brown membranous scale leaves present at each node.

  5. Both terminal bud and axillary buds are present.

  6. Axillary buds grow into daughter corms.

  7. Adventitious roots arise at the base.

👉 By these features, the specimen is identified as a Corm of Colocasia, another underground stem modification.

3️⃣ Stem Tuber (Example: Potato – Solanum tuberosum)

✨ Features of Stem Tuber:

  1. Roots absent.

  2. Swollen and fleshy due to food storage (swollen apex of underground branch).

  3. Covered with a brown corky layer.

  4. Nodes and internodes are present.

  5. Eyes on the surface represent nodes.

  6. Each eye shows a leaf scar and an axillary bud.

👉 By these features, the specimen is identified as a Stem Tuber of Potato, an underground stem modification.

4️⃣ Bulb (Example: Onion – Allium cepa)

✨ Features of Bulb:

  1. Stem is a reduced disc-like structure.

  2. Adventitious roots form on the lower side of the disc.

  3. Numerous fleshy leaf bases arise on the upper side of the disc.

  4. Fleshy leaf bases are concentrically arranged.

  5. Function = Food storage.

  6. Outer leaves = scaly, dry, membranous.

  7. Dry scales give protection.

  8. Both terminal and axillary buds are present.

👉 By these characters, the specimen is identified as a Bulb of Onion, an underground stem modification.


5️⃣ Offset (Example: Pistia)

✨ Features of Offset:

  1. The plant shows a rosette of leaves on the upper surface and a tuft of adventitious (balancing) roots on the lower side.

  2. From the axil of leaves, lateral branches arise. These are weak, elongated, horizontal, and contain only one internode each.

  3. At the tip of each branch, a cluster of leaves above and a tuft of roots below is produced.

👉 By these features, the specimen is identified as an Offset of Pistia (a free-floating hydrophyte).
👉 Offset is a sub-aerial modification of the stem.

📌 Quick Revision – Stem Modifications (till now):

  • Rhizome (Ginger) → horizontal, branched underground stem.

  • Corm (Colocasia) → vertical, solid underground stem with ridges.

  • Stem Tuber (Potato) → swollen underground stem with eyes & buds.

  • Bulb (Onion) → reduced disc with fleshy leaf bases & dry scales.

  • Offset (Pistia) → short, horizontal branch with 1 internode, helps in vegetative propagation.

6️⃣ Stem Tendril (Example: Passiflora)

✨ Features:

  1. Thin, wiry, leafless and spirally coiled structure called tendril arises from the axil of a leaf.

  2. The tendril is a modified axillary bud.

  3. Helps the plant in climbing.

👉 Identified as Aerial stem modification – Stem Tendril.

7️⃣ Thorns (Example: Bougainvillea)

✨ Features:

  1. A hard, woody, pointed structure called thorn is present.

  2. Thorns are present in the axils of leaves → modified axillary buds.

  3. Help in climbing and protection.

👉 Identified as Aerial stem modification – Thorns.

8️⃣ Phylloclade (Example: Opuntia)

✨ Features:

  1. Stem is aerial, green, flattened, thick, fleshy → called phylloclade.

  2. Performs photosynthesis in place of leaves.

  3. Nodes and internodes are present.

  4. At every node, there is a group of spines.

  5. Spines are modified leaves (barbs, hairs, or sharp structures).

  6. A group of spines + barbs at a node = areole.

  7. Adaptation for xerophytic habitat (desert plants).

👉 Identified as Phylloclade (Opuntia) → modified aerial stem.

📌 Quick Classification of Stem Modifications (recap):

  • Underground: Rhizome (Ginger), Corm (Colocasia), Tuber (Potato), Bulb (Onion).

  • Sub-aerial: Offset (Pistia).

  • Aerial: Stem Tendril (Passiflora), Thorns (Bougainvillea), Phylloclade (Opuntia).


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