Morphology of Stem – Class 11 (NCERT Biology) by BHS

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Characteristics of the Stem

Develops from the plumule of the embryo.

Bears nodes (points where leaves and branches arise) and internodes (region between two nodes).

Has buds:

Terminal bud (at the tip, helps in elongation).

Axillary bud (in the axil of leaves, can form branches).


Stem is usually green and photosynthetic when young.

Exhibits positive phototropism and negative geotropism.



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Modifications of the Stem

Besides its normal function of support and conduction, stems are modified for storage, perennation, vegetative propagation, climbing, and defense.

1. Underground Stem Modifications

These look like roots but are stems (distinguished by nodes, internodes, and buds).

Rhizome: Horizontal, underground stem storing food (Ginger, Turmeric).

Tuber: Swollen tip of an underground stem with eyes (Potato).

Corm: Condensed, vertical stem storing food (Colocasia, Gladiolus).

Bulb: Small disc-like stem surrounded by fleshy scale leaves (Onion, Garlic).


👉 Functions: Storage of food, perennation, vegetative propagation.


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2. Subaerial Stem Modifications

These grow partly underground and partly aerial.

Runner: Creeping stem with nodes that form new plants (Grass, Oxalis).

Stolon: Long arching branches that touch the ground and root (Strawberry).

Sucker: Arises from underground and grows obliquely upward (Mint, Chrysanthemum).

Offset: Short, thick internode with rosette of leaves (Water hyacinth, Pistia).


👉 Functions: Vegetative propagation.


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3. Aerial Stem Modifications

Tendrils: Slender, coiled structures for climbing (Cucumber, Grapevine, Passion flower).

Thorns: Hard, pointed structures for defense (Bougainvillea, Citrus).

Phylloclade: Flattened/ cylindrical green stem performing photosynthesis (Opuntia, Euphorbia).

Cladode: Single internode modified for photosynthesis (Asparagus).

Bulbils: Modified axillary buds functioning as reproductive structures (Agave, Dioscorea).



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Functions of the Stem

Provides support to aerial parts.

Conduction of water, minerals (xylem) and food (phloem).

Storage of food in modified forms.

Photosynthesis (in green stems or modifications like phylloclade).

Protection (thorns).

Vegetative propagation through stem modifications.



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Quick Revision

Normal functions → support + conduction.

Underground modifications → rhizome, tuber, corm, bulb.

Subaerial modifications → runner, stolon, sucker, offset.

Aerial modifications → tendrils, thorns, phylloclade, cladode, bulbils.



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✨ Conclusion:
The stem is not just a structural framework but also a highly versatile organ. Its various modifications highlight the adaptive strategies of plants for survival, storage, photosynthesis, reproduction, and defense.

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