CDS Biology · Plant Biology
Plant Organs
How to tell a modified stem from a modified root, what a leaf does and how leaves are arranged, and how fruits form and seeds are carried away.
Why this matters
Seven CDS questions from 2018 (II) to 2024 (I), five of them in the 2018 (II) paper. Two ask which vegetable is really a stem, and the others ask about the root, the leaf, true fruits and seed dispersal.
Concept 1 of 3: Modified roots and stems
Many vegetables that grow underground are not roots at all. A stem has nodes, and at each node there is a bud or a leaf. A root never has buds or leaves. So look for buds, scale leaves and rings of nodes: if you find them, the organ is a stem, however deep it grows.
Definition
Stem or root (NCERT Class 11, Morphology of Flowering Plants):
- The root absorbs water and minerals from the soil through its root hairs; it has no nodes or buds.
- A stem has nodes and internodes, with buds at the nodes.
- Underground stems store food: potato (tuber; its eyes are nodes with buds), ginger and turmeric (rhizome), onion and garlic (bulb: a short disc-shaped stem with fleshy scale leaves), Colocasia (corm).
- Modified roots that store food: carrot, radish, turnip, beetroot and sweet potato.
- Some parts are modified leaves: the tendril of pea (a leaflet), the spines of cactus, the fleshy scales of onion.
| Plant part | What it really is |
|---|---|
| Potato | Underground stem (tuber); eyes are nodes with buds |
| Onion and garlic | Underground stem (bulb) with fleshy scale leavesQ CDS 2024 (I): potato, onion and garlic is the set of modified stems; carrot and turnip are roots. |
| Ginger and turmeric | Underground stem (rhizome) |
| Carrot, radish, turnip, sweet potato | Modified root |
| Pea tendril | Modified leaflet |
| Cactus spines | Modified leaves |
| Root hairs | The main absorbing surface of the plantQ CDS 2018 (II): the main absorbing organ of a plant is the root only. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2018 · CDS (II) 2018 — General Knowledge · Q33Moderate
Example 1 · Plant Biology · Plant Organs
Which one of the following features is an indication for modification of stem of a plant?
Underground does not mean root
Potato, ginger, onion and garlic grow under the soil but are stems. Carrot, radish, turnip and sweet potato are roots. Sweet potato and potato sit in different groups.
Onion scales are leaves, the bulb is a stem
The fleshy scales of an onion are leaves, so a scale is not a sign of a stem. The onion bulb as a whole is an underground stem, built on a short flat disc of stem at its base.
Concept 2 of 3: The leaf and its work
A leaf is the plant's kitchen. It is flat and thin to catch light, it has tiny pores to let gases in and out, and water evaporates from it all the time. Carrying water up from the roots and food down to them is not the leaf's job: the pipes of the stem and root do that.
Definition
The primary functions of a green leaf:
- Photosynthesis: making food.
- Exchange of gases through the stomata.
- Transpiration: loss of water as vapour, which also cools the leaf.
- Long-distance conduction of water and food is done by the xylem and phloem of the stem and root, not by the leaf.
- Phyllotaxy is the pattern in which leaves are arranged on the stem: alternate (one leaf at a node, as in mustard), opposite (two at a node, as in guava) or whorled (more than two at a node, as in Alstonia).
- Venation, the pattern of veins: reticulate (a net) in dicots, parallel in monocots such as grasses.
| Term | Meaning |
|---|---|
| Phyllotaxy | Arrangement of leaves on the stemQ CDS 2023 (II): phyllotaxy is the pattern of arrangement of leaves. |
| Alternate | One leaf at each node (mustard) |
| Opposite | Two leaves at each node, facing each other (guava) |
| Whorled | More than two leaves at a node (Alstonia) |
| Reticulate venation | Net of veins; dicots |
| Parallel venation | Veins side by side; monocots |
| Stomata | Pores for exchange of gases and loss of water vapour |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2018 · CDS (II) 2018 — General Knowledge · Q13Moderate
Example 2 · Plant Biology · Plant Organs
Which of the following is not a primary function of a green leaf?
Conduction is the stem's job
A leaf makes food, exchanges gases and loses water. Carrying water and food over long distances is the work of the xylem and phloem in the stem and root, so it is the one that is not a primary function of the leaf.
Phyllotaxy is leaves, not flowers
Phyllotaxy (phyllon means leaf) is the arrangement of leaves. The arrangement of flowers on a stalk has its own name, inflorescence.
Concept 3 of 3: Fruits and seed dispersal
After fertilisation the ovary of a flower swells into a fruit and the ovules inside become seeds. When only the ovary forms the fruit, it is a true fruit. When another part of the flower, usually the thalamus, also swells and makes the fleshy part, it is a false fruit. Then the seeds must travel away from the parent, and wind, water and animals carry them.
Definition
Fruits and seeds (NCERT Class 10 and 12):
- After fertilisation the ovary becomes the fruit and the ovules become the seeds.
- A true fruit develops only from the ovary: mango, tomato, pea pod.
- A false fruit has other floral parts in it, usually the thalamus (receptacle): apple, strawberry, cashew.
- A parthenocarpic fruit forms without fertilisation and has no seeds, such as the banana.
- Seed dispersal agents: wind (light seeds with wings or hairs: drumstick, maple, cotton), water (coconut, which floats), animals (hooks and spines as in Xanthium, or juicy fruits that are eaten), and explosion of the fruit (balsam, castor).
| Kind or agent | Examples |
|---|---|
| True fruit (from the ovary only) | Mango, tomato, pea podQ CDS 2018 (II): a true fruit is one where only the ovary grows into the fruit. |
| False fruit (thalamus included) | Apple, strawberry, cashew |
| Seedless (parthenocarpic) fruit | Banana |
| Dispersal by wind | Drumstick and maple (wings), cotton (hairs) |
| Dispersal by water | Coconut |
| Dispersal by animals | Xanthium (hooks), fleshy fruits that are eatenQ CDS 2018 (II): wind, water and animals disperse seeds; a fungus does not. |
| Dispersal by explosion | Balsam, castor |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2018 · CDS (II) 2018 — General Knowledge · Q14Moderate
Example 3 · Plant Biology · Plant Organs
Which one of the following denotes a 'true' fruit?
Thalamus and receptacle are the same part
The thalamus and the receptacle are two names for one part of the flower. A fruit that grows from either one is a false fruit. Only the ovary makes a true fruit.
Fungi do not disperse seeds
Seeds travel by wind, water and animals (and by bursting fruits). A fungus is a decomposer or a disease-maker; it has no way to carry seeds.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Modified roots and stems7 rows
| Plant part | What it really is |
|---|---|
| Potato | Underground stem (tuber); eyes are nodes with buds |
| Onion and garlic | Underground stem (bulb) with fleshy scale leavesQ CDS 2024 (I): potato, onion and garlic is the set of modified stems; carrot and turnip are roots. |
| Ginger and turmeric | Underground stem (rhizome) |
| Carrot, radish, turnip, sweet potato | Modified root |
| Pea tendril | Modified leaflet |
| Cactus spines | Modified leaves |
| Root hairs | The main absorbing surface of the plantQ CDS 2018 (II): the main absorbing organ of a plant is the root only. |
The leaf and its work7 rows
| Term | Meaning |
|---|---|
| Phyllotaxy | Arrangement of leaves on the stemQ CDS 2023 (II): phyllotaxy is the pattern of arrangement of leaves. |
| Alternate | One leaf at each node (mustard) |
| Opposite | Two leaves at each node, facing each other (guava) |
| Whorled | More than two leaves at a node (Alstonia) |
| Reticulate venation | Net of veins; dicots |
| Parallel venation | Veins side by side; monocots |
| Stomata | Pores for exchange of gases and loss of water vapour |
Fruits and seed dispersal7 rows
| Kind or agent | Examples |
|---|---|
| True fruit (from the ovary only) | Mango, tomato, pea podQ CDS 2018 (II): a true fruit is one where only the ovary grows into the fruit. |
| False fruit (thalamus included) | Apple, strawberry, cashew |
| Seedless (parthenocarpic) fruit | Banana |
| Dispersal by wind | Drumstick and maple (wings), cotton (hairs) |
| Dispersal by water | Coconut |
| Dispersal by animals | Xanthium (hooks), fleshy fruits that are eatenQ CDS 2018 (II): wind, water and animals disperse seeds; a fungus does not. |
| Dispersal by explosion | Balsam, castor |
Watch out for (6)
- Underground does not mean root→ Modified roots and stems
- Onion scales are leaves, the bulb is a stem→ Modified roots and stems
- Conduction is the stem's job→ The leaf and its work
- Phyllotaxy is leaves, not flowers→ The leaf and its work
- Thalamus and receptacle are the same part→ Fruits and seed dispersal
- Fungi do not disperse seeds→ Fruits and seed dispersal
Test yourself on Plant Biology
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.