UPSC Prelims 2018 Q2: Environment & Ecology – Ecosystems & Ecology Concepts
- (a) 2 and 3 only
- (b) 2 only
- (c) 3 only
- (d) 1, 2 and 3
Answer: (d) 1, 2 and 3
Explanation
Xerophytic Adaptations: Desert Plant Leaf Modifications
Plants in arid zones (xerophytes) have evolved multiple leaf modifications to reduce transpiration water loss:
| Modification | Present in deserts? | Mechanism |
|---|---|---|
| 1. Hard and waxy leaves | YES | Thick cuticle with wax layer reduces evaporation. Examples: Nerium (oleander), Calotropis, Aloe. |
| 2. Tiny leaves | YES | Smaller surface area to volume ratio cuts transpiration. Examples: Casuarina (scale-like leaves), Tamarix, Acacia. |
| 3. Thorns instead of leaves | YES | Leaves modified into spines (as in Opuntia/cactus, Euphorbia); photosynthesis shifts to the green fleshy stem (phylloclade). |
All three are classic xerophytic strategies described in NCERT Biology (Class 11 - Morphology of Flowering Plants). Other adaptations include sunken stomata, CAM photosynthesis, deep tap roots, hairy leaves and succulence.
Answer: (d) 1, 2 and 3.
Why each option is right or wrong
- (a) Incorrect. Hard, waxy leaves (modification 1) are also a classic desert adaptation, seen in Nerium, Aloe and Calotropis.
- (b) Incorrect. Tiny leaves alone is not the complete answer - waxy coating and thorns are equally important.
- (c) Incorrect. Thorns alone leave out the equally common waxy-cuticle and tiny-leaf strategies.
- (d) CORRECT. All three - waxy leaves, tiny leaves and thorn modification - are established xerophytic adaptations to inhibit water loss.
Key point
Xerophyte checklist: Thick waxy cuticle, tiny/reduced leaves, spines (thorns), sunken stomata, CAM pathway, succulence, deep roots. Cactus (Opuntia) shows spines + phylloclade + CAM together.
Remember
Desert plants (xerophytes) reduce water loss through several leaf modifications: thick waxy cuticles (Nerium, Aloe), reduced or tiny leaves (Casuarina, Acacia) and complete conversion of leaves into thorns while photosynthesis moves to green stems (Cactus, Euphorbia). All three modifications listed in this question are well-established xerophytic adaptations.
Common mistakes
- Assuming thorns are only for defence against herbivores - in xerophytes their primary role is drastic reduction of transpiring surface area.
- Confusing phylloclade (modified stem doing photosynthesis, as in Opuntia) with cladode (modified stem branch, as in Asparagus).
- Thinking waxy cuticle is only present in aquatic plants - in xerophytes the wax layer is much thicker to prevent water loss, not entry.
Exam tip
Rule: Any leaf modification that cuts surface area or blocks evaporation qualifies as a desert adaptation. Waxy layer, tiny leaves, thorns and hairs all satisfy this.
Sources
- NCERT Class 11 Biology - Morphology of Flowering Plants (Chapter 5)
- NCERT Class 12 Biology - Organisms and Populations (Chapter 13)