▶️ Answer/Explanation
(a) Adaptation refers to the process resulting from natural selection where populations become better suited to their environment over many generations.
Explanation: Adaptations are characteristics that help organisms survive and reproduce in their specific environment. These traits develop over long periods through natural selection, where individuals with beneficial traits are more likely to survive and pass on their genes. In the case of xerophytes, their adaptations help them conserve water in arid environments.
(b)
Feature 1: Spines/needles/thorns
Explanation: These structures reduce the surface area of the plant, minimizing water loss through transpiration. They also serve as a defense mechanism against herbivores that might otherwise consume the plant’s precious water reserves.
Feature 2: Fleshy/thick/swollen stem
Explanation: The enlarged stem acts as a water storage organ, allowing the cactus to store large amounts of water during rare rainfall events and slowly utilize it during prolonged dry periods.
(c)(i) 33,600 stomata
Explanation: Calculation: 8 cm² = 800 mm² (since 1 cm² = 100 mm²). The ice plant has 42 stomata per mm² on its lower surface, so total stomata = 800 × 42 = 33,600.
(c)(ii) Xerophytes generally have fewer stomata than non-xerophytes to reduce water loss through transpiration. The stomata are often concentrated on the lower leaf surface which is cooler and more shaded, further minimizing water loss. The data shows that xerophytes like the tongue leaf plant and ice plant may have no stomata on their upper surfaces at all.
Detailed Explanation: The table demonstrates how xerophytes have evolved to minimize water loss while still allowing for gas exchange. The oak tree (non-xerophyte) has many more stomata (503 per mm² on lower surface) compared to the xerophytes (15-42 per mm²). Some xerophytes like the tongue leaf plant have no stomata on their upper surface at all (0 per mm²). This adaptation reduces transpiration while still allowing some gas exchange through the fewer stomata on the cooler, shaded lower surface.
(d) Reasons to conserve xerophytic ecosystems include:
- Maintaining biodiversity and genetic diversity
- Preventing extinction of unique species
- Ensuring stability of food chains and nutrient cycling
- Protecting vulnerable ecosystems that may have important ecological functions
Detailed Explanation: Xerophytic ecosystems, while appearing harsh, support unique biodiversity that may have undiscovered benefits for medicine, science, or agriculture. They often contain species found nowhere else. Conservation helps maintain ecological balance, protects potential resources, and preserves these unique environments for future generations. Additionally, many xerophytic plants are important carbon sinks and help stabilize soils in fragile environments.
▶️ Answer/Explanation
(a) An inherited (structural) feature that helps an organism to survive or reproduce in its environment.
Explanation: Adaptive features are characteristics that organisms develop over generations through natural selection. These features enhance the organism’s ability to survive in its specific environment, such as the water lily’s floating leaves that allow it to thrive in aquatic conditions.
(b)(i)
A – palisade mesophyll
B – spongy mesophyll
C – air spaces
Explanation: The palisade mesophyll (A) contains tightly packed chloroplast-rich cells for photosynthesis. The spongy mesophyll (B) has loosely arranged cells with air spaces (C) between them to facilitate gas exchange.
(b)(ii) The air spaces reduce the density of the leaf, allowing it to float on water and be near the surface where it can access light and carbon dioxide for photosynthesis.
Explanation: The extensive air spaces in water lily leaves serve multiple adaptive functions. They make the leaf buoyant, keeping it at the water’s surface where sunlight is abundant. This positioning also ensures access to atmospheric CO₂ for photosynthesis while the lower surface remains submerged.
(c)(i)
Tomato plants have more stomata on their lower epidermis (129/mm²) than upper epidermis (10/mm²), while water lilies have all stomata on the upper epidermis (475/mm²) and none on the lower epidermis.
Tomatoes reduce water loss by having most stomata on the shaded lower surface. Water lilies, floating on water, don’t need to conserve water and have stomata only on the upper surface exposed to air.
Explanation: Terrestrial plants like tomatoes evolved stomatal distribution to minimize water loss through transpiration, hence more stomata on the cooler, shaded lower surface. Hydrophytes like water lilies, with constant water supply, maximize gas exchange through numerous upper-surface stomata since their lower surface is underwater where stomata would be useless.
(c)(ii) Guard cells
Explanation: Guard cells are specialized kidney-shaped cells that flank each stoma. By changing their turgor pressure through osmotic water movement, they can open or close the stomatal pore, regulating gas exchange and transpiration.