Adaptations of Plants in Different Habitats, practice questions

B, Xerophytes live in dry habitats, where the central challenge is minimising water loss through transpiration.

B, The moist air pocket around sunken stomata lowers the rate of water loss by transpiration.

C, Because its leaves are reduced to spines, a cactus's thick green stem takes over photosynthesis.

C, Aerenchyma provides buoyancy and gas storage, a feature specific to hydrophytes.

B, The upper surface stays in contact with air, so stomata there can function normally.

C, A mesophyte faces neither severe water shortage nor an excess of water, so it shows balanced features.

B, The water around a hydrophyte buoys its stems and leaves, so little structural support is needed.

B, A long taproot penetrates deep enough to reach a permanent water table below the dry surface soil.

B, Aerenchyma also stores oxygen that diffuses to tissue that cannot exchange gases with the surrounding water.

C, This combination of features reduces water loss and is characteristic of a xerophyte.

• (a) The plant is most likely a xerophyte, adapted to a dry habitat.
• (b) The thick, waxy cuticle physically blocks water from evaporating through the surface, reducing water loss.
• (b) The sunken stomata trap a layer of moist air close to the stomata, lowering the rate of water loss by transpiration.
• (c) The stem most likely carries out photosynthesis, because the leaves have been reduced to spines and can no longer perform this role effectively.

• (a) The plant is a hydrophyte.
• (b) The air spaces (aerenchyma) provide buoyancy, keeping the leaves and flowers near the water surface for light and pollination.
• (b) The air spaces also store oxygen that diffuses slowly to underwater tissue, which cannot exchange gases directly with the surrounding water.
• (c) The upper surface is in contact with air, so stomata there can exchange gases normally; the lower surface is in contact with water, where stomata would be blocked and unable to function.

• (a) Species P's shallow, widely spreading roots can absorb surface rainwater quickly over a large area before it evaporates or drains away.
• (a) Species Q's long taproot grows deep enough to reach a permanent water table far below the surface, which is available even when the surface soil is completely dry.
• (b) Species P's root system would be more effective after a light, brief rainfall that wets only the upper soil layer, since a taproot reaching deep water would gain no additional benefit from that shallow moisture.

An adaptation is a structural, physiological or behavioural feature that helps an organism survive and reproduce successfully in its particular habitat. In this chapter, adaptation almost always refers to a visible structural feature of a plant that solves a specific problem posed by its habitat. Examples include a thick cuticle in a xerophyte or large air spaces in a hydrophyte, each solving a different survival problem.

Xerophytes are plants adapted to dry habitats, where the main challenge is minimising water loss through transpiration. Typical features include a thick, waxy cuticle, sunken stomata, leaves reduced to spines or needles, fewer stomata overall, hairy leaf surfaces, and fleshy stems or leaves that store water.

Hydrophytes are plants adapted to living in or on water, where drying out is not a risk but staying afloat and obtaining oxygen can be. Typical features include large internal air spaces for buoyancy and gas storage, a thin cuticle, stomata on the upper surface of floating leaves, and reduced supporting tissue.

Mesophytes grow in habitats with a normal, moderate water supply and show a balanced set of features rather than the extreme adaptations seen in xerophytes or hydrophytes. They are often used as the reference point when comparing the more extreme adaptations of the other two groups. Most common flowering plants, such as hibiscus or mango trees, belong to this mesophyte group.

Every adaptation in this chapter solves a specific habitat problem: sunken stomata trap a layer of moist air to slow transpiration in a dry habitat, while large air spaces provide buoyancy in a water habitat. Marks are given for stating the problem, not just naming the feature. This same principle applies well beyond this chapter, across almost every topic in plant and animal physiology in the syllabus.

Xerophyte features work by reducing the surface area exposed to air, trapping moist air close to the stomata, or reducing the number and activity of stomata themselves. A thick cuticle also physically blocks water from evaporating through the leaf or stem surface.

Some xerophytes, such as cacti, store water in fleshy stems or leaves to survive long periods without rain. This stored water is used gradually between rare rainfall events, which is why such plants can survive in habitats other plants cannot. This water-storage tissue is usually found in a thick, fleshy stem or leaf, making it easy to identify in a diagram.

Hydrophytes rely on large air spaces (aerenchyma) in their stems and leaves both to keep leaves and flowers at or above the water surface for light and pollination and to store oxygen for underwater tissue, which cannot easily obtain gases directly from the water.

Without the right adaptations, a plant introduced to a habitat different from its own would lose too much water, fail to obtain enough light or oxygen, or be unable to reproduce successfully. Adaptations are the reason a wide range of habitats, from deserts to lakes, can support plant life at all. This also explains why moving a plant to an unsuitable habitat, without any adjustment, usually causes it to wilt and die.

The two extreme groups are opposites in almost every feature: xerophytes minimise water loss with sunken stomata and thick cuticles, while hydrophytes have no such restriction and instead maximise gas exchange and buoyancy. This direct contrast is the single most common way this chapter is examined.