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Adaptations of Plants in Different Habitats, worked answers

Fully worked answers for Adaptations of Plants in Different Habitats, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Matching a plant adaptation to its habitat and the specific problem it solves.
Comparing xerophytes and hydrophytes in a table of features and the problem each one addresses.
Explaining how a named feature reduces water loss or increases buoyancy, in full biological terms.
Identifying whether an unfamiliar plant is more likely a xerophyte, hydrophyte or mesophyte from a description of its features.
Explaining why a mesophyte does not need the extreme adaptations of a xerophyte or hydrophyte.
Linking a structural feature to the underlying process it affects, such as transpiration or gas exchange.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

Explain how sunken stomata help a xerophyte reduce water loss.

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**Sunken stomata** are located in pits or grooves below the leaf surface, which **traps a layer of moist air** close to the stomata. This moist air pocket **reduces the difference in water vapour concentration** between the leaf's internal air spaces and the surrounding air, so **less water diffuses out** through transpiration.

sunken stomatapits or groovestraps moist airreduces concentration differenceless water diffuses out

Explain why a cactus has leaves reduced to spines, and state which part of the cactus performs photosynthesis instead.

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A cactus lives in a **dry habitat** where a broad leaf would lose too much water through **transpiration**. By reducing its leaves to **spines**, the cactus greatly **lowers its surface area** and therefore its water loss. Because it no longer has broad leaves, its **thick, green stem** takes over **photosynthesis** and also **stores water** in its fleshy tissue.

dry habitatspineslowers surface areathick green stemphotosynthesisstores water

Explain two different root adaptations that allow different xerophytes to obtain enough water in a dry habitat.

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Some xerophytes have an **extensive, shallow root system** that spreads widely just below the surface to **absorb rainwater quickly** before it evaporates or drains away. Other xerophytes instead grow a very **long taproot** that penetrates deep into the soil to reach a **permanent water table**. Both solve the same problem of obtaining water in a dry habitat, but suit different patterns of rainfall.

shallow root systemabsorb rainwater quicklylong taprootpermanent water tablesame problemdifferent rainfall patterns

Explain why large air spaces (aerenchyma) are important to a hydrophyte.

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**Aerenchyma**, large air spaces running through the stem and leaves, provides **buoyancy**, keeping leaves and flowers near the **water surface** for light and pollination. It also **stores oxygen**, which slowly diffuses to **underwater tissue** that cannot exchange gases directly with the surrounding water.

aerenchymabuoyancywater surfacestores oxygenunderwater tissue

State and explain why a hydrophyte does not need a thick cuticle or well-developed supporting tissue.

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A hydrophyte has a **thin cuticle** because it lives **in or on water** and is never at risk of **drying out**, so it does not need protection against water loss. It also has **reduced supporting tissue** because the **surrounding water itself holds the plant up**, buoying its stems and leaves so little mechanical strength is required.

thin cuticlein or on waternever dries outreduced supporting tissuewater holds plant up

A mesophyte does not show the extreme adaptations of a xerophyte or hydrophyte. Explain why.

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A mesophyte grows in a habitat with a **normal, moderate and reliable water supply**, so it faces **neither severe water shortage nor an excess of water**. Because it does not need to solve either extreme problem, it shows a **balanced set of features** instead of the extreme adaptations seen in xerophytes or hydrophytes.

normal moderate water supplyneither shortage nor excessbalanced set of features

Explain what would happen if a typical mesophyte were planted in a desert habitat without any adjustment.

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A typical mesophyte lacks the features that reduce water loss in a **xerophyte**, such as a **thick cuticle** or **sunken stomata**. In a desert, it would **lose water faster than its roots can replace it** through transpiration, and without enough water to keep its cells **turgid**, it would eventually **wilt and die**.

lacks xerophyte featuresloses water faster than replacedcells not turgidwilt and die

Phrasing that earns marks

Adaptation: 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: 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: 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: 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.
Structure suits function in habitats: 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.
Reducing water loss: 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.
Storing water: 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.
Buoyancy and gas exchange in water plants: 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.

Frequently asked questions

What are the adaptations of a xerophyte?

A xerophyte lives in a dry habitat and is adapted to reduce water loss. Common features include a thick, waxy cuticle, leaves reduced to spines to lower the surface area, sunken stomata that trap moist air, fewer stomata, hairs on the leaf surface, and thick stems or tissues that store water. Together these keep the plant alive where water is scarce. Being able to state the problem behind each feature, not just list them, is what earns full marks.

How is a water plant (hydrophyte) adapted to its habitat?

A hydrophyte lives in or on water and does not risk drying out, so it has a thin cuticle and can have stomata on the upper surface of floating leaves. It often has large air spaces in its tissues to help it float and to store gases, and little supporting tissue because the water holds it up. These same features would be a serious disadvantage on dry land, which shows how closely each adaptation matches its own habitat.

Why does a cactus have spines instead of broad leaves?

Broad leaves have a large surface area, which would lose a lot of water by transpiration in a hot, dry habitat. By reducing its leaves to spines, a cactus greatly lowers the surface area and therefore water loss, while its thick green stem carries out photosynthesis and stores water. This trade-off between surface area and water conservation is the core idea behind almost every xerophyte adaptation.