B15.8 Adaptations in Plants

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: how plants survive hot, dry, windy, shady and nutrient-poor places. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain why water loss is a problem for plants in hot, dry or windy places
• Describe and explain the adaptations of cacti
• Describe adaptations of other plants, such as marram grass, shade plants and carnivorous plants
• Calculate rates of water loss from plant data

Key words

• Transpiration: The loss of water vapour from the leaves, mainly through the stomata.
• Stomata: Tiny pores, mostly on the underside of leaves, that let carbon dioxide in and water vapour out. Each is opened and closed by two guard cells.
• Waxy cuticle: A waterproof layer on the outer surface of a leaf or stem.
• Succulent: A plant with thick, fleshy parts that store water, such as a cactus.
• Spines: Narrow, pointed leaves with a very small surface area.

The water problem

Plants need water for photosynthesis, to keep cells firm, and to carry mineral ions. They lose water by transpiration: water evaporates inside the leaf and diffuses out through the open stomata.

Transpiration is faster when it is hot, dry, windy or bright. In a desert, water is scarce and conditions make water loss fast, so plants must cut water loss and collect as much water as possible.

Key fact: Every plant has a trade-off: stomata must open to let in carbon dioxide for photosynthesis, but open stomata let water escape.

Cacti

Cacti are the best-known desert plants. Almost every part of a cactus is adapted to save or store water.

• leaves reduced to spines; How it helps very small surface area, so little water lost; spines also stop animals eating the cactus for its water
• swollen, fleshy, green stem; How it helps stores water after rain, and carries out photosynthesis instead of the leaves
• thick waxy cuticle; How it helps waterproof, so less evaporation from the surface
• few stomata, often sunken; open at night; How it helps less water vapour escapes; night air is cooler and more humid
• shallow, wide roots and/or very deep roots; How it helps absorb rain quickly over a large area, or reach water far below the surface
• rounded, compact shape; How it helps small surface area to volume ratio, so less water lost

Other plant adaptations

Marram grass grows on sand dunes, where it is windy and the sand drains fast. Its leaves roll up in dry weather, so the stomata are on the inside, surrounded by trapped, moist air. Less water diffuses out.

• Conifers such as pines have needle-shaped leaves with a thick cuticle, which lose little water in cold, dry winds.
• Shade plants on a woodland floor often have large, thin, dark green leaves with lots of chlorophyll to absorb as much of the dim light as possible.
• Carnivorous plants such as sundews grow in boggy, nutrient-poor soil. They trap and digest insects to get the mineral ions (e.g. nitrate) that the soil lacks.

Worked example: rate of water loss

A potted cactus and its pot have a mass of 528.0 g. After 18 hours the mass is 524.4 g.
Water lost = 528.0 − 524.4 = 3.6 g. Rate = 3.6 ÷ 18 = 0.2 g per hour.

Answer: 0.2 g/h

Exam tip: To compare leaves of different sizes, divide the water lost by the surface area (e.g. mg per cm² per hour).

Common mistakes

• Saying cacti have “no leaves”. Their leaves are reduced to spines.
• Saying a waxy cuticle “stops all water loss”. It reduces evaporation; some water is still lost.
• Forgetting the cost: closing stomata also reduces carbon dioxide uptake, so photosynthesis slows.
• Comparing total water loss from leaves of different sizes without dividing by area.

Revision questions

Try these, then check your answers below each one.

Question 1. Why do cacti have spines instead of broad, flat leaves? (Choose from: Spines have a small surface area, so less water is lost, Spines carry out more photosynthesis, Spines store water, Spines absorb water from the air.)

Answer: Spines have a small surface area, so less water is lost

Working: Spines are leaves with a tiny surface area, so little water is lost by transpiration. They also stop animals eating the cactus.

Question 2. True or false? Opening its stomata at night instead of during the day helps a cactus reduce water loss.

Answer: True

Working: True. At night the air is cooler and more humid, so less water evaporates through the open stomata.

Question 3. A potted plant had a mass of 315.0 g. After 9 hours its mass was 309.6 g. Calculate the mean rate of water loss in g per hour.

Answer: 0.6 g/h

Working: 315.0 − 309.6 = 5.4 g; 5.4 ÷ 9 = 0.6 g/h.

Related lessons

• Previous lesson: B15.7 Adaptation in Animals
• Next lesson: B15 RP Required Practical: Field Investigations
• In this chapter: B15.1 The Importance of Communities
• In this chapter: B15.2 Organisms in Their Environment
• In this chapter: B15.3 Distribution and Abundance

Continue in the full course

This lesson is section B15.8 (AGCSC1508) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.