Leaf structure and plants in extreme environments

Leaves are built for photosynthesis and gas exchange. Plants in very dry places have extra adaptations to save water.

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Inside a leaf

Waxy cuticle: reduces water loss.
Upper epidermis: thin and transparent, so light passes through.
Palisade mesophyll: tall cells near the top, packed with chloroplasts to absorb as much light as possible.
Spongy mesophyll: loosely packed cells with air spaces, so gases diffuse easily.
Stomata (mostly on the lower surface) let carbon dioxide in and oxygen and water vapour out. Guard cells open and close them.
Veins: xylem brings water; phloem carries sugars away.
Leaves are broad (large surface area for light) and thin (short distance for gases).

Surviving dry conditions

Plants such as cacti live where water is scarce:
Spines instead of leaves reduce the surface area for water loss.
Thick waxy cuticle reduces evaporation.
Fewer stomata, often sunken in pits, and in some species opened only at night when it is cooler.
Swollen stems store water.
Long or widespread roots reach deep water or collect rain quickly.

Other examples

Marram grass on sand dunes has rolled leaves with stomata on the inside, and hairs. These trap moist air, which reduces the concentration gradient so less water is lost.
In general, small or narrow leaves lose less water than large, broad ones.
Worked example

Explain how rolled leaves help marram grass survive on dry sand dunes.

  1. The stomata are on the inside of the rolled leaf.
  2. Moist air is trapped inside the roll.
  3. The water vapour concentration gradient is smaller, so less water diffuses out.

Answer: Rolling traps humid air around the stomata, reducing the concentration gradient, so less water is lost by transpiration.

Key idea

Leaves have a waxy cuticle, palisade cells full of chloroplasts, air spaces for diffusion and stomata. Desert plants reduce water loss with spines, thick cuticles, fewer or sunken stomata, water-storing stems and long roots.

Key words
Photosynthesis
The process in which plants use light energy to make glucose and oxygen from carbon dioxide and water.
Diffusion
The net movement of particles from a higher to a lower concentration. It is passive and needs no energy from respiration.
Stomata
Pores, mostly on the underside of leaves, that let gases in and out. Guard cells open and close them.
Xylem
Dead, hollow plant tubes strengthened with lignin that carry water and mineral ions up from the roots.
Phloem
Living plant tissue that carries sucrose and amino acids around the plant in both directions (translocation).

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Extra practice

2 more questions

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  1. Why are palisade cells tightly packed near the top of the leaf?

    1. To store water
    2. To allow gas exchange
    3. To carry sugar
    4. To absorb as much light as possible
    Show the answer

    Answer: To absorb as much light as possible

    More chloroplasts near the light.

  2. How do the air spaces in the spongy layer help photosynthesis?

    1. They store water
    2. They carry sugars
    3. They absorb light
    4. Carbon dioxide can diffuse to the cells
    Show the answer

    Answer: Carbon dioxide can diffuse to the cells

    Gases move quickly through air spaces.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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