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How do cacti store water?

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Quick answer

Cacti store water in their thick, fleshy stems. Shallow roots soak up rain quickly, and the water is held in spongy storage cells and slimy mucilage. Pleated ribs let the stem swell when full. Waxy skin, spines instead of leaves and breathing mostly at night stop the stored water evaporating in the desert heat.

Cacti store water in thick, fleshy stems packed with spongy tissue whose cells and gel-like mucilage soak up and hold moisture after rain. Pleated ribs let the stem swell and shrink like an accordion, while waxy skin, spines instead of leaves and night-time gas exchange keep that stored water from escaping.

Step by step

  1. When rain falls, a cactus's shallow spreading roots absorb water rapidly from the topsoil.
  2. The water flows up into the stem and fills large storage cells and water-holding mucilage.
  3. The stem's ribs unfold and the stem visibly swells as its water reserves increase.
  4. Waxy skin and spines minimise surface water loss while the stomata stay closed through the hot day.
  5. During long droughts the plant slowly uses its stored water, and the ribs fold inward as the stem shrinks.

Key things to understand

  1. Shallow, widespread roots quickly absorb rainwater from a large area of soil before it evaporates or drains away.
  2. Water moves into the stem, where large storage cells and sticky mucilage hold it like a sponge.
  3. Ribs or pleats along the stem expand as it fills and contract as water is used, without the skin tearing.
  4. A thick waxy cuticle and leaves reduced to spines greatly cut water lost from the plant's surface.
  5. Most cacti open their stomata at night, taking in carbon dioxide when cooler air means less water evaporates.

Labelled diagram

  1. Shallow roots

    Spread widely just below the surface to catch brief rainfall quickly.

  2. Storage tissue

    Spongy cells and mucilage inside the stem hold absorbed water.

  3. Pleated ribs

    Fold and unfold so the stem can swell or shrink safely.

  4. Waxy skin

    Thick cuticle seals the surface against evaporation in hot sun.

  5. Spines

    Modified leaves that lose little water and deter grazing animals.

How a cactus collects and keeps water

Real-world examples

  • Saguaro cacti in the Sonoran Desert visibly fatten after summer storms as their pleated stems fill with water.
  • Prickly pear, known in India as nagphani, grows as a hardy hedge plant across dry parts of Rajasthan and the Deccan.
  • A potted barrel cactus on a sunny balcony can survive weeks without watering by drawing on its stem reserves.
  • Cactus-like euphorbias lining fields in the Thar Desert store water the same way despite not being true cacti.

Common mistake

✗ Myth: A cactus is full of drinkable water you can tap in a desert emergency.

✓ Fact: The water is bound in fibrous, often bitter pulp rather than a liquid pool, and many species contain compounds that cause vomiting. Eating it can worsen dehydration instead of relieving it.

People also ask

How does a cactus survive without water?
A cactus survives dry spells by drawing on water stored in its fleshy stem after rain. It limits losses with a waxy skin, spines instead of broad leaves and stomata that open only at night. Its shallow roots then refill the stem quickly whenever the next rain arrives.
Where do cactus store water?
Cacti store water in their stems, not in leaves, which are reduced to spines. Inside the stem, large thin-walled storage cells and thick mucilage hold the water. Some species also store water in swollen underground roots, and pleated ribs allow the stem to expand as it fills.
Why do cactus have thick stems?
A thick stem gives a cactus a large volume for storing water relative to its surface area, which reduces evaporation. Because the leaves are reduced to spines, the green stem also takes over photosynthesis, so one organ handles water storage, food making and protection against heat.
Is it safe to drink water from a cactus?
Generally no. Cactus pulp is not a reservoir of clean water, and in many species it contains acids and alkaloids that can cause vomiting or diarrhoea, worsening dehydration. Survival experts advise against relying on it.
Why do cacti have spines instead of leaves?
Spines are modified leaves that lose almost no water because they have no stomata or broad surface. They also deter thirsty animals from eating the moist stem, and dense spines can shade the stem and trap a layer of still air.
How do cacti photosynthesise at night?
They use crassulacean acid metabolism, or CAM. At night, stomata open and carbon dioxide is stored as malic acid. During the day stomata close, and the acid releases carbon dioxide inside the stem for photosynthesis.
Are all spiny desert plants cacti?
No. Many succulent euphorbias in Africa and India look remarkably like cacti but belong to a different plant family. They evolved similar water-storing stems independently, an example of convergent evolution.

Part of the guide: Nature & Earth: Living Things and Rocks →