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A short class with Sarah and her students. The other ways are under More.
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Look closely at this cactus. Can anyone find its leaves?
I can only see a thick green stem covered in spines.
Those spines are the leaves. They have been changed into thin, sharp points. Why might a desert plant do that?
Because broad leaves lose a lot of water through their stomata in the heat.
Exactly. Thin spines have much less surface for water to escape from. What else could spines be useful for?
Keeping away animals that want to eat the juicy stem.
If the leaves are spines, which part makes food for the cactus?
The stem, because it is green and has chlorophyll.
Right. Now feel how thick and fleshy it is. What is stored inside?
Water! The stem must be like a water tank.
Yes. Its spongy tissue holds water with the help of a sticky substance called mucilage. And these ribs let the stem swell after rain and shrink in dry months.
So the stem works like an accordion that fills up and squeezes in.
Most plants open their stomata in the day. A cactus usually opens them at night. Why?
Because nights are cooler, so less water evaporates through the open stomata.
Good. It takes in carbon dioxide at night, stores it as an acid and uses it for photosynthesis in the day with its stomata closed. Now, why are its roots shallow but spread wide?
So they can quickly soak up rain over a large area before it dries up.
Exactly. A waxy skin, few stomata and night-time breathing save water, the stem stores it, and the roots collect it fast.
the leaves of a cactus are reduced to spines, cutting water loss and protecting the stem.
the stem contains chlorophyll and takes over photosynthesis from the leaves.
the fleshy stem stores water in spongy tissue with mucilage, and its ribs let it swell and shrink.
a thick waxy coat and a small surface reduce transpiration.
stomata open at night to take in carbon dioxide, which is used for photosynthesis in the day (CAM).
shallow, wide roots soak up rainwater quickly before it is lost.
The prickly pear (Opuntia), common in dry parts of India, has flat green stem pads that make food, while its leaves are reduced to spines.
After a heavy shower, the ribbed stem of a barrel cactus swells as it stores water, and it slowly shrinks again in the dry months.
A cactus opens its stomata at night, when the air is cool, so it loses far less water than a plant that opens them in the hot afternoon.
A mango tree has broad leaves with many stomata, but a cactus has no broad leaves and a thick waxy stem, so it transpires much less.
Spines for leaves and a stem to store, stomata open when the sun's no more.
A thirsty camel once tried to nibble a cactus, but the spines stopped it. That night, while the camel slept, the cactus quietly opened its stomata to breathe in the cool air, and when the rare rain came, its wide roots drank it up and its ribbed stem swelled like a balloon.
"Spines save, stems store, stomata snooze by day": the three main water-saving tricks of a cactus.
C.A.C.T.U.S. - Cuticle thick, Acid store of carbon dioxide at night, Chlorophyll in the stem, Thorny spines for leaves, Underground roots spread wide, Stem stores water.
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What are the spines of a cactus?
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Which part of a cactus carries out photosynthesis?
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What helps the cactus stem hold water?
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When does a cactus usually open its stomata, and why?
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Why are many cactus roots shallow but widespread?
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A plant adapted to survive in very dry conditions, such as a cactus.
A sharp, pointed structure; in a cactus, a leaf that has been modified to reduce water loss and give protection.
A waxy layer on the outer surface of a plant that reduces water loss.
Tiny pores, mainly in leaves or green stems, through which gases enter and leave and water vapour escapes.
The loss of water as vapour from the aerial parts of a plant, mainly through the stomata.
A thick, sticky substance in the cactus stem that helps it hold water.
Crassulacean Acid Metabolism, a way of photosynthesis in which stomata open at night and carbon dioxide is stored for use in the day.
A flattened green stem that looks and works like a leaf, as in the prickly pear (Opuntia).
The cactus is the classic desert plant: its spines, fleshy green stem, waxy skin, night-time stomata and spreading roots help it collect, store and save water where rain is rare.
This lesson uses the cactus as a worked example of the desert adaptations described in the case study. It explains why leaves turned into spines cut down transpiration, how the green stem takes over photosynthesis and stores water, how a thick waxy cuticle and stomata that open at night save water, and how wide-spreading roots catch rainwater quickly. Each adaptation is linked back to the chapter's main ideas: how water is absorbed by roots, carried up through the xylem, lost through stomata, and how food made in green parts is carried through the phloem.
In a desert, rain is rare, days are hot and the air is dry. A plant absorbs water through its roots, carries it up through the xylem and loses most of it as water vapour through the stomata in transpiration. In hot, dry air, transpiration is very fast, so an ordinary leafy plant would soon wilt. The cactus survives because almost every part of it is built to collect water quickly, store it and lose as little as possible.
The sharp spines of a cactus are modified leaves. Broad leaves have a large surface and many stomata, so they lose a lot of water. By reducing its leaves to thin spines, the cactus greatly cuts down the surface through which water can escape. The spines also protect the juicy stem from thirsty animals, and in some cacti a dense covering of spines shades the stem and slows the drying wind near its surface.
With no proper leaves, the stem takes over their work. It is green because it contains chlorophyll, so it carries out photosynthesis and makes the plant's food. In the prickly pear (Opuntia), which grows in dry parts of India, the stem is flattened into green pads that look and work like leaves. The stem is also thick and fleshy. Its spongy tissue stores large amounts of water, held by a thick, sticky substance called mucilage. Many cacti have ribs or pleats along the stem that let it swell after rain and shrink during drought, like an accordion.
The stem is covered by a thick, waxy cuticle that water cannot easily pass through. The cactus also has relatively few stomata. Its rounded or column-like shape gives it a small surface compared with the volume of water it holds, which also reduces water loss.
Most plants open their stomata in the daytime to take in carbon dioxide. A cactus usually does the opposite. Its stomata open at night, when the air is cooler and less water evaporates, and the carbon dioxide taken in is stored in the cells as an acid. In the daytime the stomata stay closed, and the stored carbon dioxide is used for photosynthesis in sunlight. This way of photosynthesising is called CAM (Crassulacean Acid Metabolism).
Many cacti have shallow roots that spread widely just below the surface of the soil. When a shower falls, these roots soak up water quickly before it evaporates or drains away. Some cacti also have a deeper taproot, and other desert plants rely mainly on very deep taproots to reach water far underground.
Water absorbed by the roots still travels up through the xylem, but much of it is stored in the stem instead of being lost through stomata. Because transpiration is so slow, the cactus uses its water very sparingly. Food made in the green stem is carried by the phloem to the roots and other growing parts. The cactus shows how a plant can change the usual pattern of absorbing, carrying and losing water to survive in a harsh place.