Environmental adaptations

What are succulents?

Succulent plants come in all different shapes and sizes, and they are quite commonplace in the world. Around 3-5% of all flowering plants are succulents.

They can be tiny plants growing in between the cracks of rocks, long plants dangling from the branches of trees, or even growing into trees themselves like the magnificent baobab.

Despite the differences in how they grow, succulents have one thing in common. They have all adapted, through the process of evolution by natural selection, to store more water than most other plants.

Sometimes succulent plants have specialised cells for storing water. Water storage cells don’t have the green chlorophyll that allows plants to make their energy for photosynthesis. These cells have elastic walls, so they can expand to hold more water when the rains finally come. Other succulents are able to retain more water in their normal cells.

This means that succulents can survive in environments that would kill many plants, particularly places that are arid and have unpredictable or low rainfall.

If you think about where cacti and succulents live, perhaps you would picture a hot, dry desert. But succulents do not just grow in environments like this! In fact, many true deserts, like the Sahara, are too dry all year round even for succulent plants.

The best environments for succulents have predictable rainfall, even if it is in small amounts. Semi-deserts that are arid but do experience rain are usually rich with succulents.

Succulents grow outside of the tropics as well, for example in mountainous regions, or on volcanoes like the Canary Islands. Succulent plants can be found on every continent except Antarctica – there are even some native to the UK!

Almost all cacti are native to the Americas. This suggests they evolved after the prehistoric supercontinent of Gondwana split up, separating the Americas from the rest of the world. The few cacti which do live in the Old World (Rhipsalis baccifera) might have been carried across the ocean by birds: this is still being researched.

Things to think about

• Why would a plant living on a tall, cold, European mountain evolve to become succulent?Higher altitudes are cold, topped with ice and snow.  Frozen water is not available for the plant to use, making these places dry like a desert.
• Why might a volcanic island be an arid environment?Volcanoes create new igneous rocks. As they are young they haven’t broken down much yet, so there isn’t much soil and water drains off quickly.

Cacti and succulents – what’s the difference?

You can categorise succulents by which part of the plant has the water storing parts. Stem succulents have increased water storage in their stems, and leaf succulents have increased water storage in their leaves. A third type, called caudiciform succulents have a special structure called a caudex, which is a swollen base to the plant.

Cacti are just one of the many kinds of stem succulent. All cacti are succulents, but not all succulents are cacti! Most cacti have lost their leaves through the process of evolution and photosynthesise through their stems instead.

Shrinking succulents

Another adaptation in succulents that helps them to manage periods of drought is their ability to shrink down when dry and refill their tissues with relatively little damage. Cacti with ribs are particularly well-adapted to this. The ribs act a bit like a concertina, being able to close up when the cactus is shriveling and folding open again when it is well-watered.

A side-effect of plants shrinking is that they can end up sinking into the ground. When this happens, much of the plant will be protected from the sun, exactly when it needs it!

Some cacti, like Blossfeldia liliputana, can “come back to life” when they have completely dried up, with no water at all in their cells (which would kill almost all other living things)! We call plants with this ability resurrection plants.

Things to think about

• Can you think of another reason why a cactus might want to “keep a low profile” and stay close to the ground?Staying low down makes it more difficult for grazing animals to eat the plant.
• What do you think might happen if a cactus suddenly got a lot of water after it had been very shrivelled up?Rehydrating too quickly can cause the cactus to split. This usually doesn’t harm it as it dries over and heals but it does make the plant less attractive if you grow them!

Shady spines

Cactus spines help to protect plants from being eaten, but they often serve a second purpose. As well as the large protective spine that sticks out of the centre of the areole, many cacti have a circle of spines that lie more parallel to the skin of the cactus. These radial spines are good at reducing the amount of direct sunlight the plant gets (a bit like sitting under a shade mesh).

A second effect of having lots of spines is that they can help to catch water vapour (dew) when things cool down at night. When the water droplets are large enough, they run down the plant and trickle to the base and the roots.

Things to think about

• What colour of spine would be best for reflecting sunlight (think about how hot you get in certain clothes in summer)?White is the most reflective colour.

Weatherproof wax

Another way of reducing your exposure to the sun is putting on some suntan lotion. Some succulent plants have their own version of this, as well! They develop a layer of dusty-looking wax over their leaves, called farina or epicuticular wax. The wax reduces the amount of ultraviolet radiation that gets to the plant’s leaves. The succulent Dudleya brittonii has the most reflective surface of any plant!

Unfortunately, farina does rub off very easily if the leaves are touched. This doesn’t happen as much in the wild but it’s very easy to do if you are growing plants!

Things to think about

• Succulents grow all over the world. But where would you expect plants with adaptations protecting against sunlight to have evolved?These plants usually live in in the tropics, near the equator, where the light from the sun is most intense.
• Can you think of anything else a plant might grow which would reduce the sunlight hitting its surface?Hair! There are many hairy cacti, and succulents with hairy leaves.

Night owls

Photosynthesis, the way that plants create their energy, is a chemical reaction. Using the light of the sun inside their specialised cells called chloroplasts, plants transform carbon dioxide and water into glucose (sugar) and oxygen. Photosynthesis can only happen during the day, when the light of the sun falls onto the plants.

The glucose is the source of a plant’s energy. After it has been produced, to access the energy it has to be broken down through respiration. This transforms the glucose and oxygen back into carbon dioxide and water (and creates energy in the process). Many plants respire during the day and at night: this means they have energy available all the time. But cacti and many succulents mostly just respire at night. Why might this be?

Both carbon dioxide and oxygen are gases and get into and out of the plant through pores that open and close, called stomata. Stomata are usually found on the underside of plant leaves. In cacti, they are on the stem of the plant instead. Plants also lose water vapour through their stomata when they are open, through the process of evapotranspiration. Heat makes the water loss happen faster.

This means that in arid environments, having the stomata open during the hottest parts of the day would risk drying the plants out. They have evolved a different kind of photosynthesis (called CAM) which keeps their stomata closed during the day. This reduces the amount of water lost and helps them to survive in arid environments. Cacti also have a reduced number of stomata in total, when compared with other plants.

Things to think about

• How do you think only producing energy at night affects the growth of the cacti and succulents that use CAM photosynthesis?It slows growth. Cacti and succulents are usually very slow-growing plants.