Plant defences

Selective pressure

One of the main forces behind evolution is selective pressure. If something in the environment makes it more difficult to survive, that creates selective pressure. Random mutations in the genes of plants can sometimes create an advantage against selective pressure. Over time and enough generations carrying the mutated genes, they become permanent changes – creating new species of living things.

Plant-eating animals are a major selective pressure, particularly for plants that take a while to grow to a size where they can reproduce. For many plants, the selective pressure results in evolutionary adaptations that defend against predation. Cacti are well known for some of their defenses – their spines!

Spine-sational cacti

Cactus spines actually bring several benefits to plants. They make the plants more difficult to eat, increasing their chances of survival. If there are enough spines, they can also shield the plant against the strong sunlight they experience in the wild, preventing sunburn.

Cactus spines grow out of a part of the plant called an areole. Although other plants might have sharp defenses like thorns and prickles, areoles are unique to cacti – no other plants have them!

Areoles are raised areas on the body of the cactus. They can be fluffy, hairy, or velvety. Not every cactus will have spines but when they do, they always grow out of the areole. Young spines are softer and harden up as they age.

Larger spines often point straight out of the centre, called a central spine, and the ones that point in other directions are called radial spines. Areoles aren’t all about spines though – flowers develop here too and eventually fruit if the flower is pollinated.

Cactus growers often select their plants for the beauty of their spines. They are very varied, coming in all sorts of shapes and sizes.

Teeth, thorns and prickles

Many other succulent plants have other kinds of weaponry to make themselves less edible. If you look at some of the other stem succulents you will see that many of them look a lot like cacti at a first glance. This is because they have similar evolutionary adaptations. But they are not closely related to cacti, or to each other.

When different living things arrive at a similar end result from evolution, but following different paths, it is called convergent evolution. Bats and birds both having wings is a good example of this in the animal kingdom.

Look at the pictures to the right and spot the difference: one of these plants is a cactus, and one is a completely different kind of stem succulent called a Euphorbia. They both look very similar overall, as they are cylindrical with succulent stems and have arms that come off as branches. But looking very closely reveals a key difference.

The cactus has spines that emerge from areoles. The Euphorbia has some nasty-looking spines, but they come out of something called a stipule, part of where its leaves come out. In fact, this Euphorbia is more closely related to a passion fruit than it is to the cactus!

Leaf succulents can also have sharp defenses, although they aren’t usually quite as vicious as the spines of cacti. For example, Agave plants can have quite tough, vicious teeth down the sides of their leaves and a terminal spine at the very tip.

Aloe plants can also have spines on their leaves. They tend to be a lot softer than other kinds of spine and come straight out of the leaf itself.

Things to think about

• Is there any advantage to having spines that look like they might hurt but that aren’t actually very dangerous?Yes! Appearing harmful can put an animal off eating you.

Blending in and hugging the ground

Weaponry is not the only evolutionary adaptation succulent plants have developed in response to pressure from herbivores. Many plants have developed a form of camouflage – if you are harder to spot, then you are less likely to be eaten!

Lithops are one of the masters of disguise of the succulent world. These succulents from South Africa have the common name “living stones” because they have evolved to look just like the rocks that surround them. There are many different coloured forms of Lithops and they often match the rocks in the exact location they evolved in.

Of course, having a bright yellow or white flower does ruin the disguise when the flowering season comes. But the evolutionary importance of creating seeds and continuing the next generation is greater than the survival of an individual plant.

Things to think about

• Are there any disadvantages to plants not having as much green pigmentation (chlorophyll)?They won’t produce as much energy from the sun through photosynthesis.

Another adaptation Lithops have to protect themselves is the window they have at the top of their pairs of leaves. The part poking out of the ground is the “tip of the iceberg” – most of their bodies remain underground. As they grow from the centre of the plant, even if the tip of the leaf is damaged, they can refresh their leaves the next growing season.

Lithops aren’t the only succulent plants to hide underground. This is quite a common adaptation. Haworthia are particularly well known for it, and many have windows at the ends of their leaves just like Lithops, so the parts poking out above the soil can capture more of the sun’s energy.

Things to think about

• What is the other advantage to hiding most of your body underground if you are a succulent plant living in an arid environment?It is cooler and shaded underground which means the plant will lose less water.
• Can you think of anything that might cause problems for a plant that only has a small window peeking up to the surface?Getting covered over with dust or sand!  Usually this wouldn’t be much of a problem as it would be blown away again before too long.

Many succulent plants contain toxic or irritating substances to make them even less attractive to eat. Euphorbia release a milky sap called latex when they are cut or damaged, which can be irritating to the eyes, mouth, and digestive system.

Other succulents have different chemical defenses. Some Kalanchoe plants contain tiny, sharp crystals of calcium oxalate, which is the same thing that forms kidney stones, and not something you want in your digestive system (ouch).

Dracaena (commonly known as Sansevieria) contain saponins, which are bitter tasting and mildly toxic. They can give people stomach upsets but can seriously harm or even kill smaller animals.

Many Crassula and related succulents are also poisonous. Some Tylecodon contain cotyledoside which is very toxic and causes nenta poisoning in sheep and goats.

Things to think about

• By the time an animal has been poisoned, the plant has already been damaged or eaten. What is the evolutionary advantage of a defence like this?Small amounts of damage might not kill the plant.  But even if it does, the animal will be less likely to eat other similar plants.  The whole species, rather than the individual, gains benefits from the evolutionary adaptation!