Plant defences

ENVIRONMENT

“ Plants may go in for chemical warfare, with leaves that are unpalatable or even poisonous”

Cacti are infamous for their spiky exterior that defends their water stores

Plant defences explained

The plant kingdom has evolved some canny ways to see off a variety of threats

Leaves are vital to the survival of plants. They catch the sunlight that plants use to power food production from oxygen and water in the process called photosynthesis. Stems hold the leaves in a position that maximises the light they catch, much like an array of green solar panels. Anything that damages the leaves or stems reduces the amount of sunlight the plant can collect, slowing its growth and impacting on the plant’s overall health.

Oddly, too much sunlight can damage plants too. Chlorophyll, the green chemical which reacts with sunlight in photosynthesis, is easily damaged by high intensities of direct sunlight. Plant cells therefore contain chemicals which act like sunblock, letting in just the right amount of light for photosynthesis. Plants that always grow in shady woods don’t need sunblock, but they may die if we replant them into sunny gardens. Drought also damages plant growth, because the leaves of a wilted plant are not best arranged to catch the Sun – so getting enough water is essential.

However, the main threat to plant photosynthesis is from animals that eat the leaves or stems. Plants therefore invest a lot of energy in keeping grazers away. Some plants use different kinds of armour. Their leaves might have a tough, waxy coat that makes leaves difficult to eat, or a beard of hairs to stop insects settling on them. Other defences might include their stems and leaves, which may have spines or prickles that make it uncomfortable to eat the leaves or even get too close.

Many plants also go in for chemical warfare, with chemicals in their leaves that are unpalatable or even poisonous to grazing animals. A few species can also cheat; they don’t produce poisons themselves but instead look like other plants that animals know are toxic, and so avoid getting eaten by proxy.

DID YOU KNOW?

Bracken isn’t to be messed with: young leaves contain cyanide, while older ones can cause cancer and blindness

Chemical warfare

Plants use thousands of different poisons and distasteful chemicals to stop animals eating their leaves. Many are derived from the lignins used to harden cell walls, or tannins which may help regulate plant growth. The grain of Indian millet is an important food in tropical countries, but its leaves contain a chemical called dhurrin. When an animal tries to eat its leaves, the plant releases enzymes which break down the dhurrin, which in turn release cyanide – one of the deadliest compounds known.

Plants like poison ivy (pictured below) release chemicals which cause a painful skin rash in any animal brushing against them. The sap of the African blister bush sensitises the skin to sunlight, so animals suffer painful sunburn.

Some plants can warn neighbours of attack too. When an animal grazes certain African acacias, they produce poisons and ethylene gas. The ethylene triggers all the acacias within the surrounding 45 metres (150 feet) to produce poisons, in case the animal attacks them too.

Stinging nettle up close

We zoom in to the painful defensive weapon used by this common weed

Stinging hairs

The leaves and stems of common nettle are covered in stinging hairs – glass-like tubes made of silicate.

Hypodermic needle

Without its cap, the hair’s sharp tip acts like a hypodermic syringe, penetrating the animal’s skin and injecting its contents.

Bulbous cap

The bulbous cap of the hair has a fragile fracture zone beneath it, and readily snaps off at the lightest touch.

Irritant injection

Irritant liquids including acetylcholine, formic acid and histamine are injected into the animal – and, as we know, that hurts!

Six types of plant protection in focus

1Prickly leaves Tough, prickly-edged leaves like holly discourage grazing animals, but it takes energy to produce them.

Leaves higher on the plant have no prickles, but, if an animal tries to eat them, it grows replacement leaves with prickles. 2Spines Many plants have spiny leaves. In cacti, the stem is green and photosynthetic, and its leaves are reduced to very tough, sharp spines. These have evolved to stop animals biting into the swollen stems to steal the stored water.

4Silica blades Humans sometimes put broken glass on top of walls to repel climbers – and many meadow grasses use the same technique. The edges of their leaves are protected by a line of microscopic, sharp blades of silica. 5Camoufl age It is diffi cult to hide leaves which are green and exposed to the Sun, but in the desert, some plants have leaves that look just like pebbles, helping to disguise them from animals seeking food and moisture. 3Thorns Thorns are short, highly modifi ed side stems, which make it uncomfortable for animals trying to eat the plant. But in plants like the blackberry, the thorn’s job is to attach to surrounding vegetation so the plant can scramble over it.

6Insect guardians Some acacia bushes have glands that secrete nectar to attract ants, and hollow thorns in which ants can nest.

The ants want these resources for themselves, so they mount a ferocious attack on any intruders.