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 carbon dioxide 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.

Six types of plant protection

1. Prickly 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.

2. Spines

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.

3. Thorns

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.

4. Silica 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.

5. Camouflage

It is difficult 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.

6. Insect 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.


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