Why plants need physical defences

1. Plants have no circulating white blood cells and make no antibodies, so they cannot hunt down a pathogen once it is inside them.
2. A plant is also rooted in one place, so it cannot move away from infected neighbours or from the insects carrying disease.
3. Its defences are therefore built into its structure, and a physical defence is a barrier that stops the pathogen reaching living tissue in the first place.
4. To infect a leaf a pathogen must cross two barriers in turn, first the waterproof layer on the outside and then the wall around each cell.

The waxy cuticle

1. The waxy cuticle is secreted by the epidermal cells and forms a continuous waterproof sheet over the upper and lower surfaces of every leaf.
2. Because it repels water, rain runs straight off instead of sitting on the surface as a film.
   1. Fungal spores need a film of surface water to germinate, and many bacteria need it to swim towards a stoma, so removing the water removes their route in.
3. The wax is also a solid barrier in its own right, and a fungal thread cannot push through an intact cuticle to reach the epidermal cells beneath.
4. Plants growing in exposed or dry places build a much thicker cuticle, which is why a holly leaf feels hard and glossy while a lettuce leaf tears easily.
5. The cuticle is broken by hail, wind damage, grazing and careless pruning, and almost every leaf infection begins at a break of this kind.

Cell walls

1. Every plant cell is enclosed in a cellulose cell wall, so a pathogen that gets past the cuticle meets a second barrier at every single cell.
2. Cellulose is built from long chains bundled into fibres that criss-cross in layers, which makes the wall extremely strong for its thickness.
   1. A fungal thread grows by pushing forwards with great force, and the cross-linked fibres of the wall resist that pressure.
3. The wall also keeps the pathogen away from the cytoplasm, and a virus or bacterium that cannot reach the cytoplasm cannot reproduce.
4. Some walls are strengthened further with lignin, a hard waterproof substance found in xylem vessels and in the woody parts of stems and roots.
   1. Lignified walls are much harder for enzymes to digest, so a pathogen that dissolves its way through ordinary cell walls is stopped by them.
5. A plant can also lay extra material down inside the walls around a wound, which thickens them and seals the infected area off from healthy tissue.

Bark and layers of dead cells

1. The stems and roots of woody plants are covered by bark, which is made of layers of dead cells whose walls are soaked in a waterproof substance.
2. These cells contain no cytoplasm, so a virus or bacterium that reaches them finds nothing it can use to reproduce.
3. Bark is shed continuously in flakes and plates, so any pathogen that has settled on the outside is carried away before it can work inwards.
4. Deciduous trees use the same trick on a yearly cycle, dropping every leaf in autumn and taking the season's fungal infections down with them.
   1. The tree then opens fresh, uninfected leaves in spring, which is why gardeners are told to rake up and burn fallen leaves rather than leave them under the tree.

Defences against animals that damage plants

1. Thorns, spines and stinging hairs make a plant painful to eat, so a herbivore moves on to something easier.
   1. This protects the plant against disease as well as against grazing, because every bite leaves a wound through which pathogens can enter.
2. Dense hairs on a leaf surface hold small insects away from the epidermis, so an aphid carrying a virus cannot reach the phloem to feed.
3. Leaves that droop or curl when touched, as in the sensitive plant, throw feeding insects off and make the plant look smaller and less worth eating.
4. Closing the stomata shuts the one opening in the cuticle, which denies pathogens their easiest route into the leaf.
5. Mimicry works by appearance rather than by force, and some passion flower species grow small yellow spots that look like butterfly eggs.
   1. A female butterfly avoids laying on a leaf that already appears to carry eggs, so no caterpillars hatch there and the leaf is never eaten.
6. Almost no plant relies on a single defence, and a rose bush combines a waxy cuticle, cellulose walls, woody stems and thorns at the same time.