What immunity is, and the walls and barriers of plants
Life is under constant threat from pathogens: disease-causing organisms such as some insects and other animals, protozoa, fungi, bacteria and viruses. So every kind of living thing has evolved defences. Immunity is the ability of the body, through the immune system, to resist pathogens, either by keeping them out or by attacking and destroying them if they get in. There are two systems, innate (natural) immunity and acquired (adaptive) immunity, and they work together: innate immunity is needed for acquired immunity to work, and the other way round. A plant defends itself in two ways. Structural immunity is a set of barriers built into its structure. It is the first line, keeping pathogens out and stopping them spreading. Biochemical immunity uses chemicals, and it is the subject of the next lesson. The structural barriers are of two kinds: pre-existing ones, already there before any attack, and induced ones, formed as a response to an injury or infection. The epidermis of green stems and leaves is covered with a waxy layer of cutin, so water does not settle on it and fungi and bacteria find no good place to grow. Hairs or thorns on the epidermis stop some grazing animals from eating the plant. The cell wall, made mainly of cellulose, protects every cell, and when it is thickened with more cellulose, lignin, suberin or cutin it is hard for a pathogen to push through. Woody stems and trunks are covered with cork: several layers of dead cells whose walls are thickened with suberin. Cork protects against shocks, water loss and fungal and bacterial infection, and when the outer layer is cut or torn, new cork forms to close the wound.
| Barrier | Kind | How it helps |
|---|---|---|
| Waxy cutin on the epidermis | pre-existing | water does not settle, so microbes grow poorly |
| Hairs and thorns | pre-existing | stop some grazing animals |
| Thickened cell wall (cellulose, lignin, suberin, cutin) | pre-existing | hard for a pathogen to push through |
| Cork of woody stems | there already, and re-formed after a cut or tear | shields against shocks, water loss and infection; closes the wound |
| Barrier | Pre-existing or induced |
|---|---|
| A waxy cutin layer | pre-existing |
| Thorns on a stem | pre-existing |
| New cork formed after a stem is torn | induced |
| The cellulose cell wall | pre-existing |
Water-drop test, with a grown-up. Find two leaves from garden plants or from a market, one with a smooth waxy surface and one with a rough or hairy surface. Put a small drop of water on each and look at it from the side. Write whether the drop stays a bead or spreads, and say which leaf would give fungi and bacteria a poorer place to grow. Wash your hands afterwards, and do not taste the leaves.