Common poisonous plants of the hedgerow: black and white bryony, cuckoo pint, and deadly nightshade. The dangers of such plants are known to all through trial and error a very long time ago. Many food plants contain similar toxins in some parts of the plant. Most of the nightshade family are poisonous and the family includes potatoes, which are poisonous if green, and tomatoes, the entire plant of which is poisonous, except the fruit.

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Everyone knows about poisonous plants, deadly nightshade, foxgloves and yew, but the sheer scale of chemical warfare waged by natural creatures, both plants and animals, comes as a surprise. Even in Britain, a high proportion of the common plants is poisonous to a greater or lesser degree. The reason is that poison is a countermeasure in the struggle for life. Plants are poisonous to prevent them being eaten and the reason more people aren't poisoned is that most poisonous plants taste very bad. One man's meat is another man's poison, or in this context one animals' meat is another animal's poison.

The tolerance of different animals to toxic food varies widely. Rabbits can eat what would be lethal doses to humans of the Death Cap mushroom; surprisingly, a horse can be poisoned simply by eating raw barley, wheat or rye grain. It is worth remembering this variation between animals when we read reports that yet another substance in the diet has caused disease in an experimental animal.

Some of the commonest foods have dangerous aspects in some circumstances. The stones and pips of apricots, plums, cherries, peaches, apples and pears all contain glycosides which if eaten release potentially lethal doses of cyanide. The leaves of rhubarb contain large quantities of oxalic acid, which interferes with calcium absorption. The stems are safe but still contain enough oxalic acid to give that characteristic teeth-roughening experience. Many plants of the cabbage family have a tendency to prevent iodine from being used to replenish the body's thyroid hormones. In some parts of the world where iodine levels are low (a long way from the sea) this is a problem.

Green potatoes are lethal, thanks to the poison solanine which potatoes share with other members of their botanical family, deadly nightshade and henbane, and of course the tomato, every part of which is highly poisonous, except the fruit. Besides these natural toxins, there are of course the hazards of microbial contamination, in fact the worst danger lurking in food. In the case of natural toxins the remedy is simply avoidance. In the case of microbial contamination additives come into play: in this case preservatives.

The range of potential toxins in nature is wide: honey from rhododendrons is highly toxic, fugu, the blow fish (puffer fish) the liver of which contains a poison so deadly it has been said that 'the element of risk in eating fugu may be one of the reasons why the dish is so popular with Japanese gourmets'. Then there is cyanide in cassava and almonds (there are official limits for cyanide concentration in marzipan!). The amines in cheese, chocolate, sauerkraut, and wine, tyramine, histamine serotonin, can cause unpleasant reactions in many people. Very many substances are harmful in very large doses, even benign substance such as vitamins have caused problems, and even death, when abused. Where toxins cannot entirely be eliminated from a food, e.g. rhubarb, what matters is whether the actual doses received are harmful. The same applies to additives, which are used in very small quantities.