plants that produce and accumulate poisons during vital activity; the plants are toxic to animals and humans. There are more than 10,000 known species of poisonous plants, found mainly in the tropics and subtropics. Many species also occur in countries with temperate and cold climates. There are about 400 poisonous species in the USSR.

Poisonous plants include many mushrooms, equisetics, club mosses, ferns, gymnosperms, and angiosperms. In countries having a temperate climate they are most broadly represented in the families Ranunculaceae (crawfoot), Papaveraceae (poppy), Eu-phorbiaceae (spurges), Asclepiadaceae (milkweeds), Apocina-ceae (dogbane), Solanaceae (nightshade), Scrophulariaceae (figwort), and Araceae (arum). Many plant poisons are valuable medicinal substances, for example, morphine, strychnine, atropine, and physostigmine.

The principal active substances in poisonous plants are alkaloids, glycosides (including saponins), essential oils, and organic acids. They are generally found in all the parts of the plant, but frequently in different quantities. Therefore, although the entire plant is toxic, some parts are more toxic than others. For example, the rhizome is especially poisonous in water hemlock and species of aconite and hellebore. The most toxic part of the potato plant is the flower; of poison hemlock, the fruit; of sophora, corn cockle (Agrostemma githago), and heliotrope, the seeds; and of digitalis, the leaves. Some poisons develop and accumulate in only one organ in the plant. For example, the glycoside amygdalin is found only in the seeds of almonds, cherries, and plums. Sometimes parts of poisonous plants are not toxic, for example, the tuber of the potato, the arils of the yew, and the seeds of the opium poppy.

The content of poisonous substances in plants depends on germination conditions and the phase of development of the plant. As a rule, poisonous plants growing in the south accumulate more active substances than those growing in the north. Some plants are most toxic before blooming, others during blooming, and still others during fruit formation. Plants are most toxic in fresh form. Toxicity can decrease or even be lost completely during drying, cooking, or ensiling. However, toxicity is retained in most poisonous plants even after processing. For this reason, the inclusion of such plants in forage is frequently the cause of severe poisoning of agricultural animals. When grasses are ensiled with admixtures of hellebore, the alkaloids from the latter are leached out, permeating the silage mass and rendering it poisonous. As a rule, animals do not eat poisonous plants, but with a shortage of fodder or after long-term stall-feeding, animals will greedily consume any fresh green, including poisonous ones. Animals that are moved to regions where there are unfamiliar poisonous plants are frequently poisoned.

Plants with absolute properties of toxicity apparently do not exist in nature. For example, belladonna and datura are toxic to man but not to rodents or chickens, thrushes, or other birds. Squill is toxic to rodents but not to other animals, and pyrethrum is toxic to insects but not to vertebrates.

Poisoning occurs not only by ingestion of the plant but also by breathing pulverized particles of poisonous plants or volatile substances produced by the plants. Poisoning also can occur through the skin as a result of contact with a poisonous plant or its juices. In humans, poisoning through the breathing passages is usually work-related. It is observed in hops harvesters, in carpenters who work with certain types of wood (for example, Euonymus), and in persons who work with certain medicinal plants (for example, belladonna, Securinega, Schizandra). Poisoning by volatile substances produced by poisonous plants is less common. Large bouquets of magnolias, lilies, European birdcherry (Padus), poppies, or tuberose can cause dysphoria, dizziness, and headaches. There are numerous cases of children being poisoned by ingesting visually tempting toxic fruits.

The toxic effects of eating poisonous plants sometimes appear within several minutes, as in the case with the consumption of yew cones. In other cases, symptoms may appear after several days or even weeks. Some poisonous plants, for example, Ephedra, are toxic only after prolonged consumption, because the active agents are accumulated by the organism rather than destroyed or excreted.

Most poisonous plants act on various organs simultaneously, but one organ or center is usually most strongly affected. Poisonous plants are grouped into different categories on the basis of which part of the body is affected: the central nervous system (aconite, meadow saffron, henbane, poison hemlock, anemone, water hemlock), the heart (lily of the valley, digitalis, silk vine), the liver (heliotrope, senecio, lupine), and the respiratory and digestive organs (field mustard, blister cress, Trichodesma inca).

The public should be educated about the danger of poisonous plants. Poisonous plants growing in pastures must be destroyed for the protection of animals. Many plant poisons are used in small doses for medicinal purposes (for example, cardiac glycosides obtained from digitalis and lily of the valley and atropine from henbane). Insecticides are produced from some poisonous plants; for example, Dalmation insect powder is obtained from the Dalmatian pyrethrum (Pyrethrum cinerariaefolium).