There are a great many diíferent kinds of plant defenses. Certain types protect the plant from all kinds of dangers, while others are effective only against certain enemies. There are defensive devices that serve only to repel enemies; others may do considerable harm to any animals that may attempt to attack the plant.
Plant poisons are efficient defenses
Among the most efficient plant defenses are the plant poisons, which take effect when the plant is touched or eaten. It is interesting to note that not all parts of the plant are equally toxic, or poisonous. Most of the poisons are concentrated in the plant parts that are most susceptible to attack: the leaves, in which food is manufactured, and the fruits and flowers, which serve for reproduction.
Different poisons produce varying effects on diíferent animals. Poison ivy (Rhus toxlcodendron), for example, brings about a very unpleasant skin rash in human beings, but it serves as the main source of food for certain insects which devour the leaves of the plant.
The many kinds of plant poisons
The plant poisons are made up of com-plex organic compounds — alkaloids, glu-cosides, resinoids and organic acids. They produce a variety of symptoms in the vic-tims. Plañís that are poisonous by con-tact, such as poison ivy, secrete a number of poisons belonging to the alkaloid group. The main effect, as we pointed out, is a rash — the condition known as contact dermatitis.
Plants such as sorghum, arrowgrass and certain species of wild cherries secrete a glucoside-lype poison called prussic acid. This poison kills within minutes after animals feed on a plant containing it. Another active ingredient in many plant poisons is the compound known as tremetol, which is found in the white snakeroot and the rayless goldenrod. Tremetol causes the disease called "trembles" in grazing animals; it is characterized by muscular weakness and shaking. This form of animal poisoning is of particular concern to mankind, because the poison is very soluble in cow's milk and can be transmitted to man through that medium. In the human form, the disease is commonly known as "milk sickness."
Plants that secrete photosensitizing poisons
Some plants secrete what are known as photosensitizing poisons. These substances make an animal highly sensitive to light. If the animal remains in the shade, it will be unaffected, but if it steps into strong sunlight, it will develop a wide variety of symptoms; death may result. Photosensitizing poisons are secreted by such plants as the buckwheat and St.-John's-wort.
Some poisonous plants do not manufacture their poisons at all, but take them up directly from the soil. These plants are often referred to as seleniferous because their poison is so commonly the element selenium. They can absorb this element through their roots from certain types of shale. The locoweed, wild aster and prince's-plume are plants of this type. They cause the conditions known as alkali disease and "blind staggers."
