propagation of plants is effected in nature chiefly sexually by the seed and the spore, less often by rhizomes and other methods (see reproduction). Vegetative means include cutting, layering, grafting, tissue culture, and division of the roots (see perennial) and of the tubers (see potato). Most farm and garden crops are propagated by seed, but some plants will not breed true from seed and must be propagated by various vegetative methods, depending on the type of plant.

medicinal plants, plants used as natural medicines. This practice has existed since prehistoric times. There are three ways in which plants have been found useful in medicine. First, they may be used directly as teas or in other extracted forms for their natural chemical constituents. Second, they may be used as agents in the synthesis of drugs. Finally, the organic molecules found in plants may be used as models for synthetic drugs. Historically, the medicinal value of plants was tested by trial and error, as in the Doctrine of Signatures. Modern approaches to determining the medicinal properties of plants involve collaborative efforts that can include ethnobotanists, anthropologists, pharmaceutical chemists, and physicians. Many modern medicines had their origin in medicinal plants. Examples include aspirin from willow bark (Salix spp.), digitalis from foxglove (Digitalis purpurea), and vinblastine from Madagascar periwinkle (Vinca rosea) for the treatment of childhood leukemia. See also herbal medicine.

insectivorous plants: see bladderwort; pitcher plant; Venus's-flytrap.

house plants, varied group of plants grown indoors and requiring no special care. They are usually grown singly in pots, but can also be grouped and planted together in dish gardens and terrariums. Some are cultivated for their flowers, such as geraniums and African violets, while others, such as philodendron and sansevieria (snake plant), are grown for their decorative foliage. Growing house plants successfully can be greatly facilitated in many cases by maintaining a cool temperature and raising the atmospheric moisture either by the use of humidifiers or simply by placing evaporating pans on radiators.

diseases of plants. Most plant diseases are caused by fungi, bacteria, and viruses. Although the term disease is usually used only for the destruction of live plants, the action of dry rot and the rotting of harvested crops in storage or transport is similar to the rots of growing plants; both are caused by bacteria and fungi. Any environmental factor that favors the growth of parasites or disease transmitters or that is unfavorable to the growth of the plants will lead to increases in the likelihood of infection and the amount of destruction caused by parasitic disease. Parasitic diseases are spread by dissemination of the agent itself (bacteria and viruses) or of the reproductive structures (the spores of fungi). Wind, rain, insects, humans, and other animals may provide the means for dissemination.

Most names for plant diseases are descriptive of the physical appearance of the affected plant, e.g., blight (a rapid death of foliage, blossom, or the whole plant); leaf spot, fruit spot and scab, and stem canker (localized death of an organ); wilt (loss of turgor); gall (overgrowth of cells); witches'-broom (growth of abnormal shoots); stunting (underdevelopment); and leaf curl, mosaic, and yellows (resulting from chlorosis, or lack of chlorophyll). Many of these abnormalities are caused by different agents on different plants; when parasites are involved, each individual parasite usually invades only certain plant species and specific organs. Dutch elm disease, chestnut blight, rust, smut, certain mildews, and ergot are caused by various fungi (see fungal infection). Clubroot diseases are caused by slime molds, and water molds cause downy mildew (a disease of grapes), blue mold of tobacco, and sudden oak death (also known as ramorum leaf blight or ramorum dieback). Sudden oak death, caused by Phytophthora ramorum, was identified in 1995 in California, where it caused the deaths of many oaks. The disease, which affects many plant species besides oaks, has since been found in Oregon, and is also found in Europe; there, it was identified in 1993 in Germany, where it affected rhododendrons and viburnums. The water mold P. infestans was the cause of the late blight of potatoes that resulted in the Great Potato Famine in Ireland (1845-49). Both slime molds and water molds are now usually considered protists, rather than fungi. Most mosaic diseases and many other types of chlorosis are caused by viruses (see virus).

Plant diseases are more often classified by their symptoms than by the agent of disease, because the discovery of microscopic agents such as bacteria dates only from the 19th cent. (see Louis Pasteur). The Irish potato blight stimulated the development of plant pathology. The identification of tobacco mosaic virus in 1892 was the starting point of all modern knowledge about viruses.

Control

Plant diseases are controlled by methods of cultivation (e.g., crop rotation and the plowing under or burning of crop residue); by application of chemicals, e.g., fertilizers (to correct mineral deficiencies in the soil), spray or dust fungicides, bactericides, and insecticides; by development of disease-resistant strains by genetic methods; by use of alternative species that are not susceptible to the disease; by eradication of diseased plants or of their alternate hosts (e.g., barberries, which harbor wheat-stem rust); and by quarantine measures by state and federal governments to prevent the introduction of foreign plant diseases. Field and orchard crops are more susceptible to destruction than are wild plants, because the close proximity of large numbers of a single species (monoculture) makes possible the rapid spread of disease to epidemic proportions.

carnivorous plants: see bladderwort; pitcher plant; Venus's-flytrap.

alpine plants, high-altitude representatives of various flowering plants (chiefly perennials) that because of their dwarf habit, profuse blooming, and the preference of many for shady places are cultivated in alpine and rock gardens. Some species require specially constructed gardens duplicating mountain terrain, including systems for supplying cool water underground, comparable to the melting snows of their natural habitat. Others thrive without special care in favorable conditions (e.g., cool climate, short growing season, and alkaline, rocky soil). Alpine species of gentians, saxifrages, and stonecrops are among those most commonly planted. Many garden plants (e.g., roses, irises, and primroses) have alpine representatives. The edelweiss is a popular alpine.