Dirty Dozen

The list of the twelve most persistent organic pollutants (POPs) banned at a United Nations convention in May 2001. Nine of the chemicals in this list are crop protection chemicals and, of these, eight are insecticides. Discontinuity

See: Discontinuous pathosystem. Discontinuous epidemic

See: Discontinuous pathosystem. Discontinuous pathosystem

A pathosystem in which the parasitism is intermittent because there is a complete absence of host tissue at periodic intervals, such as during a tropical dry season, or a temperate winter. Discontinuous pathosystems involve seasonal host tissue, and they occur typically with annual plants, and the leaf parasites of deciduous trees and shrubs. Discontinuity confronts the parasite with three difficult problems, because it must survive the absence of host tissue, it must find a new host individual when tissue is again available and, if vertical resistances occur, it must match the host that it does find. Allo-infection is of primary importance in discontinuous pathosystems, and vertical resistance has a high survival value. See also: Continuous pathosystem. Discontinuous variation

In genetic terms, variation among individuals may be continuous or discontinuous. Continuous variation means that there is every degree of difference between two extremes. Discontinuous variation means that a character is either present or absent, and there are no intermediates. Continuous variation results from quantitative inheritance, while discontinuous variation results from qualitative inheritance.

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