able for sessile animals and for organisms in a restricted area such as an island, where dispersal is limited and it is possible to sample marked individuals throughout their lifetimes.

For more mobile animals and populations containing several generations, a life table can be constructed from knowledge of the survival of individuals of known age over a single time interval. Such a time-specific, vertical, or static life table requires a knowledge of the ages of individuals, and also makes the assumption that the survivorship rates and the numbers in each cohort remain the same from year to year, and that birth rates, death rates, and population size are static. In a dynamic-composite life table, instead of a true cohort, a composite number of the animals marked over a period of years is used and the data are pooled. This method again makes the same assumption as the static life table.

life zone A zone in which the flora and fauna are typical of a particular latitude or range of altitude. These zones reflect environmental gradients. The term is most commonly used in relation to relatively local changes such as the zonation of communities on a mountainside. Compare biome.

light Electromagnetic radiation in the visible spectrum, i.e. with wavelengths ranging from 400 nm (extreme violet) to 770 nm (extreme red). The wavelength of light is the distance from one peak of the electromagnetic wave to the next. The spectrum of light is the span of wavelengths visible to the human eye. White light contains light of all wavelengths. Objects appear colored because they reflect certain wavelengths of light, but not others. While light may be considered as waves of electromagnetic energy, it may also be viewed as discrete packets of energy that carry the electromagnetic field, called photons. A photon has no charge or mass. Its energy depends on the wavelength of the light, shorter wavelengths having higher energy. All photons travel at the speed of light, about 299,492 kps per second. When light is reflected or refracted by certain substances, it is separated into two or more components, such that the wave action of each is concentrated in a different plane. This is called polarized light. See electromagnetic energy.

light attenuation instrument (transmis-someter) A device that measures the attenuation of light in water due to suspended particles. The degree of attenuation of light is related to the concentration of suspended materials, but not to the number of suspended particles: a single large particle might produce the same at-tentuation as hundreds of smaller ones.

light bottle See dark bottle.

light compensation point See compensation point.

light meter An instrument for measuring the intensity of incident or reflected light, consisting of a photosensitive cell and a milliammeter or millivoltmeter calibrated in light levels.

lightning A discharge of electricity between clouds and the earth, which produces a brilliant flash and the sound of thunder. It occurs when electrostatic charges build up on the undersurface of clouds. See nitrogen cycle.

light pollution The emission of light into the night sky by street lights and other artificial lighting, which produces a glow in the sky and reduces the visibility of the stars.

light reaction See photosynthesis.

light saturation point See compensation point.

lignin See wood.

limestone A sedimentary rock containing at least 50% by weight of calcium carbonate, and sometimes also magnesium carbonate. It may be organic, having been formed from the calcareous skeletal remains of living organisms, such as corals or

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