cone

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cone
1. (in botany) A reproductive structure occurring in gymnosperms, known technically as a strobilus. It consists of sporophylls bearing the spore-producing sporangia. Gymnosperms produce different male and female cones. The large woody female cones of pines, firs, and other conifers are made up of structures called ovuliferous scales, which bear the ovules. Cones are also produced by clubmosses and horsetails.

2. (in animal anatomy) A type of light-sensitive receptor cell, found in the retinas of all diurnal vertebrates. Cones are specialized to transmit information about colour (see colour vision) and are responsible for the visual acuity of the eye. They function best in bright light. They are not evenly distributed on the retina, being concentrated in the fovea and absent on the margin of the retina. Compare rod.

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cone Solid, geometric figure swept out by a line (generator) that joins a point moving in a closed curve in a plane, to a fixed point (vertex) outside the plane. In a right circular cone, the vertex lies above the centre of a circle (base), and the cone's generators join the vertex to points on the circle. Such a cone has a volume 1/3πr2h and a curved surface area πrs, where h is vertical height, s the slant height, and r the radius of the base.

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cone In a vertebrate eye, the less common (compare ROD) of the two types of light-receptor cell, which is sensitive only to relatively high levels of stimulation. Many cones are connected each to a single bipolar cell and thence to a ganglion cell whose axon forms a nerve fibre in the optic tract. Since cones respond only to high levels of stimulation and since each stimulation produces an individual nerve impulse, cones provide sharp images.

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cone An inflorescence of Coniferae. See also STROBILUS.

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