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06413_18 - R race 1(in biology A category used in...

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R race 1. (in biology) A category used in the * classi Ü cation of organisms that con- sists of a group of individuals within a species that are geographically, ecologi- cally, physiologically, or chromosomally distinct from other members of the species. The term is frequently used in the same sense as * subspecies. Physiolog- ical races , for example, are identical in ap- pearance but differ in function. They include strains of fungi adapted to infect different varieties of the same crop species. 2. (in anthropology) A distinct human type possessing several character- istics that are genetically inherited. The major races are Mongoloid, Caucasian, Negroid, and Australoid. racemate See racemic mixture . raceme A type of * racemose in Û ores- 5 3 1 2 4 6 6 4 2 1 3 5 spike (e.g. plantain) raceme (e.g. lupin) 1 3 5 6 4 2 corymb (e.g. candytuft) 1 3 5 6 4 2 umbel (e.g. hogweed) capitulum (e.g. daisy) 1 3 5 4 2 5 3 2 1 4 1 = oldest flower Types of racemose inflorescence
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cence in which the main Û ower stalk is elongated and bears stalked Û owers. An example is the lupin. See also panicle . racemic mixture (racemate) A mixture of equal quantities of the (+)- or d - and (–)- or l - forms of an optically active com- pound. Racemic mixtures are denoted by the pre Ü x ( ± )- or dl -. A racemic mixture shows no * optical activity. racemization A chemical reaction in which an optically active compound is converted into a * racemic mixture. racemose in Û orescence (inde Ü nite in Û orescence) A type of Û owering shoot ( see inflorescence ) in which the growing region at the tip of the Û ower stalk con- tinues to produce new Û ower buds during growth. As a result, the youngest Û owers are at the top and the oldest Û owers are at the base of the stalk. In a Û attened in- Û orescence, the youngest Û owers are in the centre and the oldest Û owers are on the outside. Types of racemose in Û ores- cence include the * capitulum, * catkin, * corymb, * raceme, * spadix, * spike, and * umbel (see illustration). Compare cymose inflorescence . rachis (rhachis) 1. The main axis of a compound leaf or an in Û orescence. 2. The shaft of a * feather. 3. The back- bone. rad See radiation units . radar (radio detection and ranging) A method of detecting the presence, posi- tion, and direction of motion of distant objects (such as ships and aircraft) by means of their ability to re Û ect a beam of electromagnetic radiation of centimetric wavelengths. It is also used for navigation and guidance. It consists of a transmitter producing radio-frequency radiation, often pulsed, which is fed to a movable aerial from which it is transmitted as a beam. If the beam is interrupted by a solid object, a part of the energy of the ra- diation is re Û ected back to the aerial. Sig- nals received by the aerial are passed to the receiver, where they are ampli Ü ed and detected. An echo from a re Û ection of a solid object is indicated by a sudden rise in the detector output. The time taken for a pulse to reach the object and be re Û ected back ( t ) enables the distance 685 radiant exitance r range signal strength range
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