tetrad
(ˈtɛtrəd)
[ad. Gr. τετράς (τετραδ-) a group of four, the number four.]
1. A sum, group, or set of four; four (things, etc.) regarded as a single object of thought.
| 1653 H. More Conject. Cabbal. (1713) 82 It was a solemn Oath..to swear by him that delivered to them the mystery of the Tetractys, Tetrad, or number Four. 1653 [see tetractys]. 1832 Coleridge Table Talk 24 Apr., The adorable tetractys, or tetrad, is the formula of God. 1895 Athenæum 2 Feb. 151/1 The great tetrad of senior wranglers of 1840 to 1843. |
2. In spec. uses. a. Chem. An element, compound, or radical having a combining power of four units, i.e. of four atoms of hydrogen; a tetravalent element, etc.
| 1865 Reader 1 Apr. 372/3 A tetratomic atom or tetrad. 1866 Roscoe Elem. Chem. xxvii. 242 As in mineral chemistry we have radicals some of which are monads, and some dyads, triads, or tetrads. 1868 Fownes' Chem. (ed. 10) 259 Silicium and titanium are tetrads. |
b. Biol. (a) A group of four cells, e.g. spores, pollen-grains. (b) A group of four chromosomes formed by the division of a single chromosome. (c) A quaternary unit of organization differentiated from a triad.
| 1876 tr. Schützenberger's Ferment. 52 In the tetrads arranged in the form of a cross, we observe, also, two plane surfaces at right angles. 1882 Vines Sachs' Bot. 456 The cavity of the sporangium becomes filled with a granular plasma in which lie the mother-cells and the tetrads of spores... All the spores of the sixteen tetrads formed in the microsporangia reach maturity. 1883 [see 3]. 1895 Oliver tr. Kerner's Nat. Hist. Plants II. 101 In Rhododendron hirsutum all the pollen-tetrads of an anther-cavity are held together by a mass of sticky viscin. a 1909 Wilson (cited in C.D. Suppl. in sense b) 1909 J. W. Jenkinson Exper. Embryol. 108 Granules of chromatin took the place of the tetrads and were unequally distributed to the spindle poles. |
c. Mus. A chord of four notes (after triad).
| 1881 Broadhouse Mus. Acoustics 332 The great majority of major tetrads in Palestrina's Stabat Mater are in the positions 1, 10, 8, 5, 3, 2, 4, 9. |
d. In ancient systems of arithmetical notation: A group or series of four characters corresponding to successive powers of ten.
| 1883 Sir E. C. Bayley Geneal. Mod. Numerals ii. 90 They [the Greeks] had however a system of ‘octads’ and ‘tetrads’ for expressing numbers of very high value. |
e. Math. (See quot.)
| 1889 Cayley Math. Papers XII. 590 The term ‘tetrad’ is used in two distinct..senses, viz. a tetrad denotes any four points; and it also denotes the four vertices of a self-conjugate tetrahedron in regard to a quadric surface... Two or more tetrads, in regard to one and the same quadric surface, are called similar tetrads. |
f. Ecol. (See quot. 1976.)
| 1963 Hawkes & Readett in P. J. Wanstall Local Floras 37 We soon realized that it would be impossible to record from every basic square in the county and we modified the method by considering the squares in blocks of four (‘tetrads’) and selecting one square at random from each tetrad for surveying. 1968 Watsonia VI. 351 This involved the detailed survey of 1 km squares as the unit of recording, one square at random being selected from each block of four or ‘tetrad’. 1976 J. G. Dony Bedfordshire Plant Atlas 10/1 It has become usual in the survey of areas as small as Bedfordshire to divide the ten-kilometre grid squares into 25 smaller squares each 2 km. × 2 km. known as tetrads. Each tetrad has an area of four square kilometres. 1983 Natural World Spring 18 Distributional maps based on 2 × 2 kilometre squares, or tetrads. |
3. attrib., as tetrad metal, tetrad term; tetrad-deme Biol., an aggregation of tetrads: see 2 b (b) and deme2 2.
| 1866 Odling Anim. Chem. 17 The fourth or tetrad term of our series of typical hydrides. 1868 Fownes' Chem. (ed. 10) 445 Tin is a tetrad metal. 1883 P. Geddes in Encycl. Brit. XVI. 843/2 Starting from the unit of the first order, the plastid or monad, and terming any undifferentiated aggregate a deme, we have a monad-deme integrating into a secondary unit or dyad, this rising through dyad-demes into a triad, this forming triad-demes, and these when differentiated becoming tetrads, the Botryllus-colony with which the evolution of compound individuality terminates being a tetrad-deme. |