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A computational approach to visual perception is outlined. The investigation centres on two problems. One concerns the shape, and the other the edges, of dot figures. In determining edges it is not possible to delimit arbitrarily the region on which each point computes. So in this context a global computation is defined. This leads to a novel way of investigating clustering. When the edges of a figure have been sequenced to yield the boundary it is then possible to determine its shape. The study of shape has been confined to convex figures. In these an attempt is made to extract global units which correspond to corner-like regions. Finally, it is suggested that a theory which evolves in the manner described has a more unitary aspect than other approaches.

In a trial on an irrigated sunflower (Helianthus annuus) crop in northern Victoria, plots to which bees had access produced in excess of 60 per cent more seed than plots from which bees were excluded. The number of seeds set per cm2 of flower head, the percentage germination and the oil content of seed from plots serviced by bees were significantly greater than from plots without bees. Individual seeds were heavier in plots without bees than in those serviced by bees.

The duration of phases of the cell cycle are measured in five species and one hybrid of the genus Crepis(Compositae), after pulse labeling with tritiated thymidine. The results indicate significant variations in the duration of post-synthetic interphase (G2), which affords a possible explanation for the abnormal chromosome morphology first described in Crepishybrids as neutral amphiplasty.

Mutants of E. coliK12 with deletions of the β-galactosidase gene (lacZ) can reacquire the ability to hydrolyze β-galactosides during prolonged intense selection for growth on lactose. Full lactose competence is restored through a sequence of at least five mutations. Cell extracts of these derived strains hydrolyze o-nitrophenyl-β-D-galactoside, the standard substrate for assay of β-galactosidase. The enzyme responsible for this activity differs in its immunological, kinetic, and sedimentation characteristics from the lacZβ-galactosidase of wild-type E. coli. Its genetic determinant, designated ebg-5, maps at 59 min on the E. colichromosome, whereas the lacoperon maps at 10 min. We suggest that a gene not involved in lactose utilization has been progressively changed into a form capable of specifying a β-galactosidase and that this process is similar to that whereby genes with new functions are evolved by natural selection.

Fifty-six amber mutations of the β-galactosidase gene of Escherichia coliwere suppressed by crossing into a stock containing the supDsuppressor gene. The resultant enzymes, differing only in the position of the inserted serine, were tested for stability at 57 C. Most of the suppressed enzymes were either as stable to heat as the normal enzyme or very unstable. Tests of enzymes produced by the action of other suppressors showed that the degree of stability was characteristic of a particular position in the polypeptide chain of the amino acid substitution and independent of the amino acid inserted. The mutations were placed in linear order in the gene by deletion mapping and three-point linkage tests. The consequent order of the serine substitutions disclosed an alternating pattern of stable and unstable regions over the amino-terminal two-thirds of the enzyme; the carboxy-terminal third of the enzyme was generally unstable. Considerations of coding relations and enzyme structure suggested that serine and glutamine suppression usually result in a change in the hydrophilic nature of the side chains on the outside of the enzyme molecule. It was shown that the potentially unstable regions of the enzyme are probably not indicative of stretches of α-helix or of sites of association. The apparent position of the substrate binding sites was correlated with the location of some of the potentially unstable regions, which may mark the parts of the polypeptide chain in proximity with the substrate.

Crystals containing multiple species of tRNA give single crystal X-ray diffraction patterns with a minimum spacing of 3 Å. These crystals display a simplicity of lattice symmetry and unit cell dimensions that makes them particularly suitable for structure analysis to near atomic resolution. The finding of such crystals confirms the marked similarity of tRNAs at the level of tertiary structure.

X-ray diffraction and circular dichroism studies indicate that poly d(I–C).poly d(I–C) exists in a new polynucleotide configuration—an eight-fold helix, possibly left-handed.

MAN AND HIS HABITAT. ESSAYS PRESENTED TO EMYR ESTYN EVANS, edited by R. H. Buchanan, Emrys Jones and Desmond McCourt. London: Routledge and Kegan Paul, 1971. xi + 279 pp. £5.50.A SYSTEMATIC GEOGRAPHY OF IRELAND, by Desmond Gillmor. Dublin: Gill and MacMillan, 1971. 214 pp. £1.05.L'IRLANDE. 1, MILIEU ET HISTOJRE, by J. Guiffan, J. Verriere and P. Rafroidi. Paris: Armand Colin, 1970. 320 pp.AIR TEMPERATURE IN IRELAND, 1931-1960. Dublin: Department of Transport and Power, Meteorological Service, 1971. 68 pp. 35p.MONTHLY, SEASONAL AND ANNUAL, MEAN AND EXTREME VALUES OF DURATION OF BRIGHT SUNSHINE IN IRELAND, 1931-1960. Dublin: Department of Transport and Power, Meteorological Service, 1971. 26 pp. 15p.THE APPLICATION OF GEOGRAPHICAL TECHNIQUES TO PHYSICAL PLANNING. Dublin: An Foras Forbartha, 1971. 163 pp. £2.00.AN TSUIRBHEIREACHT AR GHAELTACHT NA GAILLIMHE/THE GALWAY GAELTACHT SURVEY. Galway: University College, Social Sciences Research Centre, 1969. Volume 1, 81 pp. Volume 2, 44 figures.TRANSPORTATION IN DUBLIN. Dublin: An Foras Forbartha (The National Institute for Physical Planning and Construction Research), No Date. 48 pp. £1.THE CORK SUB-REGIONAL PLANNING STUDY, by F. B. Gillie. Dublin: An Foras Forbartha, 1971. 125 pp. £1.50.CORK: AN ENVIRONMENTAL STUDY OF CORK, by Peter Dovell. Dublin: An Foras Forbartha, 1971. 42 pp. 50p.THE SPADE IN NORTHERN AND ATLANTIC EUROPE, edited by Alan Gailey and Alexander Fenton. Belfast: Ulster Folk Museum and Institute of Irish Studies, 1970. xiii + 257 pp. £1.65.DESERTED MEDIEVAL VILLAGES, edited by Maurice Beresford and John G. Hurst. London: Lutterworth Press, 1971. 340 pp. £8.00.REGIONAL ARCHAEOLOGIES: ULSTER, by L. N. W. Flanagan. London: Heinemann Educational Books, 1970. 56 pp. 90p.IRISH PASSENGER STEAMSHIP SERVICES. Volume 2: SOUTH OF IRELAND, by D. B. McNeill. Newton Abbot: David and Charles, 1971. 240 pp. £2.50.THREE HUNDRED YEARS OF IRISH PRINTED MAPS. Belfast: Ulster Museum, 1972. 32 pp.JOURNAL OF THE KERRY ARCHAEOLOGICAL AND HISTORICAL SOCIETY, No. 4, 1971. 194 pp. Not on public sale.JOURNAL OF THE OLD ATHLONE SOCIETY, Vol. 1, No. 2, 1970-71. 76 pp. 75p.GEOLOGY OF BELFAST AND THE LAGAN VALLEY, by P. I. Manning, J. A. Robbie and H. E. Wilson. H.M. Stationery Office, 1970. 242 pp. £2.MAP REVIEWGEOLOGY OF NORTHERN IRELAND, Sheet 36 (Belfast). 1:63,360. 1966GEOLOGY OF BELFAST AND DISTRICT. 1:21,120. 1971. Both published by the Ordnance Survey, Southampton, for the Institute of Geological Sciences.

IRELAND : A GENERAL AND REGIONAL GEOGRAPHY, by T. W. Freeman, Fourth edition. London : Methuen, 1909. xx + 558 pp. £5.THE IRISHNESS OF THE IRISH, by E. Estyn Evans. Belfast: the Irish Association for Cultural, Economic and Social Relations. 1908. pp. 8. 2s. 6d.ENCYCLOPAEDIA OF IRELAND. Dublin : Allen Figgis, 1968. 463 pp. 120s.AN INTRODUCTION TO MAP READING FOR IRISH SCHOOLS, by R. A. Butlin. Dublin : Longmans, Browne & Nolan Limited, 1968. 123 pp. with four half-inch O.S. map extracts. 10s.AN OUTLINE OF THE RE-TRIANGULATION OF NORTHERN IRELAND, by W. R. Taylor. Belfast: Her Majesty's Stationery Office, 1907. 27 pp. 4s. 6d.A REVIEW OF DRUMLIN SOILS RESEARCH, 1959-1966, by J. Mulqueen and W. Burke. Dublin : An Foras Taluntais, 1967. 57 pp. 5s.FAMILY AND COMMUNITY IN IRELAND, by Conrad M. Arensberg and Solon T. Kimball. Harvard : the University Press, 2nd edition, 1968. 417 pp. $7.95.LONDONDERRY AREA PLAN. James Munce partnership. Belfast, 1968. 156 pp. 32s 6d.AN AGRICULTURAL ATLAS OF COUNTY GALWAY, by J. H. Johnson and B. S. MacAodha. Social Sciences Research Centre, University College, Galway, Research Papers Numbers 4 and 5. Dublin : Scepter Publishers Ltd., 1967. 66 pp.LIFE IN IRELAND, by L. M. Cullen. London : B. T. Batsford Ltd. New York : G. P. Putnams's Sons. 1968. xiv + 178 pp. 25s.PHASES OF IRISH HISTORY, by Eoin MacNeill. Dublin : Gill, 1968. 364 pp. 10s 6d.ANGLO-IRISH TRADE, 1660-1800, by L. M. Cullen. Manchester : the University Press, 1968. 252 pp. 60s.IRISH PEASANT SOCIETY, by K. H. Connell. Oxford: The Clarendon Press, 1968. 167 pp. 35s.THE COUNTY DONEGAL RAILWAYS (Part One of a History of the Narrow-Gauge Railways of North-West Ireland), by Edward M. Patterson. Newton Abbot: David and Charles : 2nd edition, 1969. 208 pp. 40s.THE IRISH LIGHTHOUSE SERVICE, by T. G. Wilson. Dublin: Allen Figgis, 1908. 149 pp. 42s.REPORT OF THE DEPUTY KEEPER OF THE PUBLIC RECORDS, 1960-65. Cmd. 521. 1908. 244 pp. 17s Cd. SOURCES FOR THE STUDY OF LOCAL HISTORY IN NORTHERN IRELAND. 102 pp. 2s 6d. IRISH ECONOMIC DOCUMENTS. 37 pp. 1s. All published by Her Majesty's Stationery Office, Belfast.IRISH JOURNAL OF AGRICULTURAL ECONOMICS AND RURAL SOCIOLOGY, Volume I, numbers 1 (1967), 2 and 3 (1968). Dublin : An Foras Taluntais (Agricultural Institute). Each number 10s.JOURNAL OF THE KERRY ARCHAEOLOGICAL AND HISTORICAL SOCIETY. No. 1, 1968, 116 pp. No. 2, 1969, 150 pp.Maps and map cataloguesTHE KINGDOME OF IRELAND, by John Speed. Dublin : Bord Failte Eireann, 1966. Obtainable from the Library, Trinity College, Dublin. 12s. 6d.MAP CATALOGUE. Ordnance Survey of Northern Ireland. Belfast: Her Majesty's Stationery Office, 1908. 40 pp. 5s.CATALOGUE OF SMALL SCALE MAPS AND CHARTS. Ordnance Survey of Ireland. Dublin : Government Publications Office, 1968. 11pp. 1s.EIRE. Dublin : Ordnance Survey office. 1:350,000. 1968. 58 × 43 in. £5 10s.NORTHERN IRELAND, Sheet 4 (the south-east). 1:126,720. 1968. 40 × 30 in. Paper, flat, 5s. Ordnance Survey of Northern Ireland, Belfast.WICKLOW AND DISTRICT. Teaching extract. l:63,360, fully coloured. 1968. 1s.ICAO. Aeronautical chart: Ireland 1:500,000. 1968. Two sheets, 38 in. 29 in and 40 in. × 29 in. 5s.ICAO. World aeronautical chart: Ireland. 1:1,000,000. 1968. 21 1/2 in. × 27 in. 5s.INTERNATIONAL MAP OF THE WORLD. Ireland. 1:1,000,000. 1968. 183/4 in. 29 1/4 in. 5s.

REVIEWS OF BOOKSMAN AND HIS HABITAT. ESSAYS PRESENTED TO EMYK ESTYN EVANS, edited by R. H. Buchanan, Emrys Jones and Desmond McCourt. London: Koutledge and Kcgan Paul, 1971. xi + 279 pp. £3.5O. Reviewed by: T. J. HughesA SYSTEMATIC GEOGRAPHY OF IRELAND, bv Desmond Gillmor. Dublin: Gill and MacMilian, 1971. 214 pp. £1.06, Reviewed by: David LangridgeL'IRLANDE. 1, MILIEU ET HISTOTKE, by J. Guiffan, J. Verriere and P Rafroidi. Paris: Armand Colin, 1970. 320 pp. Reviewed by: J. P. HaughtonAIR TEMPERATURE IN IRELAND, 1931-1960. Dublin: Department of Transport and Power, Meteorological Service, 1971. 08 pp. 35p; MONTHLY, SEASONAL AND ANNUAL. MEAN AND EXTREME VALUES OF DURATION OF BRIGHT SUNSHINE IN IRELAND, 1931-1900. Dublin: Department of Transport and Power, Meteorological Service, 1971. 20 pp. l5p. Reviewed by: James E. KillenTHE APPLICATION OF GEOGRAPHICAL TECHNIQUES TO PHYSICAL PLANNING. Dublin: An Foras Forbartha, 1971. 105 pp. £2.00. Reviewed by: A. A. Horner AN TSUIRBHEIREACHT AR GHAEI.TACHT NA GAILLIMHE/THEGALAVAY GAELTACHT SURVEY. Galway: University College, Social Sciences Research Centre, 1969. Volume 1, 81 pp. Volume 2, 44 figures. Reviewed by: D. McCourtTRANSPORTATION IN DUBLIN. Dublin: An Foras Forbartha (The National Institute for Physical Planning and Construction Research), No Date. 4S pp. £1. Reviewed by: James E. KillenTHE CORK SUB-REGIONAL PLANNING STUDY, by F. B. Gillie. Dublin: An Foras Forbartha. 1971. 125 pp. £1.50. Reviewed by: E. M. FahyCORK: AN ENVIRONMENTAL STUDY OF CORK, by Peter Dovell. Dublin: An Foras Forbartha, 1971. 42 pp. 50p. REviewed by: E. M. FahyTHE SPADE IN NORTHERN AND ATLANTIC EUROPE, edited by Alan Gailey and Alexander Fenton. Belfast: Ulster Folk Museum and Institute of Irish Studies, 1970. xiii + 257 pp. £1.65. Reviewed by: F. H. A. AalenDESERTED MEDIEVAL VILLAGES, edited by Maurice Beresford and John G. Hurst. London: Lutterworth Press, 1971. 340 pp. £8.00. Reviewed by: J. H. AndrewsREGIONAL ARCHAEOLOGIES: ULSTER, by L. N. W. Flanagan. London: Heinemaan Educational Books, l970. 56 pp. 90p. Reviewed by: R. E. GlasscockIRISH PASSENGER STEAMSHIP SERVICES. Volume 2: SOUTH OF IRELAND, by D. B. McNeill. Newton Abbot: David and Charles, 1971. 240 pp. 0.50. Reviewed by: J. A. K. GrahameTHREE HUNDRED YEARS OF IRISH PRINTED MAPS. Belfast: Ulster Museum, 1972. 32 pp. Reviewed by: J. H. AndrewsJOURNAL OF THE KERRY ARCHAEOLOGICAL AND HISTORICAL SOCIETY, No. 4, 1971. 194 pp. Not on public sale; JOURNAL OF THE OLD ATHLONE SOCIETY, Vol. 1, No. 2, 1970 71. 76 pp. 75p. Reviewed by: J. A. K. GrahameGEOLOGY OF BELFAST AND THE LAGAN VALLEY, by P. I. Manning, J. A. Robbie and H. E. Wilson. H.M. Stationery Office, 1970. 242 pp. £2. Reviewed by: D. B. PriorMAP REVIEWGEOLOGY OF NORTHERN IRELAND, Sheet 30 (Belfast). 1:63,360. 1966; GEOLOGY OF BELFAST AND DISTRICT. 1:21,120. 1971. Both published by the Ordnance Survey. Southampton, for the Institute of Geological Sciences. Reviewed by: D. B. Prior

Experimental evidence is consistent in showing that a considerable proportion of the heterotic expression for a character is a function of the temperature. There is relatively little heterosis at temperatures below the growth optimum, but as the temperature is increased above the optimum, so also is the heterosis. This is so for growth rate in Arabidopsis; and in Drosophila for frequency of heterozygous chromosomes, for wing and thorax length, for body weight and for various components of fitness (larval survival, total viability, egg production, longevity). The temperature-heterosis relation also holds for fluctuating temperatures (larval survival in Drosophila) and for high temperature shocks (growth rate in Arabidopsis and survival in Drosophila). The genetic basis for this phenomenon can be found in the common occurrence of high temperature sensitive alleles in natural populations. Such alleles have been demonstrated in Arabidopsis and in a large number of microorganisms. Different strains of these organisms possess different series of temperature unstable alleles. These alleles constitute a burden unavoidable by the organism because they are a frequent consequence of mutation and are relatively resistant to natural selection. The fraction of temperature sensitive alleles produced by forward mutation probably exceeds one-tenth of the total mutations. Many of the loci whose activity is restored by back-mutation are unstable to high temperature. The molecular basis of temperature sensitive mutants rests on the production of unstable enzymes by the affected loci. Such enzymes are usually unstable at any temperature, but the high thermal coefficient for inactivation leads to mutant expression only at elevated temperatures. It is suggested that mutational alteration of the amino acids of an enzyme, other than those comprising the catalytic center, sensitizes the enzyme to thermal denaturation. This interpretation of heterosis is intended only for organisms whose internal temperature adjusts to the environmental temperature.

Langridge, Patricia(Oregon State University, Corvallis), and Richard Y. Morita. Thermolability of malic dehydrogenase from the obligate psychrophile Vibrio marinus. J. Bacteriol. 92:418–423. 1966.—The thermolability of malic dehydrogenase in whole cells of Vibrio marinusMP-1 grown at 15 C was compared with that of cell-free extracts and partially purified fractions. The intracellular enzyme was found to be stable between 0 C, and the organism's optimal growth temperature, 15 C. In cell-free extracts, considerable lability was noted even at 0 C, and this lability did not increase further until the enzyme was exposed to temperatures above the organism's maximal growth temperature (20 C). Twenty-fold purified enzyme was stable between 15 and 20 C, but both above and below this there was considerable inactivation. A 5-min exposure of both cold- and heat-inactivated enzyme to 15 C allowed reactivation, although to a different extent. Ammonium sulfate was found both to stimulate enzyme activity and to reactivate temperature-inactivated enzyme.

The amount of air-borne pollen was measured continuously over several days at peak flowering in apple, peach, and cherry orchards using an automatic volumetric spore trap. The maximum number of grains of pollen per m3 recorded in the apple, cherry, and peach orchards were 68, 48, and 13 respectively High concentrations were associated with high ambient temperatures and low relative humidities. In a second experiment, pollen concentrations were measured over five days inside and outside a 1/10- inch mesh insect-proof cage placed in a lemon orchard. Significantly (P<0.01) more pollen was recorded outside the cage than inside.

The availability of pollen for fertilization of Moss' Early cherries and its dispersal by wind and by honeybees was studied in the laboratory and in a cherry orchard. Ripening and dehiscence of anthers was promoted by higher temperatures and lower relative humidities. Airborne pollen concentrations in the orchard were greater between 10.00 a.m. and 4.00 p.m. than during the remainder of the day. Flight activity of bees was related to ambient temperatures and bee visits to cherry flowers were comparable with those for apples. Enclosing trees in beeproof cages caused a marked decline in fruit yields.

The availability of pollen for fertilizing Packham's Triumph pear flowers and its dispersal aerially and by honeybees was studied in the laboratory and in a pear orchard. Ripening and dehiscence of anthers were promoted by higher temperatures and lower relative humidities. More airborne pollen was in the orchard between 10 a.m. and 4 p.m. than during the remainder of the day. Flight activity of bees was related to ambient temperature but not to visits to pear trees. Enclosing trees in beeproof cages did not cause significant effects on yield of fruit or seed counts. The apparent failure of bees to effect pollination of Packham's Triumph pear flowers is discussed.

Considering the relatively high concentrations of airborne apple pollen in apple orchards at flowering time, the present study was undertaken to determine the relative roles of airborne and insect borne pollen in the pollination of apples. Enclosing trees of Yates apples, each grafted with a limb of Jonathan, in bee-proof cages caused significant reduction in the number of fruit set, the weight of fruit harvested, and the number of seeds per Fruit as compared with uncaged trees, although airborne apple pollen concentrations were 4.07 times higher inside the cages than outside. Concentrations of airborne apple pollen are greatest in late morning and early afternoon. Flight activity of bees was related to ambient conditions, temperature having a major effect. There appears to be a threshold of flight activity in the region of 13C. Ripening and dehiscence of anthers of Yates apple are promoted by higher temperatures and lower humidities.

An outbreak of a septicaemic condition of bees occurred in East Gippsland, Victoria, during the autumn, winter, and spring of 1958. The condition appears to conform generally with cases of septicaemia recorded in Canada, United States, and France. A causative organism has been isolated from septicaemic bees in Victoria which appears to fall into the type of the Pseudomonas species of Landerkin and Katznelson (1959). This organism has been proved pathogenic to bees when inoculated by dipping or injection.

A high yield of crystals of yeast tRNA has been obtained from unfractionated preparations. It appears that many of the tRNAs in the population possess such markedly similar conformational features that they exhibit common long range order.

This protein binds sulfate strongly and is implicated in sulfate transport in Salmonella typhimurium. It has a molecular weight of 32,000 and an axial ratio of 4:1. Crystals are elongate prisms up to 0.5 millimeter. X-ray diffraction photographs give discrete crystalline reflections to a spacing of at least 2 angstroms. The unit cell is orthorhombic P212121, with four molecules per unit cell of 40.8 by 47.5 by 136 angstroms. This is consistent with a highly asymmetric molecule such as the prolate ellipsoid suggested by the other physical measurements. Addition of sulfate had minimum effects on the physical properties as measured by light absorption, optical rotary dispersion, circular dichroism, fluorescence and its depolarization, nuclear magnetic resonance, and sedimentation velocity.

Virus aggregates with a unique fivefold axis have been observed in the electron microscope.

Cohen-Bazire and Kunisawa1have reported that the chromatophores of Rhodospirillum rubrum can be separated into two fractions by centrifugation in a sucrose density gradient. From their relative positions in the gradient, they were designated ‘heavy’ and ‘light’ chromatophores.

It has recently been discovered1that a mixture of gibberellin A and gibberellic acid greatly accelerates the initiation of flowers in unvernalized Hyoscyamus niger L., a vernalizable long-day biennial plant. Even more recently, it has been found2that this mixture of compounds will cause flower formation under short-day conditions in Bryophyllum crenatum Baker, a plant which requires long-day for internode elongation, short day for floral initiation, and long-day again for the development of functional flowers3.

THE photosynthetic flowering plants possess a greater capacity for synthesis of crystalloids than any other group of organisms. Their complete exogenous requirements for growth consist of only fourteen elements and the two compounds carbon dioxide and water. No intact flowering plant, with the exception of those lacking chlorophyll, has been shown to require any organic compound apart, of course, from carbon dioxide. I have now attempted to examine plant biosynthesis by the ‘mutational block’ method, which has been of great success in the analysis of metabolism in haploid micro-organisms.

Three-dimensional structure of DNA from PBS2 bacteriophage, which contains uracil in place of the more usual thymine, is the same as that of "normal" DNA. The difference in secondary structure between DNA and RNA thus appears to be due to the 2′-hydroxyl group alone.

X-RAY diffraction was first applied to the analysis of blood clotting by Bailey, Astbury and Rudall1. The apparent persistence of the α-type wide-angle X-ray diagram when fibrinogen is converted to fibrin led these investigators to conclude that “fibrinogen is no other than an insoluble modification of fibrin without any fundamental change in molecular plan”.