Showing papers by "University of Leeds published in 1973"

Closure temperature in cooling geochronological and petrological systems

Abstract: Closure temperature (Tc) of a geochronological system may be defined as its temperature at the time corresponding to its apparent age. For thermally activated diffusion (D=Doe−E/RT it is given by $$T_c = R/[E ln (A \tau D_0 /a^2 )]$$ (i) in which R is the gas constant, E the activation energy, τ the time constant with which the diffusion coefficient D diminishes, a is a characteristic diffusion size, and A a numerical constant depending on geometry and decay constant of parent. The time constant τ is related to cooling rate by $$\tau = R/(Ed T^{ - 1} /dt) = - RT^2 /(Ed T/dt).$$ (ii) Eq. (i) is exact only if T−1 increases linearly with time, but in practice a good approximation is obtained by relating τ to the slope of the cooling curve at Tc.

The chitin system

Abstract: SUMMARY 1The view is supported that chitin is not found in Deuterostomia because of the absence of chitin synthetase, and is not found in higher plants because of the absence of glucosamine. In Fungi, control mechanisms are present affecting the synthesis of glucosamine; chitin is often present, but when it is absent this probably results from a failure to synthesize glucosamine. 2A review of conformation maps for cellulose and chitin indicates the possibility of a slightly right-handed twist in small groups of chitin chains. 3The occurrence of α, β and γ-forms of chitin in the peritrophic membranes of various insects is described. Gamma chitin seems to be the commonest form. 4In several beetles, optical and electron-microscope studies trace the formation of chitinous cocoon fibres from larval peritrophic membrane and define the discrete ribbon-like nature of the, β chitin produced in the mid-gut. 5By studying apodemes it is found that orthopteroid insects are most varied, different molecular structures being present in levator, depressor and pretarsal tendons. By contrast, Hymenoptera and Coleoptera show very similar structures in all three apodemes as well as in other parts of the cuticle. Apodemes are regarded as sampling the cuticle at their varying points of origin; they provide especially favour able material for diffraction studies. 6In arthropod cuticles there is evidence for the widespread occurrence of α chitin micelles which are three chains thick in the direction of the c axis. This is compared with the structure of γ chitin where the chains repeat in groups of three along the c axis. 7Changes in the diffraction pattern are related to the series of proteins defined by Hackman. The chitin-protein complex is not affected by water or neutral salt extrac tion, but is disrupted by treatment in urea. 8Electron microscopy defines the unit of structure as a composite microfibril: a core of chitin surrounded by adsorbed proteins. This consists of ‘primary’ protein (often repeating as regular units along the fibrils) and a quantity of ‘satellite’ protein which obscures the imaging of the regularly arranged ‘primary’ protein. There are apparent ‘bridges' between the microfibrils. 9New diffraction data give information about the size and arrangement of micro-fibrils. These fibrils may be arranged in layers of ‘rods’, or as an hexagonal arrange ment of ‘rods’.

Phosphate flow into mycorrhizal roots

Abstract: Ectotrophic mycorrhizas have been studied intensively and their value to some tree species is well known. Endotrophic mycorrhizas have received less attention and their potential value to plants has only been fully substantiated within the last 12 years. Responses of the host to mycorrhizal infection seem always to be associated with improved phosphorus nutrition of the plant. The mechanism whereby this occurs is considered. There is evidence that the ultimate limitation on phosphorus uptake by a simple cylindrical root is the diffusion impedance in the soil around it, and that widely spreading hyphae effectively short-circuit this impedance. Some data on hyphal length, and estimated net flux of phosphorus through the hyphae are given, with a discussion of possible mechanisms driving this flux.

Structural components of alginic acid. II. The crystalline structure of poly-alpha-L-guluronic acid. Results of x-ray diffraction and polarized infrared studies.

Abstract: A structural investigation of the marine algal polysaccharide poly-α-L-guluronic acid is described. The molecular chains consist of 1 4 diaxially linked L-guluronic acid residues in the 1C chair conformation and are stabilized in a twofold helix conformation by an intra-molecular O(2)H … O(6)D hydrogen-bond. The X-ray fiber diffraction photograph has been indexed to an orthorhombic unit cell in which a = 8.6 A, b (fiber axis) = 8.7 A, c = 10.7 A. A structure corresponding to the space group P212121 is proposed, in which all intermolecular hydrogen bonds interact with water molecules and in which all oxygen atoms except for the inaccessible bridge oxygens are involed. The relationship between the shape and structure of the polyguluronic acid molecule and its biological function is discussed.

Fluviatile fining-upwards cycles and the magnitude of palaeochannels

Abstract: The bankfull width (w) of modern meandering rivers (sinuosity >1.7) is related to bankfull depth (h) by the equation w = 6.8h1.54. The equation may be used to calculate limits for the bankfull widths of palaeochannels that deposited alluvial fining-upwards cycles. Calculated bankfull widths and measured bankfull widths from epsilon cross stratification are further used to state ranges of values for meander wavelength and mean annual discharge using existing empirical equations. The empirical equations directly relating width/depth ratios to sediment load and sinuosity are shown to be generally valid only for small rivers because of the increase of width/depth ratios as meandering rivers increase in size.

Studies on the enzymology of purified preparations of brush border from rabbit kidney.

Abstract: 1. A method for the preparation of brush border from rabbit kidneys is described. Contamination by other organelles was checked by electron microscopy and by the assay of marker enzymes and was low. 2. Seven enzymes, all hydrolases, were substantially enriched in the brush-border preparation and are considered to be primarily located in this structure. They are: alkaline phosphatase, maltase, trehalase, aminopeptidase A, aminopeptidase M, γ-glutamyl transpeptidase and a neutral peptidase assayed by its ability to hydrolyse [125I]iodoinsulin B chain. 3. Adenosine triphosphatases were also present in the preparation, but showed lower enrichments. 4. Alkaline phosphatase was the most active phosphatase present in the preparation. The weak hydrolysis of AMP may well have been due to this enzyme rather than a specific 5′-nucleotidase. 5. The two disaccharidases in brush border were distinguished by the relative heat-stability of trehalase compared with that of maltase. 6. The individuality of the four peptidases was established by several means. The neutral peptidase and aminopeptidase M, both of which can attack insulin B chain, differed not only in response to inhibitors and activators but also in the inhibitory effect of a guinea-pig antiserum raised to rabbit aminopeptidase M. This antiserum inhibited both the purified and the brush-border activities of aminopeptidase M. The neutral peptidase and γ-glutamyl transpeptidase were unaffected but aminopeptidase A was weakly inhibited. The characteristic responses to Ca2+ and serine with borate served to distinguish aminopeptidase A and γ-glutamyl transpeptidase from other peptidases. 7. No dipeptidases, tripeptidases or carboxypeptidases were identified as brush-border enzymes. 8. Incubation of brush border with papain released almost all the aminopeptidase M activity but only about half the activities of maltase, γ-glutamyl transpeptidase and aminopeptidase A. No release of alkaline phosphatase, trehalase or the neutral peptidase was observed.

Properties of Ultra-high Modulus Linear Polyethylenes

Abstract: We have recently been studying the effect of molecular weight and molecular weight distribution on the cold drawing behaviour of linear polyethylene. Some of the results seem to be sufficiently unusual to warrant reporting at this stage.

Mesomorphism in the System Disodium Chromoglycate-Water

Abstract: The system has been studied by a combination of polarization microscopy, using a hot stage, and X-ray diffraction analysis. The sequence of condensed phases occurring at room temperature with increasing concentration of disodium chromoglycate (DSCG) is: (i) a mesophase N; (ii) a mesophase M; (iii) crystalline solid solutions SS. A variety of evidence has shown that the molecule of DSCG retains an essentially planar configuration throughout the system. The mesomorphic phases are optically negative, indicating a parallel, or approximately parallel, arrangement of the molecular planes. (A parallel arrangement of these planes in the SS solid solutions had been previously indicated by a preliminary X-ray analysis, which pointed to there being only one molecule in the unit cell; and in conformity with such an arrangement the crystals were found to be optically negative.) The transformations between the N and M phases and between the M and SS phases take place pseudomorphically, and on reversal the orig...

A mechanical analysis of a hind leg of a frog (Rana temporaria)

Abstract: An attempt is made to relate the structure and properties of the principal extensor muscles and bones of the frog leg, to their performance in jumping and swimming. The following data are presented in turn. First, the muscles are described and their dimensions, and moment arms about the joints, are given. Secondly, the maximum isometric tension of each muscle, measured in tests with excised muscles, is reported. Stresses are calculated from these forces and the dimensions previously given. Thirdly, measurements are presented of the changing angle of each joint as the leg extends in jumping, and of the magnitude and direction of the forces exerted on the ground (measured by means of a force platform). The angles are used, in conjunction with the moment arms, to calculate the range and rate of shortening of each muscle, and hence the strain and rates of strain of their component fibres. The forces are used, with the moment arms, to calculate the forces and stresses exerted by the muscles. Fourthly, measurements taken from film of frogs swimming are used to estimate the force exerted on the water during the propulsive stroke. Muscle performance in swimming and jumping is compared. Finally, measurements of the stiffness and strength in bending of the femur and tibiofibula are presented, and related to the forces which act in jumping. The Discussion section of the paper includes an attempt to define the optimal working conditions for a muscle subjected to inertial loading.

Towards more intelligent teaching systems

Abstract: This paper suggests criteria against which the “intelligence” of a teaching machine can be judged. With the electronic computer in mind, distinctions are made between pre-structured, generative, adaptive, and self-improving teaching systems. From the work which has been carried out at Leeds, examples are taken which illustrate the characteristics and intelligence of these systems and the requirements for their implementation.

Deterministic one-counter automata

Abstract: The equivalence problem for deterministic one-counter automata is shown to bedecidable. A corollary for schema theory is that equivalence is decidable for Ianov schemas with an auxiliary counter.

Resistance to Arsenic Compounds Conferred by a Plasmid Transmissible Between Strains of Escherichia coli

Abstract: A plasmid, R773, which confers resistance to arsenic compounds, is transmissible between strains of Escherichia coli. It is a member of compatibility group F(1).

Precambrian Africa and South America as a Single Continent

Abstract: Palaeomagnetic evidence suggests that the principal cratonic areas of Africa were approximately in their present relative positions and orientations as early as 2,200 m.y. BP. This implies that the younger orogenic belts between them did not result from convergence of widely separated forelands which previously belonged to unrelated plates. South America and, until about 1,000 m.y. BP, North America may have been joined to Africa, and the concentration of all continental crust in one large mass until that time remains a possibility.

Peasant classes and primordial loyalties

Abstract: To establish a theoretical framework for the analysis of the social contexts of peasant political action, this article examines critically various approaches in social anthropology and Marxism. In the context of peasant societies, it considers the problematics comprehended in a distinction between a class‐in‐itself (an economic category) and a class‐for‐itself (a political group) recognising that the process of transformation of the one into the other is mediated by primordial ties such as those of kinship. For the analysis of such ties various approaches in social anthropology are examined, and emphasis is given to underlying conceptual problems in structural‐functional holism and methodological individualism in the light of a Marxist conception of the dialectical unity of man and society. It outlines an approach which seeks to extend the framework of class analysis.

Viscosity of Basic Magmas at Varying Pressure

Abstract: THE viscosity of magmas is an important parameter in problems concerning the generation and emplacement of magmatic rocks1,2, but no experimental data exist on hydrous basaltic melts. Here I describe laboratory determinations of the viscosity of some basic melts at one atmosphere and at high water pressures and their relationship to melt chemistry3. Basic rocks ranging in composition from basic andesite to olivine melanephelinite had viscosities between 4,000–100 (±5%) poise at temperatures near the liquidus (1,200° C) at one atmosphere. At higher temperatures (1,400° C) viscosities ranged from 260–15 (±5%) poise (Fig. 1). These results are in fair agreement with previous work using a concentric-cylinder method4–9. I also found, in agreement with previous work9,10, that the redox state of basic melts had only a marginal effect on viscosities.

Solid hexafluorobenzene: I. The crystal structure at 120 K

Abstract: X-ray analysis of hexafluorobenzene at 120 K has shown that the space group is P21/n(C 2h 5), with six molecules per unit cell, and a = 16·82, b = 9·17, c = 5·76A, β = 95·8°. Four of the molecules in the unit cell are in general positions (Wyckoff position e) and two are in special positions at centres of symmetry (Wyckoff position c). At both types of site, the molecules are planar to within 0·01 A; in them C-C = 1·36 ± 0·02 A and C-F = 1·32 ± 0·017 A. The thermal libration tensors, calculated from the atom thermal parameters, show that molecular oscillations are less constrained at type e than at type c sites.

Palaeomagnetic Studies in the British Caledonides-VI Regional and Global Implications

Abstract: Summary Palaeozoic palaeomagnetic data from the British Isles are reviewed and are found to be consistent with erratic polar shift from the vicinity of 10° N, 180° E in the Ordovician to 0°, 145° E in the Siluro-Devonian and to 25° N, 160° E in the early Carboniferous. The Cambrian pole is less well established near 25° N, 170° E. The rates and times of shifts are discussed. World-wide palaeomagnetic data suggest the existence of three groups of continents in the Early and Middle Palaeozoic—the Gondwanan, Euramerican and Siberian groups—separated by large oceans which closed in the Late Palaeozoic along the lines of the Hercynides and Uralides. The data from Gondwana and Siberia are internally consistent and each may have been a single plate to which only peripheral slices were added subsequently. Data from Euramerica are less coherent. The most probable explanation seems to be that this group of continents was loosely-knit and consisted of the North American and the Baltic/Russian plates (which joined in the Silurian on the site of the Scandinavian Caledonides) and the British Isles sub-plate which joined in the Devonian (on previously unrecognized sutures lying in the present North Atlantic and North Sea respectively). This interpretation takes all the data at face value and assumes that time-coverage is adequate in all parts of the region. Alternatively, if time-coverage was not adequate, Britain might have been part of the North American plate, but further work is required to discriminate between these interpretations. Palaeomagnetic evidence from the British Isles also indicates that little or no closure has taken place across the British Caledonides since Early Ordovician time. If so the British Isles sub-plate may have been rotated in azimuth as well as being transported laterally before joining the Euramerican plate. Examples of rotation and remagnetization on a more local scale are cited, and are ascribed to tectonic and thermal effects of orogeny respectively. The Euramerica and Gondwana groups were already close together by early Carboniferous times, which is distinctly earlier than the Hercynian-Alleghenian orogenic belt which may mark their juncture. It may be generally true that large scale crustal drift, involving consumption of a large area of oceanic crust between two continents, significantly predates the orogenic belt which forms when the two continents have met. If this is so, large scale closure across the British Caledonides might have occurred in Precambrian or Cambrian time, but palaeomagnetic data are not yet adequate to evaluate this.

Aspects of spore production in the red algaCeramium

Abstract: Tetraspore development from the post-meiotic to the mature stage has been studied using light and electron microscopy and histochemistry. The structure of the mature carpospore is identical to that of the tetraspore suggesting a similar developmental sequence. The tetrasporangial wall consists of 3 main fibrillar layers, the origin of the inner of which appears to be the wall-plasmalemma interface. The development of furrows cleaving the protoplast into 4 results in the formation of new plasmalemma and subsequently new wall fibrils. The Golgi apparatus is important in the formation of two well-defined substances. The first is fibrillar and is secretedvia vacuole-like structures into the sporangial wall. After spore release, this functions as a protective mucilaginous layer. The second has a distinctive fine structural morphology and probably functions as an adhesive. Observations on spore releasein vivo reveals a similar process for both types of spore. Each spore is surrounded by mucilage which may assist in initial attachment prior to the secretion of the adhesive.

Atrial septal defect in adults: Clinical and hemodynamic results of surgery

Abstract: Secundum type atrial septal defects were closed in 32 adults aged 30 to 57 years, with the use of cardiopulmonary bypass. Before operation, 12 had functional class II disease and 11 class III disease (New York Heart Association classification). Eight had a history of congestive cardiac failure, and 6 had atrial fibrillation. Cardiothoracic ratio ranged between 50 and 76 percent. Sixteen patients had moderate to severe pulmonary hypertension, but none had reversed shunt. There was 1 operative and 1 late death. Neither death was related to the complications usually associated with atrial septal defect in this age group. The remaining patients were followed up for 6 months to 12 years. All but 2 had functional class I disease after operation. Postoperatively a statistically significant reduction was noted in cardiothoracic ratio and pulmonary arterial pressures, and there was consistent rise in calculated pulmonary to systemic resistance ratio. A small residual shunt of 1.6:1 was present in 2 patients. It is concluded that irrespective of preoperative complications (pulmonary hypertension, atrial fibrillation and congestive heart failure), repair of atrial septal defect with left to right shunt in adults produces considerable clinical and hemodynamic improvement, and has low mortality and morbidity rates.