Showing papers by "Simón Bolívar University published in 1991"
TL;DR: The major biogeochemical processes that control the fate of hydrophobic organic compounds in the aquatic environment are reviewed and Physico-chemical parameters used to predict the aquatic fate of such compounds are discussed.
94 citations
TL;DR: In this paper, the kinetics of growth of polypyrrole films deposited onto electrode surfaces from aqueous solutions of pyrrole have been investigated, and it is shown that the rate of film growth is limited by the low conductivity of the growing film, which was evaluated from potentiostatic current transients recorded during electrodeposition of polypolypolypyrpoly.
85 citations
TL;DR: The ability to compensate for defoliated biomass in H. rufa is possibly related to its long coevolution with large herbivores in its original African habitat and is apparently one of the causes of its success in Neotropical savannas.
Abstract: The African grass Hyparrhenia rufa has established itself successfully in South American savannas (Llanos) and displaced dominant native grasses such as Trachypogon plumosus from the wetter and more fertile habitats. Several ecophysiological traits have been related to the higher competitive capacity of H. rufa. To further analyze the behavior of both species, their growth, biomass allocation, physiological and architectural responses to defoliation and water stress were compared under controlled conditions. Although total, aerial and underground biomass decreased under defoliation in both grasses, increases in clipped-leaf biomass and area compensated for defoliation in H. rufa but not in T. plumosus. This difference was due mainly to a higher proportion of assimilates being directed to leaf and tiller production and a higher leaf growth rate in the African grass as compared to T. plumosus, which showed incrased senescence under frequent defoliation. In both species, water stress ameliorated the effects of defoliation. The ability to compensate for defoliated biomass in H. rufa is possibly related to its long coevolution with large herbivores in its original African habitat and is apparently one of the causes of its success in Neotropical savannas.
82 citations
TL;DR: Myrosmodes cochleare is a terrestrial orchid restricted to high elevations of the northern Andes and is self-compatible but not autogamous, with high flower size towards the base of the inflorescence.
Abstract: Myrosmodes cochleare is a terrestrial orchid restricted to high elevations of the northern Andes. It is self-compatible but not autogamous. Flowering begins at the apex of the inflorescence, but flower size increases steadily towards the base. Fruit set of selfed flowers was found to increase significantly from the apex to the base of the inflorescence. Open-pollinated fruit set was significantly lower than hand-pollinated fruit set overall, but the highest levels occurred in the middle of the inflorescence. Eumenid wasps and a calliphorid fly were observed to pollinate the flowers, but visits were rare and occurred only during infrequent warm periods.
62 citations
TL;DR: It was found that several variables (systolic blood pressure, the presence of atrial fibrillation, the radiologic cardiothoracic index, and left ventricular end-diastolic volume obtained by the ventriculogram), were negatively correlated with regard to survival.
57 citations
TL;DR: A thematic classification of the bibliographical material is given depending on whether work deals with the definition of theoretical details of bond graphs, applications of bond graph graphs to particular fields, or if it deals with a computer program developed using the bond graph theory.
Abstract: This paper presents a survey of the bond graph method from its conception and creation by Prof. Henry Paynter to the present. A thematic classification of the bibliographical material is given depending on whether work deals with the definition of theoretical details of bond graphs, applications of bond graphs to particular fields, or if it deals with a computer program developed using the bond graph theory. A list of journals which publish bond graph related articles, as well as textbooks is also presented. The survey, though exhaustive, does not pretend to be all inclusive. Some works may have been unintentionally omitted due to obvious limitations on accessing and listing all the relevant literature concerning the topic.
56 citations
TL;DR: The results suggested that the main advantage of using cloned peptides for chronic Chagas' disease diagnosis resided in their highly specific immunoreactive properties.
Abstract: A workshop organized by the Ibero-American Project of Biotechnology evaluated the diagnostic potential of several cloned Trypanosoma cruzi recombinant antigens for Chagas' disease serodiagnosis. A set of recombinants, Antigen 2, Antigen 13, SAPA, H49, A13, JL5, JL7, JL8, JL9, and RA1 provided by three different South American laboratories were probed with a panel of 236 South American serum samples. Antigens JL7, H49, Antigen 2, and A13 scored as the best diagnostic recombinant reagents. The results suggested that the main advantage of using cloned peptides for chronic Chagas' disease diagnosis resided in their highly specific immunoreactive properties.
50 citations
TL;DR: In this paper, a permutation-based algorithm for block clustering within the same class of block structures is proposed, where permutation arguments are used to decide where to split and when to stop.
Abstract: Hartigan (1972) discusses the direct clustering of a matrix of data into homogeneous blocks. He introduces a stepwise divisive method for block clustering within a certain class of block structures which induce clustering trees for both row and column margins. While this class of structures is appealing, the stopping criterion for his method, which is based on asymptotic theory and the assumption that the individual elements of the data matrix are normally distributed, is quite restrictive. In this paper we propose a permutation-based algorithm for block clustering within the same class of block structures. By using permutation arguments to decide where to split and when to stop, our algorithm becomes applicable in a wide variety of cases, including matrices of categorical data and matrices of small-to-moderate size. In addition, our algorithm offers considerable flexibility in how block homogeneity is defined. The algorithm is studied in a series of simulation experiments on matrices of known structure, and illustrated in examples drawn from the fields of taxonomy, political science, and data architecture.
49 citations
TL;DR: In this paper, the authors studied the behavior of a spectacled bear (Tremarctos ornatus) on cattle in the Venezuelan Andes from February 1986 to November 1988.
Abstract: Spectacled bear (Tremarctos ornatus) predatory behavior on cattle was studied in the Venezuelan Andes from February 1986 to November 1988. Only 12 of the 140 localities surveyed reported spectacled bear predation. Sixteen recent claims of predation were investigated, and predation by spectacled bear confirmed. Herding practices used in the Venezuelan Andes may increase the probability of predation and the subsequent persecution and killing of bears.
47 citations
TL;DR: In this paper, effective diffusivities in 3D, spatially periodic porous media are calculated using a three-dimensional model and the results show that the model overestimates the diffusivity parallel to the bedding plane.
Abstract: Calculations of effective diffusivities in three-dimensional, spatially periodic porous media are performed. For isotropic systems, it is found that, for a given porosity, the predicted value of the effective diffusivity matches experimental results for randomly-packed beds of spheres. Furthermore, the three-dimensional geometry yields approximately the same results as previous calculations employing two-dimensional representations, indicating a relative insensitivity of the effective diffusivity to local geometry. Regarding anisotropic systems, for which two-dimensional modes fail, it is found that there is a significant improvement in the prediction of the effective diffusivity perpendicular to the bedding plane when the three-dimensional model is employed using one adjustable parameter. However, the three-dimensional model overestimates the effective diffusivity parallel to the bedding plane.
39 citations
TL;DR: In this article, an enzyme that preferentially catalyzes the condensation of propionyl-CoA rather than acetyl CoA has been purified from the mitochondria of the parasitic intestinal nematode Ascaris lumbricoides var. suum.
TL;DR: The application of a numerical optimization scheme to the tuning of power system stabilizers is studied and the overall method is shown to be accurate and reliable.
Abstract: The application of a numerical optimization scheme to the tuning of power system stabilizers is studied. The scheme is based on minimax optimization techniques with multiple objectives given by relevant system perturbations, aggregated by means of a weighted sum. The large number of constraints is handled by separating the optimization and simulation in two levels which interchange limited information. Three different optimization techniques were tested with applications to the Venezuelan power system for the years 1989 and 2005. The overall method is shown to be accurate and reliable. >
TL;DR: In this article, the authors used dimensional analysis to establish the independent parameters that affect the static liquid holdup, and they also presented experimental data to explore the dependence of the static holdup on the E[umlt o]tv[UMlt o ]s number.
Abstract: In this paper, the authors use dimensional analysis to establish the independent parameters that affect the static liquid holdup. The authors also present experimental data to explore the dependence of the static holdup on the E[umlt o]tv[umlt o]s number; through comparison with previous theoretical works, the authors analyze the mechanisms responsible for the effect of E[umlt o]tv[umlt o]s number on the static liquid holdup.
TL;DR: The results described indicate that the CoASH modification of Ascaris propionyl-CoA condensing enzyme may be responsible for the existence of several forms of the enzyme and might represent a mode of control by chemically modulating the amount of the active forms ofThe enzyme.
TL;DR: Results of the present paper show that the ERT did not change after injecting the insect in the abdominal haemocoel with 0.9% saline solution, but ERT increased when 0.32, 0.52 or 0.69 mg/g of morphine is injected in the same place.
Abstract: The escape reaction time (ERT) of the cricket Pteronemobius sp. from the heated box begins at 48° and increases with temperature until 56°C, beyond which there is no further increase. The ERT (2.2 ± 1.39 s) from the hot box at 54°C is used as a model for studying the analgesic effects of opiates. Results of the present paper show that the ERT did not change after injecting the insect in the abdominal haemocoel with 0.9% saline solution, but ERT increased when 0.32, 0.52 or 0.69 mg/g of morphine is injected in the same place. The maximum ERT increase is reached at 90 min after drug injection, and the drug effect disappears 3 h after the injection. At 90 min after drug injections, the dose of 0.50 mg/g of morphine produces 50% of ERT increase, and it is referred to as the median analgesic dose (D50). 1.05 mg/g of morphine produces an ERT longer than 30 s that results in an irreversible damage to the insect. Sixty-four μg/g of naloxone given in addition to D50 of morphine fully blocked the effect of morphine during its 3-h action. However, more than 64 μg/g of naloxone alone also increase the ERT in the cricket, similar to what has been described for vertebrates. Four daily morphine injections of D50 decreased ERT in such a way that, at the fourth day, the ERT is similar to the ERT produced by saline solution; i.e., tolerance is shown. The suppression of daily morphine injections of D50 during the fifth day produced a hyperresponse to vibration (big jumps) not shown in the case of the injections of saline solution; i.e., addiction is shown.
TL;DR: In this article, structural data describing Trachypogon savanna relationships were used to characterize the sources of variation among savannas as a function of the environmental characteristics of the Orinoco Llanos.
Abstract: Multivariate structural data describing Trachypogon savanna relationships were used to characterize the sources of variation among savannas as a function of the environmental characteristics of the Orinoco Llanos. Twenty-six savannas including major physiognomic types of the Orinoco Llanos ranging from herbaceous to bush island savannas were ordinated on a regional scale. Results of Detrented Canonical Correspondence Analysis (DCCA) using floristics variables indicated that savannas were ordinated along two complex gradients. A soil physical and chemical gradient as expressed by changes in bulk density and magnesium concentration were evident after the analysis of the first DCCA axis. The second axis was a climatic gradient of decreasing annual precipitation and incresing monthly precipitation during the dry season. The impact of human disturbance on the savanna composition was also an explanatory variable of the second DCCA axis. Savanna sites overlapped considerably in composition and most species were widely distributed, with aboveground phytomass abundance depending upon the site.
TL;DR: Chemical analyses of three species in the Neotropical ant genus Megalomyrmex have identified this taxon as the third myrmicine genus to produce alkaloids as major venom products, and the structure was established using a micro-Hofmann degradation sequence.
Abstract: Chemical analyses of three species in the Neotropical ant genusMegalomyrmex have identified this taxon as the third myrmicine genus to produce alkaloids as major venom products. Workers ofM. leoninus and workers and ergatoids ofM. goeldii produce one or more of fourtrans-2,5-dialkylpyrrolidines previously identified in other myrmicine genera.M. modestus, on the other hand, is distinctive in producing the novel alkaloid (5E,8E)-3-butyl-5-hexylpyrrolizidine (5d), whose structure was established using a micro-Hofmann degradation sequence. The relationship ofMegalomyrmex to other alkaloid-producing ant genera is discussed along with the possible chemotaxonomic significance of the analyzed species when viewed in terms of the recognized species groups in this genus.
TL;DR: This work has identified the first clearly documented case of agamospermy derived from distyly in E. undulatum, a gametophytic apomict that produces about 15% seed set independently of pollination.
Abstract: Erythroxylum undulatum is a gametophytic apomict that produces about 15% seed set independently of pollination. Embryo sac formation is aposporous, and the plants are largely pollen-sterile, which may be a result of polyploidy and hybrid origin. It appears to be derived from a distylous ancestor, as all other known species of Erythroxylum are distylous or secondarily dioecious, and all examined individuals of E. undulatum correspond to the normal long-styled morph. This is the first clearly documented case of agamospermy derived from distyly.
TL;DR: From this study, the variation of the electric properties cannot be clearly and directly ascribed to the longitudinal variability of the total volumetric fluid content of the bone.
Abstract: A preliminary study is presented of the variability of the electric properties, in the axial, tangential and radial directions, as a function of position in the diaphysis of a femoral bovine bone. The measurements were carried out at three frequencies: 100 Hz, 10 kHz and 1 MHz. It is shown that both the conductivity and the permittivity exhibit significant variations along the diaphysis, and increase in magnitude towards the epiphyses. From this study, the variation of the electric properties cannot be clearly and directly ascribed to the longitudinal variability of the total volumetric fluid content of the bone. The results reflect the orthotropic nature of the electric properties, at any given location, and indicate a position-dependent tendency towards axis symmetry.
TL;DR: In this article, a family of statistics is presented that can be used for testing goodness of fit to a parametric family, including Mardia's measure of multivariate kurtosis and Moore and Stubblebine's test for multivariate normality.
Abstract: A family of statistics is presented that can be used for testing goodness of fit to a parametric family. These statistics include Mardia's measure of multivariate kurtosis and Moore and Stubblebine's test for multivariate normality. The asymptotic distribution of the statistics is found under mild hypotheses on the parametric family and, in the case of multivariate normality, the distribution is shown to be independent of the “true” parameter. A class of tests for multivariate normality is presented and the performance of two such tests in the bivariate case is found in simulations.
TL;DR: In this paper, the quantization of a class of dynamical systems subject to second class constraints that allow an analysis in terms of associated gauge theories with first class constraints is discussed.
Abstract: The quantization of a class of dynamical systems subject to second class constraints that allows an analysis in terms of associated gauge theories with first class constraints is discussed. The comparison with early approaches is done. The approach is applied to the self-dual formulation of spin one massive excitations in 3 dimensions. The quantum equivalence to the corresponding gauge invariant Chern–Simons topological massive model is analyzed.
TL;DR: Soldiers though appear to be quite performent in the nest-defense against invaders from other species, showing in this case an aggressiveness superior to the one observed amongst workers.
Abstract: L'etude du comportement de defense du nid chezAtta laevigata permet de classer les 4 castes physiques d'individus steriles etudies en deux groupes. Le premier groupe, constitue par les ouvrieresminor etmedia, rassemble des individus pouvant reconnaitre avec precision des intrus de meme espece. Menaces et mordus, ces derniers sont alors mutiles ou transportes puis abandonnes par les fourmis residentes aux frontieres de leur territoire. Les ouvrieresmajor et les soldats, formant le second groupe, necessitent quant a eux le marquage de l'ennemi par la pheromone d'alarme de leurs congeneres pour pouvoir prendre part aux affrontements intraspecifiques avec efficacite. Les soldats se montrent en revanche tres performants dans la defense du nid contre des envahisseurs appartenant a d'autres especes, developpant dans ce cas une agressivite superieure a celle observee chez les ouvrieres. L'âge de ces dernieres ou le type de tâche qu'elles assument ne semble pas influencer leur comportement agonistique qui est comparable dans les deux castes temporelles de recolteuses et d'architectes.
TL;DR: The eigenfunctions, eigenvalues, and typical wave-packet dynamics of a ``harmonic oscillator'' defined on a lattice, when subjected to an added constant force field \ensuremath{\alpha}.
Abstract: We obtain the eigenfunctions, eigenvalues, and typical wave-packet dynamics of a ``harmonic oscillator'' defined on a lattice, when subjected to an added constant force field \ensuremath{\alpha}. The eigenfunctions of this displaced harmonic oscillator are given in terms of Mathieu functions. In the momentum representation, the Mathieu eigenfunctions have period \ensuremath{\pi} (or 2\ensuremath{\pi}) when the bottom of the parabolic potential well corresponds to a lattice point (or is halfway between two lattice points). This occurs when \ensuremath{\alpha} is an integer (or half-integer) in units of the force needed to stretch the oscillator by one lattice constant. For an incommensurate displacement of the well (i.e., for \ensuremath{\alpha} irrational in the above units) the eigenfunctions are not periodic. In real space, the eigenfunctions are displaced zero-field eigenfunctions centered on the new equilibrium position only for the \ensuremath{\alpha} integer. The eigenvalues are obtained as the sum of a periodic function of the force \ensuremath{\alpha}, plus an overall quadratic shift in \ensuremath{\alpha}. While this quadratic term is common to the continuum limit of the theory, the first periodic one is not. As \ensuremath{\alpha} is turned on, the periodically varying relative position of the parabolic well with respect to the underlying lattice gives rise to the oscillating term for the energy eigenvalues. The time evolution of a Gaussian wave packet is followed, and, in contrast with the continuum limit, its noncoherency is displayed. Damped oscillations of this wave packet occur between the spatial limits imposed by the parabolic well, or between those given by a Bloch oscillation. The ``periods'' for these two types of motion are quite different, and each depends on both the force \ensuremath{\alpha} and on the spring constant of the oscillator. As time increases, Bloch oscillations are eventually destroyed by the inhomogeneous restoring force of the harmonic oscillator, but not by the constant force field. Finally, we compare and contrast our results with the predictions of the semiclassical solid-state model, especially those involving effects due to Bloch oscillations.
TL;DR: In this article, the authors obtained three approximation levels for the Rayleigh-type optical mixing signal intensity: the first level was an analytical solution valid only for constant pump intensity; the analytical solution corresponding to the second approximation level considers pump propagation and yields good results for a large number of cases; the third level is an exact but numerical solution.
Abstract: In a propagation treatment at all orders in the pump field and to first order in the probe and signal fields, for a homogeneously broadened line, we obtain three approximation levels for the Rayleigh-type optical mixing signal intensity: the first level is an analytical solution valid only for constant pump intensity; the analytical solution corresponding to the second approximation level considers pump propagation and yields good results for a large number of cases; the third level is an exact but numerical solution. We present and compare the results obtained with the different approximations.
TL;DR: In this paper, a new set of solutions to the low energy heterotic string field equations is obtained, and its geometrical structure is discussed in terms of supersymmetric self-dual Yang-Mills equations.
Journal Article•
TL;DR: Human semen samples were analyzed by graphite furnace atomic absorption spectrometry, using L'vov platforms and the method of standard additions, to determine their Cd, Cr, Cu, Pb and Zn content and recoveries of the added analytes showed recoveries ranging from 96 to 104%.
Abstract: Human semen samples were analyzed by graphite furnace atomic absorption spectrometry, using L'vov platforms and the method of standard additions, to determine their Cd, Cr, Cu, Pb and Zn content. The samples were analyzed directly and after conventional and microwave wet acid dissolution. Matrix modification, using magnesium and palladium nitrates and ammonium biphosphate solutions, was evaluated for the analysis of the microwave-digested samples. The best results were obtained for Cr, Cu and Zn using Pd(NO3)2, and a mixture of Mg(NO3)2 and Pd(NO3)2 solutions. The direct analysis of water-diluted semen produced inaccurate results with unacceptably high standard deviations. The results obtained for the microwave-dissolved samples showed relative standard deviation values within the range 0.63-8.4%. The analysis of spiked semen solutions showed recoveries of the added analytes ranging from 96 to 104%. The accuracy of the measurements was checked against the NIST 1,577a, bovine liver, standard reference material. Sample dissolution time was drastically reduced from 3-4 hours, using the conventional method, to approximately 8 minutes using the microwave-assisted wet acid digestion procedure.
01 Oct 1991-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this article, a decision-making criterion based on the Poisson distribution is proposed for cases where very low background counts are measured and very low intensities are assumed to be low.
Abstract: A decision making criterion based on the Poisson distribution is proposed for cases where very-low-background counts are measured and very-low-intensit
TL;DR: In this article, the distribution of a monodisperseperse solid in a semibatch bubble column with two immiscible liquid phases was explored, and experiments were conducted at atmospheric pressure and 25 °C.
Abstract: We explore the distribution of a monodisperse solid in a semibatch bubble column with two immiscible liquid phases. Experiments were carried out in a plexiglas bubble column with an internal diameter of 10 cm. They were conducted at atmospheric pressure and 25 °C. The gas used was atmospheric air. The liquid phase was kerosene and mixtures water-kerosene. The solid phase consisted of monodisperse spherical glass beads of diameter equal to 110 μm and density equal to 2,800 kg/m 3
01 Jan 1991
TL;DR: In this article, the fundamental features of mass transport to ensembles of microelectrodes are discussed, as well as the conditions and timescales whereby individual elements in an array act independently from each other.
Abstract: The fundamental features of mass transport to ensembles of microelectrodes are discussed, as well as the conditions and timescales whereby individual elements in an array act independently from each other At long times the diffusional fields around individual microelectrodes interact If all elements are held at the same potential the current becomes proportional to the area of the array With elements held at different potentials, products generated at one of the microelectrodes can be collected onto another in the ensemble Some particular geometries, the most common techniques for construction of ensembles of microelectrodes, and some of their applications are also briefly described