Showing papers by "University of Los Andes published in 1989"
TL;DR: Fog interception and rainfall were measured during one year in three elfin cloud forests on small mountains along the Caribbean coast of South America and one in the Venezuelan Andes.
Abstract: Fog interception and rainfall were measured during one year in three elfin cloud forests on small mountains along the Caribbean coast of South America and one in the Venezuelan Andes. (1) While rainfall increases from west to east in the small mountains along the coast, fog interception decreases. In 1985, the total annual rainfall and fog interception were 853 mm and 796 mm in the cloud forest of Serrania de Macuira, 1630 mm and 518 mm in Cerro Santa Ana and, 4461 mm and 480 mm in Cerro Copey. In the Andean forest of El Zum-bador, the 1985 rainfall was 1983 mm and the annual fog interception was only 72 mm. (2) Fog interception seems to be an important source of water to the elfin cloud forests of the small mountains which are surrounded by dry vegetation types and where the rainfall regime is highly seasonal. (3) Fog interception increases with altitude (in the same mountain), exposure (windward slopes) and leaf inclination. These variations of fog interception could partially explain the observed distribution of epiphytic flora in some of these cloud forests.
151 citations
TL;DR: In this paper, the problem of inducing local sliding regimes on non-linear smooth surfaces defined in the state space of general nonlinear controlled systems is treated and a suitable notion of relative degree is found to be of crucial importance in establishing the most salient properties of nonlinear systems undergoing sliding motion.
Abstract: In this paper we treat the problem of inducing local sliding regimes on non-linear smooth surfaces defined in the state space of general non-linear controlled systems. A suitable notion of relative degree is found to be of crucial importance in establishing the most salient properties of non-linear systems undergoing sliding motion. The relevance of sliding modes in several control problems as complementary ‘outer loop’ feedback is examined
88 citations
TL;DR: Differences in the daily cycles of osmotic potential of the xylem sap indicate the existence of a high resistance pathway along the vascular connection between the parasite and its host, thus ensuring an adequate water and carbon balance.
Abstract: The gas exchange and water relations of the hemiparasite Pthirusa maritima and two its mangrove host species, Conocarpus erectus and Coccoloba uvifera, were studied in an intertidal zone of the Venezuelan coast. Carbon uptake and transpiration, leaf osmotic and total water potential, as well as nutrient content in the xylem sap and leaves of mistletoes and hosts were followed through the dry and wet season. In addition, carbon isotope ratios of leaf tissue were measured to further evaluate water use efficiency. Under similar light and humidity conditions, mistletoes had higher transpiration rates, lower leaf water potentials, and lower water use efficiencies than their hosts. Potassium content was much higher in mistletoes than in host leaves, but mineral nutrient content in the xylem sap of mistletoes was relatively low. The resistance of the liquid pathway from the soil to the leaf surface of mistletoes was larger than the total liquid flow resistance of host plants. Differences in the daily cycles of osmotic potential of the xylem sap also indicate the existence of a high resistance pathway along the vascular connection between the parasite pathway along the vascular connection between the parasite and its host. P. maritima mistletoes adjust to the different physiological characteristics of the host species which it parasitizes, thus ensuring an adequate water and carbon balance.
59 citations
TL;DR: In this article, the theory of variable structure systems and their associated sliding regimes is extended to controlled dynamical systems described by first order quasilinear partial differential equations, and the authors extend the theory to the case of sliding regimes.
54 citations
TL;DR: The aetiology of cardiac failure in Chagas disease is discussed and it is suggested that parasympathetic abnormalities are a consequence of, rather of, the progressive cardiac enlargement seen in these patients.
52 citations
TL;DR: It was shown that even when they are on a high-fat diet male rats do not show body weight gain induced by systemic sulpiride, and the well known hyperprolactinemia, induced by the pituitary D2 dopamine receptor blockade, might bring about an impairment of the steroidogenesis with subsequent decrease in estrogens level, which in turn might be responsible for the hyperphagia and body weight increaseinduced by systemic injections of sulPiride.
Abstract: Long-term intraperitoneal administration of sulpiride induced body weight increase in female but not in male rats. The hypothesis that systemic sulpiride causes an endocrine unbalance which in turn causes body weight gain and hyperphagia was tested in four experiments. First, it was shown that even when they are on a high-fat diet male rats do not show body weight gain induced by systemic sulpiride. Second, sulpiride suppressed the estrous cycle. Third, gonadectomy prevented the body weight gain induced by systemic sulpiride in female rats. Fourth, estradiol simultaneously administered with sulpiride prevented the expected sulpiride-induced body weight gain. These results are discussed in terms of an hypothetical functional castration produced by systemic sulpiride. The well known hyperprolactinemia, induced by the pituitary D2 dopamine receptor blockade, might bring about an impairment of the steroidogenesis with subsequent decrease in estrogens level, which in turn might be responsible for the hyperphagia and body weight increase induced by systemic injections of sulpiride.
48 citations
TL;DR: Two cohorts of seedlings of Andropogon semiberbis were tagged and monitored for up to three years and several variables such as survivorship, plant size, seedling density, fire damage and distance to the nearest adult were measured or estimated.
Abstract: Two cohorts of seedlings of Andropogon semiberbis were tagged and monitored for up to three years. Several variables such as survivorship, plant size, seedling density, fire damage and distance to the nearest adult were measured or estimated. Two burning experiments to measure survivorship before and after fire, and two seedling-growth experiments to assess competitive interference from adult plants of three grass species were performed
35 citations
TL;DR: It was found that Chagas' disease stages showed significant myocardium damage between stages IA and II, and during ChagAs' disease evolution the most affected organelle was the sarcoplasmic reticulum.
Abstract: Myocardial damage was analyzed in the different stages of Chagas' disease. Myocardial biopsies from chagasic patients, whose clinical histories were initially unknown to the examiner, were examined and evaluated by electron microscopy using a table in which 244 characteristics were considered. When the ultrastructural results were associated with their respective clinical reports, it was found that Chagas' disease stages showed significant myocardium damage between stages IA and II:IA (normal EKG and cineventriculogram), 13.0%; IB (early left ventricular damage with normal EKG), 14.5%; II (advanced left ventricular damage, abnormal EKG), 25.5%; stage III (congestive heart failure) showed a decrement to 22.5% if compared with stage II. The acute stage of the illness was characterized by the presence of the parasites within the myocytes which were surrounded by inflammatory cell infiltrate. During Chagas' disease evolution the most affected organelle was the sarcoplasmic reticulum.
33 citations
TL;DR: In this article, the shifted 1/N expansion was used to obtain energy eigenvalues of the rotating Morse potential and the results obtained agree very well with the numerical results for the H2 molecule.
29 citations
TL;DR: In this paper, the authors analyzed the deformation potentials of chalcopyrite compounds and found that both the valence and conduction-band extrema move to higher energies on compression in these compounds.
Abstract: From the analysis of the variation of the energy gap with temperature and pressure in some ${A}^{\mathrm{I}{B}^{\mathrm{III}{C}_{2}^{\mathrm{VI}}}}$ chalcopyrite compounds, valence- and conduction-band acoustic deformation potentials are calculated. It is found that both the valence- and conduction-band extrema move to higher energies on compression in these compounds. This should explain the small band-gap pressure and temperature coefficients observed in ${A}^{\mathrm{I}{B}^{\mathrm{III}}}$${C}_{2}^{\mathrm{VI}}$ chalcopyrite compounds relative to their binary analogs.
25 citations
TL;DR: In this article, measurements of magnetic susceptibility in the temperature range 4.2-300 K were made on polycrystalline samples of the (AgIn) 1 - z Mn 2 z Te 2 and (CuIn) (1 - n Mn 2 Z Te 2 ) alloys, and the data used to give values of spin-glass transition temperature T g and Curie-Weiss paramagnetic temperature θ.
TL;DR: In this paper, the reaction of dimethyl diazomalonate with 2-alkoxy-6-methyl-3,4-dihydro-2H-pyran in hot fluorobenzene has been studied in the presence of some transition metal catalysts.
TL;DR: In this article, exact solutions of the Einstein-Maxwell field equations for the case of static and spherically symmetric distribution of charged matter are obtained through the extension of a method originally used for neutral configurations and are conveniently matched to the Reissner-Nordstrom exterior metric.
Abstract: Exact solutions of the Einstein-Maxwell field equations are obtained for the case of static and spherically symmetric distribution of charged matter. The solutions are obtained through the extension of a method originally used for neutral configurations and are conveniently matched to the Reissner-Nordstrom exterior metric. The physical plausability of the solutions is discussed and it is shown that some of them reduce in appropriate limits to known neutral or charged solutions.
TL;DR: This paper examines the variations of 17 grammatical variables within the communicative function of 51 medical English papers and provides a framework of grammatico-rhetorical relationships that characterize the style of a given sub-genre and shows similarities and differences in grammatical forms among the observed objects.
TL;DR: In this article, the first-order decomposition of EDA in 1,4-dioxane under bis-(hexafluoroacetoacetonato)copper(II) and of dirhodium tetraacetate (DRTA) has been measured in the temperature range 332-352°K.
Journal Article•
TL;DR: The present study describes morfological features of Eulimnadia colombiensis, a new Colombian species (Arthropoda, Crustacea, Conchostraca), a species of predominantly parthenogenetic reproduction.
Abstract: The present study describes morfological features of Eulimnadia colombiensis, a new Colombian species (Arthropoda, Crustacea, Conchostraca). A species of predominantly parthenogenetic reproduction. It is tipical for the pie de monte Llanero and the Llanos Orientales.
TL;DR: In this article, it is shown that the manganese ordering has negligible effect on the lattice parameters but appreciable lattice parameter differences can occur with different annealing temperatures, these differences being attributed to changes in non-stoichiometry.
Abstract: Polycrystalline samples of (CuIn)x(AgIn)yMn2zTe2 alloys are used in lattice parameter, optical energy gap E0, and differential thermal analysis (DTA) measurements and for various sections of constant x/y ratio, T(z) diagrams are determined. The results indicate that for both the zincblende and chalcopyrite phases, ordering of the manganese occurs in the lower temperature range. In contradiction of previous data, it is shown that at temperatures of 200 °C and below, the ordered chalcopyrite α′ is the equilibrium condition over the complete range of solid solubility in the adamantine structures. It is shown that the manganese ordering has negligible effect on the lattice parameter but that appreciable lattice parameter differences can occur with different annealing temperatures, these differences being attributed to changes in non-stoichiometry. However, the value of E0 is appreciably affected by the manganese ordering and it is shown that E0 gives a very convenient indication of the particular phase present in a single phase sample.
An polykristallinen (CuIn)x(AgIn)yMn2zTe2-Legierungsproben werden Messungen des Gitterparameters, der optischen Energielucke E0 und der differentiellen Thermoanalyse (DTA) durchgefuhrt und fur verschiedene Abschnitte mit konstantem x/y-Verhaltnis werden T(z)-Diagramme bestimmt. Die Ergebnisse zeigen, das sowohl fur sowohl fur die Zinkblende- als auch die Chalkopyritphase im Niedertemperaturbereich Manganordnung auftritt. Im Gegensatz zu fruheren Werten wird gezeigt, das bei Temperaturen von 200 °C und darunter, das geordnete Chalkopyrit α′ die Gleichgewichtsbedingung uber den vollstandigen Bereich der Loslichkeit in den Adamantine-Strukturen darstellt. Es wird gezeigt, das Manganordnungseinflus auf den Gitterparameter vernachlassigbar ist, das jedoch betrachtliche Gitterparameterdifferenzen bei unterschiedlichen Temperungstemperaturen auftreten konnen; diese Differenzen werden Anderungen in der Nichtstochiometrie zugeordnet. Der Wert von E0 wird jedoch betrachtlich beeinflust durch Manganordnung und es wird gezeigt, das E0 in bequemer Weise eine spezielle Phase in einer Einphasenprobe anzeigt.
TL;DR: Observation chez un nourrisson de 18 mois d'une leishmaniose cutanee apparue 2 mois apres la naissance lors d'un sejour d' un mois en Tunisie.
Abstract: Observation chez un nourrisson de 18 mois d'une leishmaniose cutanee apparue 2 mois apres la naissance lors d'un sejour d'un mois en Tunisie. Identification du zymodeme MON 24. Discussion sur la contamination francaise ou algerienne
01 Jul 1989
TL;DR: In this article, the T(x) diagram of Zn2x(AgIn)1−xTe2 alloys was determined using the melt and anneal technique.
Abstract: Polycrystalline samples of Zn2x(AgIn)1−xTe2 alloys are prepared by the melt and anneal technique. Differential thermal analysis measurements are carried out on the alloys and the T(x) diagram is determined. Guinier X-ray powder photographs are used to show the equilibrium conditions and to give lattice parameter values. Two single solid phase fields are observed in the diagram, which is shown not to be pseudobinary. Room temperature optical energy gap values are determined as a function of composition.
Journal Article•
TL;DR: Se describe una especie nueva de rana, Atelopus tamaense, del paramo el Tama, en el Estado Apure, Venezuela, que incluye una clave para the identificacion de las especies venezolanas de Atelopolis y se hacen algunas comparaciones para diferenciar la especies nuevas.
Abstract: Se describe una especie nueva de rana, Atelopus tamaense, del paramo el Tama, en el Estado Apure, Venezuela. El taxon se distingue de todos los demas miembros del genero por la combinacion de extensivo ''webbing" y coloracion distintiva. Se suministran datos sobre su morfologia externa, piel, coloracion, variacion intraespecifica, dimorfismo sexual y ecologia. Se incluye una clave para la identificacion de las especies venezolanas de Atelopus y se hacen algunas comparaciones para diferenciar la especie nueva de las especies de Atelopus colombianas y venezolanas del grupo de A. ignescens al cual pertenece A. tamaense.
TL;DR: In this article, the principle of adiabatic invariance of the action is the dynamical principle behind the propensity rule for vibrational predissociation of van der Waals molecules.
03 Apr 1989
TL;DR: In this article, an average model of the PWM controlled system, derived by an infinte sampling frequency assumption, is obtained by replacing the discontinuous FWM controller by a memoryless nonlinear controller completely specified by the feedback duty ratio function.
Abstract: In this article a design method is proposed for the specification of a variety of Pulse-Width-Modulation (PWM) feedback controllers acting on plants described by Nonlinear Dynamical Systems. It is shown that an average model of the PWM controlled system, derived by an infinte sampling frequency assumption, is simply obtained by replacing the discontinuous FWM controller by a memoryless nonlineariy completely specified by the feedback duty ratio function. The average model captures the essential qualitative stability properties of the controlled system and, thus, can be used to considerably simplify the PWM controller design task. The case of PWM regulators for linear plants can be systematically treated by traditional input-output design methods. Some examples are provided.
TL;DR: It is suggested that vitamin E prevents ventricular fibrillation in acute myocardial ischemia in rats.
Abstract: The effect of vitamin E on the ventricular fibrillation threshold was studied in an experimental model of acute myocardial ischemia. An anterior thoracotomy was performed on 23 anesthetized Wistar rats. The ventricular fibrillation threshold was measured. Vitamin E was then administered intravenously to an experimental group (n=11) and a placebo to a control group (n=12). The ventricular fibrillation threshold was measured again. FInally, the left anterior descending coronary artery was occluded, producing antero-apical myocardial ischemia. The ventricular fibrillation threshold was measured again. This threshold did not very significantly when vitamin I or the placebo was administered before occluding the coronary artery but after the occlusion a threshold decreased in the placebo group was observed, whereas no such decrease was manifested in the vitamin E-treated group. The results suggest that vitamin E prevents ventricular fibrillation in acute myocardial ischemia in rats.
TL;DR: In this paper, it was shown that an upper frequency cut-off ωc is necessary to avoid an infinite energy, which implies the breaking of Lorentz invariance because of the appearance of a friction force on polarizable particles of proper frequency ω0' when the relativistic factor γ overcomes α−/2.
Abstract: If the zero-point field is considered in a realistic sense an upper frequency cut-off ωc is necessary to avoid an infinite energy. This implies the breaking of Lorentz invariance because of the appearance of a friction force on polarizable particles of proper frequency ω0’ when the relativistic factor γ overcomes α−/2. Electron-positron pairs production requires ωc to be the Compton frequency. Hence the γ of a charged hydrogen atom (H−) in a protosyncrotron could not overcome the maximum obtainable value γ≃α−/2 where α is the fine-structure constant. The corresponding limiting energy ∼1013eV will be detectable by the designed superconducting supercollider.
TL;DR: The lesions induced many sporadic anomalies in the behavior of the rats in choosing a preferred Ta and short episodes of almost parallel variations in Thy and Ta in phase with each other suggest that some lesions induce an instability of the set-point mechanism.
TL;DR: A new species of frog of the genus Colostethus is described from Andean mountain streams and pools at Parque Nacional Yacambu, Estado Lara, Venezuela.
Abstract: A new species of frog of the genus Colostethus (Anura: Dendrobatidae) is described from Andean mountain streams and pools at Parque Nacional Yacambu, Estado Lara, Venezuela. The taxon (Colostethus yustizi sp. nov.) belongs in the Colostethus collaris Group and it is distinguished from members of this monophyletic assemblage by the presence of a dark collar with pale spots, and by its reduced foot web.