scispace - formally typeset
Search or ask a question

Showing papers by "National Autonomous University of Mexico published in 1987"


BookDOI
01 Jan 1987

297 citations


Journal ArticleDOI
01 Jun 1987
TL;DR: In this article, it has been shown that the dominant rock type within the Jaraguay and San Borja volcanic fields is a magnesian andesite, with up to 8% MgO at 57% SiO2, low Fe/Mg ratios, and high Na/K ratios, consistent with derivation from a mantle source with fractionated REE patterns.
Abstract: Late Cenozoic volcanism in Baja California records the effects of cessation of subduction at a previously convergent, plate margin. Prior to 12.5 m.y., when subduction along the margin of Baja ceased, the predominant volcanic activity had a calc-alkaline signature, ranging in composition from basalt to rhyolite. Acidic pyroclastic activity was common, and possibly represented the westermost, distal edge of the Sierra Madre Occidental province. After 12.5 m.y., however, the style and composition of the magmatic products changed dramatically. The dominant rock type within the Jaraguay and San Borja volcanic fields is a magnesian andesite, with up to 8% MgO at 57% SiO2, low Fe/Mg ratios, and high Na/K ratios. These rocks have unusual trace-element characteristics, with high abundances of Sr (up to 3000 ppm), low contents of Rb; K/Rb ratios are very high (usually over 1000, and up to 2500), and Rb/Sr ratios are low (less than 0.01). Furthermore, Lan/Ybn ratios are high, consistent with derivation from a mantle source with fractionated REE patterns. 87Sr/86Sr ratios are less than 0.7048, and usually less than 0.7040, whereas the pre-12.5 m.y. lavas have 87Sr/86Sr ratios between 0.7038 and 0.7063. We have previously termed these rocks bajaites, in order to distinguish them from other magnesian andesites. Bajaites also occur in southernmost Chile and the Aleutian Islands, areas which also have histories of attempted or successful ridge subduction. It is proposed that the bajaite series is produced during the unusual physico-chemical conditions operating during the subduction of young oceanic lithosphere, or subduction of a spreading centre. During normal subduction, the oceanic crust dehydrates, releasing volatiles (water, Rb and other large-ion lithophile elements) into the overlying wedge. Subduction of younger crust will result in a progressive decrease, and eventual cessation of the transfer of volatiles when subduction stops. Thermal rebound of the mantle may cause the slab to melt, perhaps under eclogitestable conditions. The resulting melt will be heavy-REE-depleted, perhaps dacitic, but will otherwise inherit MORB-like Rb/Sr and K/Rb ratios. The ascending melt will react with the mantle to form the source of the bajaitic rocks. Furthermore, any amphibole in the mantle, stabilised during the higher PH2O conditions of earlier subduction, will break down and contribute a high-K/Rb ratio component. The implications of this study are that firstly, the subducted slab does not contribute a highly fractionated REE component in most modern arcs (i.e. the slab does not melt); secondly, Rb has a very short residence time in the mantle, and its abundance in arc rocks is a direct reflection of the input from the dehydrating slab; and thirdly, bajaitelike rocks may provide recognition of attempted or successful ridge subduction in the geologic past.

225 citations


Journal ArticleDOI
TL;DR: The composition and productivity of two mangrove sites surrounding the Laguna de Terminos, Mexico, were studied from March 1979 to January 1984, and measurements were made of the tree composition, above-ground woody biomass changes, and litterfall production at a high-salinity fringing site and a low-saliency riverine site.

208 citations


Journal ArticleDOI
TL;DR: A review of some of the available methods to study the effects of site conditions on strong ground motion is presented in this paper, where the need of unified treatment of source, path and side effects in the assessment of seismic risk is pointed out.

202 citations


Journal ArticleDOI
TL;DR: In this paper, the mass, momentum and energy balance equations for two-dimensional natural convective flow in a tilted rectangular porous material saturated with fluid are analyzed by solving numerically the mass and momentum equations, using Darcy's law and the Boussinesq approximation.

176 citations


Journal ArticleDOI
01 Jan 1987-Icarus
TL;DR: In this article, an onion shell model for chondrite parent asteroids was assessed using metallographic cooling rates (MCR) derived from the compositions of metallic Fe-Ni grains, which indicated that the hottest materials in chondrites would have been buried the deepest and cooled the slowest.

168 citations


Journal ArticleDOI
01 Feb 1987-Ecology
TL;DR: The results of these experiments suggest that a achene behavior ranges from "low—risk" (peripheral achenes) to "high— risk" (central achene) within the progeny of a single individual, while individuals and populations vary in the proportion of offspring exhibiting each type of behavior.
Abstract: Heterosperman pinnatum is an annual composite that produces heteromorphic achenes, with the proportion of achene morphs varying geographically. Field, greenhouse, and laboratory experiments were used to examine the ecological consequences of the heteromorphism. Achenes were classified into three types based on morphology. Central, intermediate, and peripheral morphs, named for the positions occupied within the fruiting head, range in shape from long, thin—beaked achene with barbed awns (central) to short wide achenes without beaks or awns (peripheral). Considerable within— and between—population phenotypic variation in achene and head characteristics was documented. Field experiments using artificial dispersal agents showed that central achenes are the most likely to adhere to animals and that the proportion of a population's achenes presented for dispersal that adhere depends on the proportion of central achenes produced in the population and on the proportion of achenes with adhesive awns. Once achenes had reached the ground, mean and median secondary dispersal distances were short (@<20 cm). Central achenes lose innate dormancy earlier than peripheral achenes during the period between autumn achene production and the onset of the summer rainy season. Germination is inhibited by darkness, though darkness becomes less inhibitive at the onset of the germination season (earlier for central achenes). In the laboratory, peripheral achenes germinated over a slightly narrower range of temperatures than central achenes. Percent germination was greater for central than intermediate achenes, and for intermediate than peripheral achenes, in two natural germination events in the field. Seed bank samples indicated that few if any achenes remain dormant between years. No significant differences in the growth rates of young seedlings were found in the greenhouse. A demographic experiment documented trade—offs resulting from the dormancy differences: central achenes tended to germinate earlier than intermediate and peripheral ones, and early germination resulted in greater mortality, but survivors that had germinated earlier attained greater size and produced more seeds. Intermediate achenes, while intermediate in all components of fitness, had the highest total fitness. This was because they resembled peripheral achenes in germinating late and having high survival, but they resembled central achenes in having higher percent germination. The results of these experiments suggest that a achene behavior ranges from "low—risk" (peripheral achenes) to "high—risk" (central achenes) within the progeny of a single individual, while individuals and populations vary in the proportion of offspring exhibiting each type of behavior.

137 citations


Journal ArticleDOI
TL;DR: The authors' results show that R. phaseoli plasmids may be expressed in the A. tumefaciens background and direct the formation of effective, differentiated nodules in Phaseolus vulgaris and Leucaena esculenta, andTransconjugants containing the whole set of plasmid from CFN299 nodulate better and fix more nitrogen than the transconjugant carrying only the Sym plasmide.
Abstract: Rhizobium phaseoli CFN299 forms nitrogen-fixing nodules in Phaseolus vulgaris (bean) and in Leucaena esculenta. It has three plasmids of 185, 225, and 410 kilobases. The 410-kilobase plasmid contains the nitrogenase structural genes. We have transferred these plasmids to the plasmid-free strain Agrobacterium tumefaciens GMI9023. Transconjugants containing different combinations of the R. phaseoli plasmids were obtained, and they were exhaustively purified before nodulation was assayed. Only transconjugants harboring the 410-kilobase plasmid nodulate P. vulgaris and L. esculenta. Nodules formed by all such transconjugants are able to reduce acetylene. Transconjugants containing the whole set of plasmids from CFN299 nodulate better and fix more nitrogen than the transconjugants carrying only the Sym plasmid. Microscopic analysis of nodules induced by A. tumefaciens transconjugants reveals infected cells and vascular bundles. None of the A. tumefaciens transconjugants, not even the one with the whole set of plasmids from CFN299, behaves in symbiosis like the original R. phaseoli strain; the transconjugants produce fewer nodules and have lower acetylene reduction (25% as compared to the original R. phaseoli strain) and more amyloplasts per nodule. More than 2,000 bacterial isolates from nodules of P. vulgaris and L. esculenta formed by the transconjugants were analyzed by different criteria. Not a single rhizobium could be detected. Our results show that R. phaseoli plasmids may be expressed in the A. tumefaciens background and direct the formation of effective, differentiated nodules. Images

137 citations


Journal ArticleDOI
TL;DR: In this paper, the convergence of the Collatz error measures and the conditioning of the solution matrices are investigated in detail, and various ways of fitting the boundary conditions in the T-complete functions method are presented.
Abstract: The paper presents various ways of fitting the boundary conditions in the T-complete functions method. The authors point out the distinct advantages of the orthogonal collocation in comparison to the equidistant collocation and the integral fit. The convergence of the Collatz error measures and the conditioning of the solution matrices are investigated in detail.

118 citations


Journal ArticleDOI
TL;DR: Crystals of beta-lactamase II (EC 3.5.2.6., 'penicillinase') from Bacillus cereus were grown with Cd(II) in place of the natural Zn( II) cofactor and stabilized by cross-linking with glutaraldehyde to suggest the probable location of the active site of the enzyme.
Abstract: Crystals of beta-lactamase II (EC 3.5.2.6., 'penicillinase') from Bacillus cereus were grown with Cd(II) in place of the natural Zn(II) cofactor and stabilized by cross-linking with glutaraldehyde. Their space group is C2, the cell dimensions are a = 5.44 nm, b = 6.38 nm, c = 7.09 nm and beta = 93.6 degrees, and there is one molecule in the asymmetric unit. Diffraction data were collected from cross-linked crystals of the Cd(II)-enzyme, the apoenzyme and six heavy-atom derivatives. The electron-density map calculated at 0.35 nm resolution reveals the essential Cd(II) ion surrounded by three histidine residues and one cysteine residue. The position of a glutamic acid residue, modification of which destroys activity [Little, Emanuel, Gagnon & Waley (1986) Biochem. J. 233, 465-469], suggests the probable location of the active site of the enzyme. Two minor Cd(II) sites not essential for activity were also located. The structure of the apoenzyme at this resolution appears to differ from that of the Cd(II)-enzyme only in the orientation of two of the histidine residues and the cysteine residue that surround the metal ion.

98 citations


Journal ArticleDOI
TL;DR: The survival of totally defoliated juvenile and immature individuals was 50% less than that of undefoliated plants and the survival of mature individuals was not significanly affected by the defoliation.
Abstract: (1) The effects of three levels of defoliation at four stages of development on leaf production, reproduction probability, fruit production and survival of an understorey palm, Astrocaryum mexicanum, were studied from 1978 to 1981. (2) Three portions of the crown were factorized with the partial defoliation treatments. (3) Completely defoliated juvenile and immature palms produced significantly fewer leaves than control plants. In mature palms, the removal of the oldest portions of the crown increased leaf production by 30%. (4) Complete defoliation and the removal of two-thirds of the oldest leaves had the effect of reducing leaf abscission in all stages, except for seedlings. (5) Defoliation had a significant effect on reproduction during all years, except 1978. Completely defoliated palms had a lower probability of reproduction and produced no fruits for three years. The removal of a small proportion of the oldest tissue significantly increased fruit production. Defoliations greater than 33% of the total number of leaves reduced the reproductive capacity of palms, except when the oldest tissue was removed. (6) The survival of totally defoliated juvenile and immature individuals was 50% less than that of undefoliated plants. The survival of mature individuals was not significanly affected by the defoliation.

Journal ArticleDOI
TL;DR: In this paper, a systematic study of compound and solid-solution formation in the system Li2ONb2O5TiO2 has been made.

Journal ArticleDOI
TL;DR: In this paper, the genomes of two Rhizobium phaseoli strains, one Rhizibium meliloti strain, and one Agrobacterium tumefaciens strain were analyzed for the presence of repetitive DNA.
Abstract: Repeated DNA sequences are a general characteristic of eucaryotic genomes. Although several examples of DNA reiteration have been found in procaryotic organisms, only in the case of the archaebacteria Halobacterium halobium and Halobacterium volcanii [C. Sapienza and W. F. Doolittle, Nature (London) 295:384-389, 1982], has DNA reiteration been reported as a common genomic feature. The genomes of two Rhizobium phaseoli strains, one Rhizobium meliloti strain, and one Agrobacterium tumefaciens strain were analyzed for the presence of repetitive DNA. Rhizobium and Agrobacterium spp. are closely related soil bacteria that interact with plants and that belong to the taxonomical family Rhizobiaceae. Rhizobium species establish a nitrogen-fixing symbiosis in the roots of legumes, whereas Agrobacterium species is a pathogen in different plants. The four strains revealed a large number of repeated DNA sequences. The family size was usually small, from 2 to 5 elements, but some presented more than 10 elements. Rhizobium and Agrobacterium spp. contain large plasmids in addition to the chromosomes. Analysis of the two Rhizobium strains indicated that DNA reiteration is not confined to the chromosome or to some plasmids but is a property of the whole genome. Images


Journal ArticleDOI
TL;DR: Results suggest that the NO3-:NH4+ balance in the culture medium is a key factor controlling callus growth and organogenesis in rhizome cultures in Agave fourcroydes Lem.
Abstract: In vitro plant regeneration of Agave fourcroydes Lem. (Agavaceae) is described. Results suggest that the NO3 -:NH4 + balance in the culture medium is a key factor controlling callus growth and organogenesis in rhizome cultures. Stem callus showed limited organogenic capacity, but high cytokinin concentrations induced adventitious shoot formation on stem explants. When these shoots were excised and subcultured, new callus formed at their base from which new shoots arose. The shoots from stem explants and rhizome callus formed extensive root systems in vitro and were transferred to pot culture with a 90% survival rate.

Journal ArticleDOI
TL;DR: The results showed that a differential response in the production of the activities was produced, suggesting that galacturonic acid participates in the induction of pectinolytic activities of Aspergillus sp.

Journal ArticleDOI
TL;DR: As!rocaryurn mexicanurn is an example of a highly diverse pollination system in which many animals can coexist, and represents an important resource during the dry season, and provides food and space for refuge and mating.

Journal ArticleDOI
TL;DR: Particular emphasis is given to the following aspects: to motivate, from geometrical and optimization considerations, the modifications (dead zones) required to cope with the disturbances, and to guarantee the convergence properties which are essential in the establishment of stability of identification-based adaptive control systems.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the influence of rheological properties of various fluids on the translational motion of a sphere and concluded that small perturbation theories provided an adequate description of the flow field for the creeping flow regime.
Abstract: The present communication is intended as part of a systematic investigation of the influence of rheological properties of various fluids upon the translational motion of a sphere Drag measurements are presented for spheres of various diameters moving longitudinally with constant translational velocity, along the central axis of cylindrical containers of different radii Four different types of fluids (based upon measurements of shear viscosity and first normal stress difference) are considered, namely Newtonian, viscoelastic, inelastic and constant viscosity-elastic liquids The influence of each rheological property of these fluids upon the drag force is evaluated and analyzed In the case of negligible wall effects, it is concluded that small perturbation theories provide an adequate description of the flow field for the creeping flow regime The drag departure for viscoelastic liquids from the purely viscous Newtonian value has a quadratic dependence on the We number Elastic effects are of primary importance at extremely low shear rates For higher shear rates, shear thinning effects become predominant and an accurate drag prediction based on simple shear dependent viscosity values is presented For non-Newtonian fluids, wall proximity effects are considerably reduced from the purely viscous Newtonian value Both elasticity and shear thinning properties of the fluid contribute to that reduction Elastic effects on the drag may be accounted for using Caswell's wall correction term derived from small perturbation theories for creeping flow and its validity exceeds the limit of small We numbers; this is substantiated by numerical techniques Fluid elasticity diminishes wall effects but is a rapidly decreasing function, valid only for small We numbers and large sphere/container ratios Beyond this region, shear thinning effects are predominant both upon the drag force and upon the wall correction term These inelastic effects may be evaluated quite accurately using a modified version of Caswell's correction formula, which requires only a simple knowledge of the shear thinning viscosity function

Journal ArticleDOI
TL;DR: A protozoological analysis was performed from June to August, 1982 to isolate small free-living amoebae from the atmosphere by using an air vacuum sampler and several filters, finding strains, known to be pathogenic in humans.

Journal ArticleDOI
TL;DR: Differences among strains were that agonistic behavior among subordinates was higher in the Balb/cj and NIH groups than in the C57Bl/6j mice, and dominants showed higher rates of Wheel Running Activity than subordinates.

Journal ArticleDOI
TL;DR: In this article, the Burgers vector of dislocations formed during the thermo-mechanical treatment needed to induce the two way memory effect in a Cu-Zn-Al alloy was determined.

Journal ArticleDOI
TL;DR: Some of the relationships among the topological notions: Hausdorff, compact, perfect, and closed are abstracted to a more general categorical setting, where they are shown to remain intact as mentioned in this paper.

Journal ArticleDOI
TL;DR: The interhemispheric correlation patterns for each subject were observed to become similar during the communication sessions as compared to the control situations, and these effects are not due to nonspecific factors such as habituation or fatigue.
Abstract: Correlation patterns between the electroencephalographic activity of both hemispheres in adult subjects were obtained. The morphology of these patterns for one subject was compared with another subject's patterns during control situations without communication, and during sessions in which direct communication was stimulated. Neither verbalization nor visual or physical contact are necessary for direct communication to occur. The interhemispheric correlation patterns for each subject were observed to become similar during the communication sessions as compared to the control situations. These effects are not due to nonspecific factors such as habituation or fatigue. The results support the syntergic theory proposed by one of the authors (Grinberg-Zylberbaum).


Journal ArticleDOI
TL;DR: From observations of the spatial distribution of humpback whales in the Mexican Pacific between 1981 and 1986, it is possible to recognize four subregions: 1) the southern coast of Baja California; 2) the northern Gulf of California, including the Midriff Islands; 3) the mainland coast of Mexico, including Isla Isabel and Islas Tres Marias and 4) the Revillagigedo Archipelago as discussed by the authors.
Abstract: From observations of the spatial distribution of humpback whales in the Mexican Pacific between 1981 and 1986, it is possible to recognize four subregions: 1) the southern coast of Baja California; 2) the northern Gulf of California, including the Midriff Islands; 3) the mainland coast of Mexico, including the Isla Isabel and Islas Tres Marias and 4) the Revillagigedo Archipelago. The seasonal distribution of whales near the Mexican mainland and the Revillagigedo Archipelago extends from November to May and is similar to that of other winter breeding grounds, including the Hawaiian Islands. Along the southern coast of Baja California, whales have been observed from September to April, possibly indicating a shorter migratory route. In the northern Gulf of California, however, humpback whales have been reported throughout the year and are occasionally observed feeding during both summer and winter months. The degree of individual movement between the four subregions is still unknown. The number of individual humpback whales identified photographically in recent years suggests that there ate more whales in the Mexican Pacific than previously

Journal ArticleDOI
01 Apr 1987-Nature
TL;DR: In this article, the authors show that for the observed seismic wave period of 2 s, a standing-wave solution exists which explains qualitatively the damage distribution pattern, which suggests a resonance phenomenon of the waterlogged ground moving collectively.
Abstract: The destruction caused by the earthquake of 19 September 1985 in Mexico City shows three remarkable features1. First, a (concentration of damage in the former lake bed; second, a peculiar distribution of high- and low-damage areas alternating within a few city blocks, and third, a selectivity for buildings between five and fifteen storeys high. While the last point is understood by civil engineers2 and the first confirms the relevance of the soft ground, the second feature is the most puzzling. To a physicist such a pattern evokes the idea of a standing wave, with low- and high-damage areas corresponding to nodes and antinodes. This suggests a resonance phenomenon of the waterlogged ground moving collectively. From the Navier equations of classical elasticity, it is known3 that a transfer of the incoming seismic-wave energy to longitudinal waves takes place if a sharp change from hard to soft ground exists. Solving then the two-dimensional Poisson equation for the longitudinal waves within the ancient lake boundaries, we show that for the observed seismic wave period of 2 s, a standing-wave solution exists which explains qualitatively the damage distribution pattern.

Journal ArticleDOI
TL;DR: The growth of adsorbed and free bacteria in medium containing both ferrous iron and elementary sulfur indicated that T. ferrooxidans can simultaneously utilize both energy sources.
Abstract: Growth kinetics of Thiobacillus ferrooxidans in batch cultures, containing prills of elementary sulfur as the sole energy source, were studied by measuring the incorporation of radioactive phosphorus in free and adsorbed bacteria. The data obtained indicate an initial exponential growth of the attached bacteria until saturation of the susceptible surface was reached, followed by a linear release of free bacteria due to successive replication of a constant number of adsorbed bacteria. These adsorbed bacteria could continue replication provided the colonized prills were transferred to fresh medium each time the stationary phase was reached. The bacteria released from the prills were unable to multiply, and in the medium employed they lost viability with a half-live of 3.5 days. The spreading of the progeny on the surface was followed by staining the bacteria on the prills with crystal violet; this spreading was not uniform but seemed to proceed through distortions present in the surface. The specific growth rate of T. ferrooxidans ATCC 19859 was about 0.5 day/sup -1/, both before and after saturation of the sulfur surface. The growth of adsorbed and free bacteria in medium containing both ferrous iron and elementary sulfur indicated that T. ferrooxidans can simultaneously utilize both energymore » sources.« less

Journal ArticleDOI
TL;DR: In rat liver cirrhosis collagenolytic activity is shown to be significantly decreased below the normal level and to remain so for prolonged periods, when the data are corrected for the variable amounts of collagen present in the insoluble liver sediments.

Journal ArticleDOI
27 Mar 1987-Science
TL;DR: Protein isolated from sperm acrosomal granules of the marine worm Urechis caused electrical responses in oocytes with the same form, amplitude, and ion dependence as the fertilization potentials induced by living sperm.
Abstract: The earliest known response of eggs to sperm in many species is a change in egg membrane potential. However, for no species is it known what components of the sperm cause the opening of the egg plasma membrane channels. Protein isolated from sperm acrosomal granules of the marine worm Urechis caused electrical responses in oocytes with the same form, amplitude, and ion dependence as the fertilization potentials induced by living sperm. Sperm initiated fertilization potentials in oocytes when sperm-oocyte fusion, but not binding, was inhibited by clamping oocyte membrane potentials to positive values. Acrosomal protein also initiated electrical responses in clamped oocytes. These results support the hypothesis that it is the sperm acrosomal protein that opens ion channels in the oocyte membrane.