Innermost Parts Research Articles

Structural Parameters of Dwarf Galaxies in the Coma Cluster: On the Origin of dS0 Galaxies

In this paper we analyze the structural parameters of the dwarf galaxies in the Coma cluster with $-18 \le M_{B} \le -16$ and classify them in two types: those with surface brightness profiles well fitted by a single Sersic law were called dwarf ellipticals (dEs), and those fitted with Sersic-plus-exponential profiles were classified as dwarf lenticulars (dS0s). The comparison of the structural parameters of the dwarf galaxies in the Coma and Virgo clusters shows that they are analogous. Photometrically, the dE and dS0 galaxies in Coma are equivalent, having similar colors and global scales. However, the scale of the innermost parts (bulges) of dS0 galaxies is similar to the bulges of late-type spiral galaxies. In contrast, dEs have larger scales than the bulges of bright galaxies. This may indicate that dS0 and dE galaxies have different origins. While dE galaxies can come from dwarf irregulars (dIs) or from similar processes as bright Es, the origin of dS0 galaxies can be harassed bright late-type spiral galaxies.

A Mídia Impressa Local: Uma Agenda de Constrangimentos e Motivações

Neste artigo ressalta-se a prática jornalística nos mais íntimos recantos do Brasil ao apresentar um elenco de especificidades que, contraditoriamente, evidenciam aspectos perturbadores e estimuladores vividos localmente, considerando os incêndios florestais em Roraima como estudo de caso.

BLACK HOLES: OBSERVATIONAL PROPERTIES

In the article we review observational features of black holes where an influence of a gravitational field is dominant and we must use strong gravitational field approach for GR. Recent X-ray observations of microquasars and Seyfert galaxies reveal broad emission lines in their spectra, which can arise in the innermost parts of accretion disks. Simulations indicate that at low inclination angle the line is measured by a distant observer as characteristic two-peak profile. However, at high inclination angles (> 85°) two additional peaks arise. This phenomenon was discovered by Matt et al. (1993) using the Schwarzschild black hole metric to analyze such an effect. They assumed that the effect is applicable to a Kerr metric far beyond the range of parameters that they exploited. We check and confirm their hypothesis about such a structure of the spectral line shape for the Kerr metric case. We discuss how analysis of the iron spectral line shapes could give an information about an upper limit of magnetic field near black hole horizon.

A unique type B inclusion from Allende with evidence for multiple stages of melting

Abstract— A large (7 mm in diameter) Allende type B inclusion has a typical bulk composition and a unique structure: a fassaite‐rich mantle enclosing a melilite‐rich core. The core and mantle have sharply contrasting textures. In the mantle, coarse (˜1 mm across), subhedral fassaite crystals enclose radially oriented melilite laths about 500 μm long that occur at the inclusion rim. The core consists of blocky melilite grains 20–50 μm across and poikilitically enclosed in anhedral fassaite grains that are optically continuous over ˜1 mm. Another unique feature of this inclusion is that melilite laths also extend from the core into the mantle. Fassaite in both the core and mantle is very rich in fine‐grained (1–10 μm) spinel. The rim laths are normally zoned (Åk30–70) inward from the rim of the inclusion with reverse zoning over the last ˜200 μm to crystallize. A very wide range of melilite compositions is found in the core of the inclusion, where gehlenitic grains (Åk5–12) occur. These grains are enclosed in strongly zoned (Åk15–70) overgrowths. The gehlenitic cores and innermost parts of the overgrowths are Na2O‐free, but the outer parts of the overgrowths are not. In the laths at the rim, Na2O decreases inward from the rim, then increases. Fassaite in the core has the same range of Ti contents as that in the mantle: 2–9 wt% TiO2 + Ti2O3. Two melting events are required to account for the features of this inclusion. In the first event, the precursor assemblage is heated to ˜1400 °C and melts except for gehlenitic (Åk5–12) melilite and some spinel. These grains become concentrated in the core. During cooling, Na2O‐free melilite nucleates at the rim of the inclusion and on the relict grains in the core. After open system secondary alteration, the inclusion is heated again, but only to ˜1260 °C. Melilite more gehlenitic than Åk40 does not melt. During cooling, Na2O‐bearing melilite crystallizes as small, blocky grains and laths in the core and as overgrowths on relict grains in the core and at the rim. Eventually melilite co‐crystallizes with fassaite, leading to the reverse zoning observed in the laths. The coexistence in this inclusion of Na‐free and Na‐bearing melilite, plus a positive correlation between Na2O and åkermanite contents in melilite in an inclusion with a bulk Mg isotopic composition that is mass‐fractionated in favor of the heavy isotopes, are both consistent with at least two melting events. Several other recently described coarse‐grained inclusions also have features consistent with a sequence of early, high‐temperature melting, secondary alteration, and remelting at a lower temperature, suggesting that remelting of refractory inclusions was a common occurrence in the solar nebula.

When DNA puts ecological works back on the right track: genetic assessment and distribution patterns of mudsnail populations in the Evros Delta lagoons

The present study aims to assess the distribution patterns of mudsnail populations in the Evros Delta lagoons and to investigate the key environmental variables affecting them. As a correct species assignment is crucial for ecological studies, the data presented here are substantiated using DNA sequences from the mitochondrial genome to verify the systematic status of the mudsnail taxon in question. A comparative molecular analysis involving almost all brackish-water taxa of Hydrobiinae indicates that the populations from the Evros Delta belong to the Pontic species Ventrosia maritima, a taxon that was not reported before from the Mediterranean. Moreover, it appears that in previous studies, the mudsnail populations from the Evros Delta were misidentified. Principal Component Analyses using environmental variables defined station groups among which V. maritima showed a significant variation in density. Mudsnail specimens were absent at stations with an immediate access to the sea (salinity 32-34 PSU, very fine to fine sand, organic matter 0.3-1.4 %), they showed intermediate densities (up to 3,000 ind./m 2 ) in innermost lagoonal parts (34-35 PSU, very fine sand, organic matter 1.0-1.4 %) and relatively high densities (up to 7,000 ind./m 2 ) only in isolated parts (15-17 PSU, fine sand, organic matter 0.5-0.8 %). Contact with the sea, which is strongly correlated with seawater renewal rate and hydrodynamism, was found to be the most important factor affecting mudsnail distributions. The data presented, being the first detailed ecological information available for V. maritima, are compared with the available information for other mudsnail taxa. The present study not only lays the foundation for future comparative eco-genetic works of mudsnail taxa, it also stresses the absolute need for the routine usage of molecular markers in ecological studies involving frequently utilized but systematically poorly understood invertebrate species.

Utilização do laser de argônio na remoção de sutura corneana

OBJETIVO: Descrever a técnica da retirada de sutura em córnea clara, evitando-se o contato da parte externa do fio com o meio intra-ocular e avaliar se esse procedimento evitaria infecções. MÉTODOS: Foi realizado estudo retrospectivo de 1.233 casos de retirada de sutura de mononylon 10.0 em córnea clara, utilizando-se laser de argônio para cortar o fio, no qual se avaliou a incidência de infecções. RESULTADOS: Em 1.071 olhos, um tiro foi suficiente para cortar o ponto. Em 162 olhos, o tiro do laser atingiu a parte epitelial da sutura, sendo necessários disparos extras para cortar a parte intra-estromal e, assim, retirá-la sem que a parte externa do fio passasse pelo interior da córnea. Não ocorreram complicações após a retirada da sutura. CONCLUSÕES: A técnica utilizada mostrou-se eficaz na remoção das suturas e possivelmente apresenta risco menor de infecção, uma vez que a parte do fio sobrejacente ao epitélio não entra em contato com as camadas mais internas da córnea, nem com a câmara anterior, não tendo sido observado nenhum caso de infecção nesta pesquisa.

Formation of Nuclear Spirals in Barred Galaxies

We have performed smoothed particle hydrodynamics (SPH) simulations for the response of the gaseous disk to the imposed moderately strong non-axisymmetric potentials. The model galaxies are composed of the three stellar components (disk, bulge and bar) and two dark ones (supermassive black hole and halo) whose gravitational potentials are assumed to be invariant in time in the frame corotating with the bar. We found that the torques alone generated by the moderately strong bar that gives the maximum of tangential-to-radial force ratio as $(F_{Tan}/F_{Rad})_{max}= 0.3$ are not sufficient to drive the gas particles close to the center due to the barrier imposed by the inner Lindblad resonances (ILRs). In order to transport the gas particles towards the nucleus ($r<100$ pc), a central supermassive black hole (SMBH) and high sound speed of the gas are required to be present. The former is required to remove the inner inner Lindblad resonance (IILR) that prevents gas inflow close to the nucleus, while the latter provides favourable conditions for the gas particles to lose their angular momentum and to spiral in. Our models that have no IILR show the trailing nuclear spirals whose innermost parts reach close to the center in a curling way when the gas sound speed is $ c_{s} \gtrsim 15$ km s$^{-1}$. They resemble the symmetric two-armed nuclear spirals observed in the central kiloparsec of spiral galaxies. We found that the symmetric two-armed nuclear spirals are formed by the hydrodynamic spiral shocks caused by the gravitational torque of the bar in the presence of a central SMBH that can remove IILR when the sound speed of gas is high enough to drive a large amount of gas inflow deep inside the ILR. However, the detailed morphology of nuclear spirals depends on the sound speed of gas.

The Role of Water Retention in the Anatexis of Metapelites in the Bushveld Complex Aureole, South Africa: an Experimental Study

Highly restitic metapelites occur at the contact of the Rustenburg Layered Suite (Bushveld Complex). On the basis of previous experimental studies, the high ( 60%) degrees of melting required to form these restites via fluid-absent processes could not have occurred at the maximum temperatures estimated in the aureole ( 800 C). We have investigated their formation by partial melting experiments using a medium-grade and a low-grade metapelitic hornfels from the aureole that are isochemical except for water content ( 1 2 and 4 4 wt % H2O, respectively). The partial melting experiments (700–1000 C at 3 kbar) demonstrate strongly contrasting behaviour for the two samples. For the higher-grade starting composition only minor melting occurred up to 800 C ( 50%) required a temperature of 1000 C. In contrast, for the lower-grade starting composition, extensive melting ( 50– 65%) and the presence of a similar restite assemblage to that observed in the aureole rocks was achieved in all experiments run at temperatures 750 C. In the absence of fluid infiltration, the results suggest that during rapid heating in the innermost parts of high-temperature contact aureoles, extensive partial melting may occur at relatively low temperatures if the fluid from the low-grade protolith is still resident in the system. This locally renders metapelitic rocks less dehydrated and, hence, more fertile than if heating had been slower.

Texture of the zona pellucida of the mature pig oocyte. The mammalian egg envelope revisited.

The zona pellucida (ZP) of mature pig oocytes is believed to consist of a dense filamentous meshwork, less compact on the inner and outer faces. The uneven surface of the ZP is made of unordered and stretched fibrils surrounding deep funnels which are the openings of the radial canaliculi. The topography of the ZP surface may contribute to the initial interplay between male and female gametes. Using cytochemical techniques for transmission electron microscopy (TEM), such as tannic acid and ruthenium red treatments, we found that the ZP of pig oocytes was essentially made of bundles of fibrils distributed in concentric layers (except in the innermost and outer parts). A correlation appears between the dense structure of the core layer of the ZP and its texture: it is constituted of superposed layers of fibril bundles, whereas only a random meshwork is found in a very thin innermost and in the outer layer. The fascicular configuration may control the permeability of the ZP, giving its semi-rigidity and elasticity, and may facilitate sperm penetration. The liquid crystal-like design of the core layer of the ZP is similar to textures found in the the vitelline envelope (zona radiata) of other vertebrates and possibly of all the deuterostomes. Such texture is probably related to the unique ZP protein composition and to a coordinated synthesis.

Eoalpine (Cretaceous) evolution of the Oman Tethyan continental margin: insights from a structural field study in Jabal Akhdar (Oman Mountains)

ABSTRACT A structural study in Jabal Akhdar (an autochthonous window in the Oman Mountains) shows that the thrusts and imbrications, mapped in the Permian-Cretaceous formations of the Arabian Platform, have a vergence top to the north-northeast and are associated with a regional cleavage resulting from a ductile shear deformation of the same vergence. This deformation affects the entire autochthonous unit, from the southwestern edge of Jabal Akhdar to the northeastern edge of Saih Hatat, with an increasing strain intensity towards the northeast. The same domain is also affected by a HP/LT metamorphism with a northeastward increase from chlorite facies to blueschist and eclogite facies, i.e. characteristic of a northeast-dipping subduction. However, the retrograde character of the metamorphic parageneses associated with the ductile shear, as well as its north-northeast vergence, indicate that this deformation is linked to the exhumation of the autochthon during the Campanian. These observations have been synthesised in a new lithospheric-scale interpretation of the geodynamic development of the North Oman continental margins during the middle to late Cretaceous. The sequential evolution along a transect passing from southwest of Jabal Akhdar to northeast of Muscat can be summarised as follows: An intra-continental subduction zone affected the autochthon of the Arabian Platform with a basal rupture lying in the proximal part of the continental margin, to the south of the northern edge of the carbonate platform. A North Muscat microplate was created between the intra-continental subduction zone and the intra-oceanic subduction that gave rise to the Samail Ophiolite; this microplate includes the outer part of the Arabian Platform, the continental slope and the entire Hawasina Basin. From Early Turonian to Late Santonian the obduction and the intra-continental subduction were coeval and parallel. The northeast edge of the North Muscat microplate plunged below the Samail Nappe whilst the emergent southwest part overthrust the innermost parts of the Arabian Platform. The leading edge of the Samail and Hawasina Nappes advanced across the southwestern border of the North Muscat microplate just before obduction and intra-continental subduction ceased at the Santonian–Campanian boundary. Towards the end of the intra-continental subduction, the lower part of the crust of the subducted autochthon delaminated the upper part, marking the first stage of the metamorphic rocks exhumation. From Early Campanian to Early Maastrichtian, the North Muscat microplate moved to the northeast, its northeastern edge sinking by gravity into the asthenosphere. The subducted autochthon rose up, and came into contact with the base of the obducted units. The resulting uplift of the ophiolite nappes produced its emergence and partial erosion. Local crustal thickening, related to the lithospheric delamination, caused doming at Saih Hatat and subsequent erosion that locally extended to the pre-Permian sedimentary basement during the Early Maastrichtian. The present day domal shape of Jabal Akhdar is however related to Tertiary tectonic events.

Use of space, foraging behaviour and strategies of survival among three coexisting species of fairy-wrens (Malurus)

Superb, Malurus cyaneus, White-winged, M. leucopterus, and Variegated, M. lamberti, Fairy-wrens coexisting in a semi-arid environment at Booligal, New South Wales, partitioned their use of space. During drought, territories overlapped more than in good years. White-winged Fairy-wrens spent the most time sitting, showed the highest preference for sitting in bushes and allopreened the most. Superb Fairy-wrens foraged the most on the ground, moved up and down to different foraging stations the most rapidly and had the widest variety of foraging stations. White-winged Fairy-wrens foraged mostly on the outer parts of bushes; Variegated Fairy-wrens foraged on the innermost parts of bushes and spent the longest at a foraging station. White-winged Fairy-wrens flew the furthest to a new foraging site. When niche breadths for macrohabitat, foraging heights and foraging stations were represented on orthogonal axes, Variegated Fairy-wrens and White-winged Fairy-wrens are more alike in their use of space than either is to Superb Fairy-wrens and the greatest distance is between Superb and Variegated Fairy-wrens. In terms of survival strategies, Superb Fairy-wrens are ‘opportunists’, White-winged Fairy-wrens are ‘coasters’ and Variegated Fairy-wrens are ‘stayers’.

Equatorial circular orbits in the Kerr–de Sitter spacetimes

Equatorial motion of test particles in Kerr--de Sitter spacetimes is considered. Circular orbits are determined, their properties are discussed for both black-hole and naked-singularity spacetimes, and their relevance for thin accretion disks is established. The circular orbits constitute two families that coalesce at the so-called static radius. The orientation of the motion along the circular orbits is, in accordance with case of asymptotically flat Kerr spacetimes, defined by relating the motion to the locally nonrotating frames. The minus-family orbits are all counterrotating, while the plus-family orbits are usually corotating relative to these frames. However, the plus-family orbits become counterrotating in the vicinity of the static radius in all Kerr--de Sitter spacetimes, and they become counterrotating in the vicinity of the ring singularity in Kerr--de Sitter naked-singularity spacetimes with a low enough rotational parameter. In such spacetimes, the efficiency of the conversion of the rest energy into heat energy in the geometrically thin plus-family accretion disks can reach extremely high values exceeding the efficiency of the annihilation process. The transformation of a Kerr--de Sitter naked singularity into an extreme black hole due to accretion in the thin disks is briefly discussed for both the plus-family and minus-family disks. It is shown that such a conversion leads to an abrupt instability of the innermost parts of the plus-family accretion disks that can have strong observational consequences.

L1157: Interaction of the molecular outflow with the Class 0 environment

We present high angular resolution interferometric observations of the dust continuum at 2.7 and 1.3 mm, and of the HC3N () and C18O () emission around L1157-mm, a Class 0 object that drives a spectacular molecular outflow. The millimeter dust emission is clearly resolved into two components, a flattened compact source of ~ AU at 1.3 mm, and mass ~0.1 , plus an extended envelope of ~3000 AU at 1.3 mm, and mass ~1.1 . The millimeter spectral index varies throughout the region, with the lower value found toward the compact protostar, possibly indicating grain growth in the denser regions. A strong interaction between the molecular outflow and the close protostellar environment is taking place and affects the structure of the innermost parts of the envelope. This is shown by the spatial coincidence between the molecular outflow and the dust (1.3 mm continuum) and HC3N emission: both tracers show structures associated to the edges of the outflow lobes. Basically, the global picture sketched for the Class 0 object L1157-mm by Gueth et al. ([CITE]) is supported. We find possible evidence of infall, but we do not detect any velocity gradient indicative of a rotating circumstellar disk.

High-Energy Gamma Rays from Stellar Associations

It is proposed that TeV gamma-rays and neutrinos can be produced by cosmic rays (CRs) through hadronic interactions in the innermost parts of the winds of massive O and B stars. Convection prevents low-energy particles from penetrating into the wind, leading to an absence of MeV-GeV counterparts. It is argued that groups of stars located close to the CR acceleration sites in OB stellar associations may be detectable by ground-based Cherenkov telescopes.

The power spectrum amplitude from clusters revisited: 8 using simulations with pre-heating and cooling

The amplitude of density perturbations, for the currently-favoured ACDM cosmology, is constrained using the observed properties of galaxy clusters. The catalogue used is that of Ikebe et al. The relation of cluster temperature to mass is obtained via N-body/hydrodynamical simulations including radiative cooling and pre-heating of cluster gas, which we have previously shown to reproduce well the observed temperature-mass relation in the innermost parts of clusters. We generate and compare mock catalogues via a Monte Carlo method, which allows us to constrain the relation between X-ray temperature and luminosity, including its scatter, simultaneously with cosmological parameters. We find a luminosity-temperature relation in good agreement with the results of Ikebe et al., while for the matter power spectrum normalization, we find sigma8 = 0.78(-0.06)(+0.30) at 95 per cent confidence for Ohm(0) = 0.35. Scaling to the Wilkinson Microwave Anisotropy Probe central value of Ohm(0) = 0.27 would give a best-fitting value of sigma(8) similar or equal to 0.9.

Signatures of highly inclined accretion disks in galactic black hole candidates and AGNs

Recent X-ray observations of microquasars and Seyfert galaxies reveal broad emission lines in their spectra, which can arise in the innermost parts of accretion disks. Simulations indicate that at low inclination angle the line is measured by a distant observer as characteristic two-peak profile. However, at high inclination angles (>$ 85 \degr$) two additional peaks arise. This phenomenon was discovered by [CITE] using the Schwarzschild black hole metric to analyze such effect. They assumed that the effect is applicable to a Kerr metric far beyond the range of parameters that they exploited. We check and confirm their hypothesis about such a structure of the spectral line shape for the Kerr metric case. We use no astrophysical assumptions about the physical structure of the emission region except the assumption that the region should be narrow enough. Positions and heights of these extra peaks drastically depend on both the radial coordinate of the emitting region (circular hot spot) and the inclination angle. We find that the best conditions to observe this effect are realized at $\theta > 85\degr$ and $r > 5r_{\rm g}$ and may exist in microquasars or low-mass black holes in X-ray binary systems, because there is some precession (and nutation of accretion disks) with not very long time periods (see, for example, the SS433 binary system). The line profiles for different inclination angles and radial coordinates are presented. To analyze the influence of disk models on the spectral line shapes we simulate the line profiles for the Shakura–Sunyaev disk model for accretion disks with the high inclination.

Sulfate Cavity Filling in the Lower Werra Anhydrite (Zechstein, Permian), Zdrada Area, Northern Poland: Evidence for Early Diagenetic Evaporite Paleokarst Formed Under Sedimentary Cover

ABSTRACT Paleokarst developed in sulfate deposits is common, and it is usually formed along the contact with the overlying permeable rocks or it is due to near-surface dissolution of bedded evaporites. In the Lower Werra Anhydrite (Zechstein) of northern Poland the paleokarst cavities are usually filled by bluish semitransparent anhydrite and more rarely by celestite, polyhalite, halite, and carbonate. In small cavities (a few centimeters across), a rim of rod-like anhydrite crystals arranged in narrow bundles occurs, and the inner part of the cavity is filled with a mosaic aggregate of short prismatic crystals of anhydrite and celestite as well as coarse irregular anhydrite. Celestite crystals and fan-shaped aggregates as well as spherulites of anhydrite are rare. In bigger cavities (some ten centimeters across), multiple zones of fibrous anhydrite are arranged in different directions in the middle part of the cavity fill. The innermost parts of large karst cavities remain hollow in some cases, with the cavity walls encrusted by coarse, well-developed crystals of anhydrite and celestite. The karst cavities in the Lower Werra Anhydrite developed in the subsurface by dissolution of CaSO4 strata in halite-rich intervals due to gypsum dehydration water. During gypsum dehydration, dissolution of that halite would have increased the sodium chloride content of the solution and thus the solubility of calcium sulfate. Dissolved calcium sulfate was removed from a leaching zone by diffusion and/or downward flow in interstitial space, and the minerals in karst cavities precipitated from the same solutions as those solutions became oversaturated because of decreases in NaCl concentration over time. This study suggests that karst in sulfate deposits can develop in the subsurface and without uplift and/or near-surface conditions.

Detection of a Second, Strong Submillimeter HCN Laser Line toward Carbon Stars

We have searched for and found strong laser action at a frequency near 891 GHz in the J = 10-9 transition between the (1110) and (0400) vibrationally excited states of HCN toward the mass-losing carbon stars IRC +10216, CIT6, and Y CVn. This line is part of a Coriolis-coupled system that has been well studied in the laboratory during the early years of molecular laser spectroscopy. This system also includes the 805 GHz J = 9-8 transition within the (0400) state that was discovered toward IRC +10216 by Schilke, Mehringer, & Menten and that we also find to be lasing in CIT6. Toward both stars, the 891 GHz line is about an order of magnitude stronger than the 805 GHz line, and observations spaced about half a year apart provide clear evidence for temporal variability. As was concluded for the latter, given that the lines' lower energy levels are 4200 K over the ground state, they must arise from the innermost parts of the stars' circumstellar envelopes. Future high-resolution interferometric observations with the Atacama Large Millimeter Array of the HCN laser lines will yield important information on the dust formation zone of carbon stars.

Factors controlling resource allocation in mountain birch

We present a comprehensive analysis of factors affecting resource allocation and crown formation in a subarctic birch tree, Betula pubescens ssp. czerepanovii (Orlova) Hämet-Ahti. Using biomass measurements and digitized data on tree architecture, we investigated several hypotheses on various factors that may modify plant growth. We also analyzed the extent to which different mechanisms operate at different scales, ranging from individual shoots to the whole branches or trees. Different factors affected allocation at different levels of organization. Stem age had a minor effect, suggesting that similar control mechanisms operate at all stages of development. Fates of individual shoots were affected by their local growing conditions as indicated, for example, by the dependence of long shoot production on light. Buds formed in the current long shoots were likely to become new long shoots. In the innermost crown parts, radial growth had priority compared to long shoot production. Elongation of individual long shoots was controlled by two conflicting factors. Long distance from the roots suppressed growth, probably indicating costs associated with resource transportation, whereas a high level of light augmented growth. In contrast, growth of entire branches was not so clearly related to the availability of resources, but showed limitation due to allometric scaling. This set a relationship between the maximum long shoot number and the overall branch size, and may indicate allometric constraints to the way a tree is constructed. Strict allometric relationships existed also between other structural traits of mountain birch, most of them similar at all levels of branching hierarchy. However, despite the upper level restrictions set by allometry, source-sink interactions and localized responses of individual shoots operated as local processes that directed allocation towards the most favourable positions. This may be a mechanism for achieving efficient tree architecture in terms of resource intake and costs of transportation.

Density Gradients in Canadian Metropolitan Regions, 1971-96: Differential Patterns of Central Area and Suburban Growth and Change

This paper demonstrates that over the 25-year period, 1971-96, the majority of Canadian cities have undergone transition towards an increasingly decentralised urban form. The trends, however, are quite diverse, pointing to fundamental differences in the respective importance of growth in central and outer parts of the metropolitan area. On the whole, the relatively high densities observed in Canadian central cities, in comparison with US ones, appear to reflect residual centralisation rather than continued growth in metropolitan regions' innermost parts. Only Vancouver, and to a lesser extent Toronto and Victoria, exhibit indisputable evidence of post-1971 central-area growth. The predominant trend has been towards suburban-style, low-density expansion, albeit with considerable intercity variation regarding changes in central-area and suburban density. Findings presented here point to previously unidentified trends towards recentralisation in a few CMAs and, in about half of the surveyed metropolitan areas, densification of suburban tracts.