3,700 research outputs found
Functional imaging of plants: A nuclear magnetic resonance study of a cucumber plant
Functional magnetic resonance imaging was used to study transients of biophysical parameters in a cucumber plant in response to environmental changes. Detailed flow imaging experiments showed the location of xylem and phloem in the stem and the response of the following flow characteristics to the imposed environmental changes: the total amount of water, the amount of stationary and flowing water, the linear velocity of the flowing water, and the volume flow. The total measured volume flow through the plant stem was in good agreement with the independently measured water uptake by the roots. A separate analysis of the flow characteristics for two vascular bundles revealed that changes in volume flow of the xylem sap were accounted for by a change in linear-flow velocities in the xylem vessels. Multiple-spin echo experiments revealed two water fractions for different tissues in the plant stem; the spin-spin relaxation time of the larger fraction of parenchyma tissue in the center of the stem and the vascular tissue was down by 17% in the period after cooling the roots of the plant. This could point to an increased water permeability of the tonoplast membrane of the observed cells in this period of quick recovery from severe water los
Explaining quantity implicatures
We give derivations of two formal models of Gricean Quantity implicature and strong exhaustivity in bidirectional optimality theory and in a signalling games framework. We show that, under a unifying model based on signalling games, these interpretative strategies are game-theoretic equilibria when the speaker is known to be respectively minimally and maximally expert in the matter at hand. That is, in this framework the optimal strategy for communication depends on the degree of knowledge the speaker is known to have concerning the question she is answering. In addition, and most importantly, we give a game-theoretic characterisation of the interpretation rule Grice (formalising Quantity implicature), showing that under natural conditions this interpretation rule occurs in the unique equilibrium play of the signalling game
Populatiemodellen voor de effecten van bestrijdingsmiddelen
Het RIVM heeft een nieuw model ontwikkeld dat de effecten van in de tijd varierende blootstelling aan bestrijdingsmiddelen op de planten- en dierenpopulatie in oppervlaktewater kan voorspellen. Voordat een bestrijdingmiddel op de markt komt, wordt gekeken naar het zogenaamde milieurisico van dit middel. Het milieurisico wordt bepaald door het vergelijken van de mate van blootstelling van planten en dieren aan het bestrijdingsmiddel met de verwachte gevoeligheid van planten en dieren voor het betreffende bestrijdingsmiddel. Het ontwikkelde model bestaat uit een combinatie van een model dat de ontwikkeling van de populatie beschrijft en een model dat een variabele mate van blootstelling combineert met de effecten daarvan op de groei en overleving van planten en dieren. Deze combinatie biedt een oplossing voor het praktische probleem dat in het laboratorium de planten en dieren niet in alle leeftijden en bij alle mate van blootstellingen kunnen worden onderzocht. Naast de werking van het model wordt in het rapport ook aangegeven welke gegevens nodig zijn om het model te kunnen gebruiken.A model has recently been developed at RIVM to predict the effects of variable exposure to pesticides of plant and animal populations in surface water. Before a pesticide is placed on the market, the environmental risk of the substance has to be assessed. This risk is estimated by comparing the exposure of plants and animals to the pesticide with the expected sensitivity of these plants and animals to the particular pesticide. The model presented here consists of a combination of two models: one describing the development of a population and one combining the variable exposure with growth and survival of plants and animals. This combination offers a practical solution to the problem that it is not possible to study all stages and all exposure regimes in the laboratory. Separate from the model itself, indications are given about the data needed to use the model.RIV
Weak Quasi-elastic Production of Hyperons
The quasielastic weak production of and hyperons from
nucleons and nuclei induced by antineutrinos is studied in the energy region of
some ongoing neutrino oscillation experiments in the intermediate energy
region. The hyperon nucleon transition form factors determined from neutrino
nucleon scattering and an analysis of high precision data on semileptonic
decays of neutron and hyperons using SU(3) symmetry have been used. The nuclear
effects due to Fermi motion and final state interaction effects due to hyperon
nucleon scattering have also been studied. The numerical results for
differential and total cross sections have been presented.Comment: 26 pages, 10 figure
Medical device innovation in South Africa: Evolution of collaboration networks (2001-2013)
The evolution of medical device development in South Africa was investigated for the period 2001-2013. Collaboration networks for four sectors - academia, healthcare, industry, and science and support - were derived from a bibliometric study. Centrality measures identified dominant institutions. New actors entering the networks either exhibited preferential attachment to these institutions, or joined the network as part of an isolated cluster. Of the new institutions, foreign collaborators seldom stayed beyond five years, while local institutions seldom left after entering the field. Over the 13-year period, local collaboration activity persisted, while local-foreign collaborations were seen to decline. Over time, the network topology became more akin to that of a small-world network. The findings of the study may support innovation management by guiding institutional strategies for effective collaboration
Crossing the `Yellow Void' -- Spatially Resolved Spectroscopy of the Post- Red Supergiant IRC+10420 and Its Circumstellar Ejecta
IRC +10420 is one of the extreme hypergiant stars that define the empirical
upper luminosity boundary in the HR diagram. During their post--RSG evolution,
these massive stars enter a temperature range (6000-9000 K) of increased
dynamical instability, high mass loss, and increasing opacity, a
semi--forbidden region, that de Jager and his collaborators have called the
`yellow void'. We report HST/STIS spatially resolved spectroscopy of IRC +10420
and its reflection nebula with some surprising results. Long slit spectroscopy
of the reflected spectrum allows us to effectively view the star from different
directions. Measurements of the double--peaked Halpha emission profile show a
uniform outflow of gas in a nearly spherical distribution, contrary to previous
models with an equatorial disk or bipolar outflow. Based on the temperature and
mass loss rate estimates that are usually quoted for this object, the wind is
optically thick to the continuum at some and possibly all wavelengths.
Consequently the observed variations in apparent spectral type and inferred
temperature are changes in the wind and do not necessarily mean that the
underlying stellar radius and interior structure are evolving on such a short
timescale. To explain the evidence for simultaneous outflow and infall of
material near the star, we propose a `rain' model in which blobs of gas
condense in regions of lowered opacity outside the dense wind. With the
apparent warming of its wind, the recent appearance of strong emission, and a
decline in the mass loss rate, IRC +10420 may be about to shed its opaque wind,
cross the `yellow void', and emerge as a hotter star.Comment: To appear in the Astronomical Journal, August 200
Collisionless Damping of Fast MHD Waves in Magneto-rotational Winds
We propose collisionless damping of fast MHD waves as an important mechanism
for the heating and acceleration of winds from rotating stars. Stellar rotation
causes magnetic field lines anchored at the surface to form a spiral pattern
and magneto-rotational winds can be driven. If the structure is a magnetically
dominated, fast MHD waves generated at the surface can propagate almost
radially outward and cross the field lines. The propagating waves undergo
collisionless damping owing to interactions with particles surfing on magnetic
mirrors that are formed by the waves themselves. The damping is especially
effective where the angle between the wave propagation and the field lines
becomes moderately large ( to ). The angle tends naturally
to increase into this range because the field in magneto-rotational winds
develops an increasingly large azimuthal component. The dissipation of the wave
energy produces heating and acceleration of the outflow. We show using
specified wind structures that this damping process can be important in both
solar-type stars and massive stars that have moderately large rotation rates.
This mechanism can play a role in coronae of young solar-type stars which are
rapidly rotating and show X-ray luminosities much larger than the sun. The
mechanism could also be important for producing the extended X-ray emitting
regions inferred to exist in massive stars of spectral type middle B and later.Comment: 12 pages, including 7 figures, accepted for publication in Ap
Pulsational instability of yellow hypergiants
Instability of population I (X=0.7, Y=0.02) massive stars against radial
oscillations during the post-main sequence gravitational contraction of the
helium core is investigated. Initial stellar masses are in the range from
65M_\odot to 90M_\odot. In hydrodynamic computations of self-exciting stellar
oscillations we assumed that energy transfer in the envelope of the pulsating
star is due to radiative heat conduction and convection. The convective heat
transfer was treated in the framework of the theory of time-dependent turbulent
convection. During evolutionary expansion of outer layers after hydrogen
exhaustion in the stellar core the star is shown to be unstable against radial
oscillations while its effective temperature is Teff > 6700K for
Mzams=65M_\odot and Teff > 7200K for mzams=90M_\odot. Pulsational instability
is due to the \kappa-mechanism in helium ionization zones and at lower
effective temperature oscillations decay because of significantly increasing
convection. The upper limit of the period of radial pulsations on this stage of
evolution does not exceed 200 day. Radial oscillations of the hypergiant resume
during evolutionary contraction of outer layers when the effective temperature
is Teff > 7300K for Mzams=65M_\odot and Teff > 7600K for Mzams=90M_\odot.
Initially radial oscillations are due to instability of the first overtone and
transition to fundamental mode pulsations takes place at higher effective
temperatures (Teff > 7700K for Mzams=65M_\odot and Teff > 8200K for
Mzams=90M_\odot). The upper limit of the period of radial oscillations of
evolving blueward yellow hypergiants does not exceed 130 day. Thus, yellow
hypergiants are stable against radial stellar pulsations during the major part
of their evolutionary stage.Comment: 20 pages, 7 gigures. Accepted for publication in Astronomy Letter
ISSUES IN MEDICINE: Recommendations to improve the National Development Plan for Health
In November 2011, a draft National Development Plan (NDP) was released that addresses two of South Africa’s major challenges: poverty and inequity. Health and economic development are interdependent, presenting an important opportunity through the NDP to integrate health within goals of broader socioeconomic development. Reviewing the NDP identified gaps based on evidence and the epidemiological risk profile of South Africa. Recommendations to improve the NDP and to deal withpoverty and inequity should focus on prevention and addressing the social determinants of health, including: (i) a multisectoral approach to establish a comprehensive early childhood development programme; (ii) fiscal and legislative policies to bolster efforts to reduce the burden of  non-communicable diseases; (iii) promoting and maintaining a healthy workforce; and (iv) promoting a culture of evidence-based priority setting.Achieving the goal of ‘a long and healthy life for all South Africans’ will require healthy public policies, well-functioning institutional and physical infrastructure, social solidarity, and an active and conscientious civil society
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