2,768 research outputs found
An Overview of Plant Defenses against Pathogens and Herbivores
Plants represent a rich source of nutrients for many organisms including bacteria, fungi, protists, insects, and vertebrates. Although lacking an immune system comparable to animals, plants have developed a stunning array of structural, chemical, and protein-based defenses designed to detect invading organisms and stop them before they are able to cause extensive damage. Humans depend almost exclusively on plants for food, and plants provide many important non-food products including wood, dyes, textiles, medicines, cosmetics, soaps, rubber, plastics, inks, and industrial chemicals. Understanding how plants defend themselves from pathogens and herbivores is essential in order to protect our food supply and develop highly disease-resistant plant species.
This article introduces the concept of plant disease and provides an overview of some defense mechanisms common among higher plants. A close examination of plant anatomy is presented, as well as some of the ecological relationships that contribute to plant defense and disease resistance. Special care has been taken to illustrate how products used in everyday life are derived from substances produced by plants during defense responses
The Flattened Dark Halo of Polar Ring Galaxy NGC 4650A: A Conspiracy of Shapes?
Kinematics and photometry of the polar ring galaxy NGC 4650A, including new
observations of the rotation and velocity dispersion of its central stellar
disk, are used to infer the presence of a dark matter halo and to measure its
shape. Fits to the observed disk and polar ring rotation curves from detailed
mass and photometric modeling rule out a spherical dark halo. The best fit
models have halos with isodensity surfaces that are flattened to a shape
between E6 and E7 (axis ratios between 0.4 and 0.3); the asymptotic equatorial
speeds of these models are in excellent agreement with the I-band Tully-Fisher
relation. This degree of dark halo flattening is larger than that expected from
N-body collapse simulations of dissipationless dark matter. Since the
kinematics and surface brightness profile of the central luminous body indicate
that its light has an intrinsic axis ratio c/a \lsim 0.4, in NGC 4650A the
radial ``conspiracy" between the dark and luminous components that leads to
flat rotation curves may extend to the {\it shape} of the mass distribution as
well.Comment: Uuencoded compressed tar file (complete with figures). Institute for
Advanced Study number AST 94/2
Distributions of Birkeland current density observed by AMPERE are heavyâtailed or longâtailed
We analyze probability distributions of Birkeland current densities measured by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). We find that the distributions are leptokurtic rather than normal and they are sometimes heavy-tailed. We fit q-exponential functions to the distributions and use these to estimate where the largest currents are likely to occur. The shape and scale parameters of the fitted q-exponential distribution vary with location: The scale parameter maximises for current densities with the same polarity and in the same location as the average Region 1 current, whereas the shape parameter maximises for current densities with the same polarity and in the same location as the average Region 2 current. We find that current densities |J|â„ 0.2 ÎŒA mâ2 are most likely to occur in the average Region 1 current region, and second most likely to occur in the average Region 2 current region. However, for extreme currents (|J|â„ 4.0 ÎŒA mâ2), we find that the most likely location is colocated with the average Region 2 current region on the dayside, at a colatitude of 18° â 22°
The MOSDEF Survey: Kinematic and Structural Evolution of Star-Forming Galaxies at
We present ionized gas kinematics for 681 galaxies at from
the MOSFIRE Deep Evolution Field survey, measured using models which account
for random galaxy-slit misalignments together with structural parameters
derived from CANDELS Hubble Space Telescope (HST) imaging. Kinematics and sizes
are used to derive dynamical masses. Baryonic masses are estimated from stellar
masses and inferred gas masses from dust-corrected star formation rates (SFRs)
and the Kennicutt-Schmidt relation. We measure resolved rotation for 105
galaxies. For the remaining 576 galaxies we use models based on HST imaging
structural parameters together with integrated velocity dispersions and
baryonic masses to statistically constrain the median ratio of intrinsic
ordered to disordered motion, . We find that
increases with increasing stellar mass and decreasing specific SFR (sSFR).
These trends may reflect marginal disk stability, where systems with higher gas
fractions have thicker disks. For galaxies with detected rotation we assess
trends between their kinematics and mass, sSFR, and baryon surface density
(). Intrinsic dispersion correlates most with
and velocity correlates most with mass. By comparing
dynamical and baryonic masses, we find that galaxies at are
baryon dominated within their effective radii (), with Mdyn/Mbaryon
increasing over time. The inferred baryon fractions within ,
, decrease over time, even at fixed mass, size, or surface
density. At fixed redshift, does not appear to vary with
stellar mass but increases with decreasing and increasing
. For galaxies at , the median inferred baryon
fractions generally exceed 100%. We discuss possible explanations and future
avenues to resolve this tension.Comment: Accepted to ApJ. Added Figure 9, corrected sample size (main results
unchanged). 28 pages, 13 figure
The effect of axial compression and distraction on cervical facet mechanics during anterior shear, flexion, axial rotation, and lateral bending motions
© 2018 Elsevier Ltd. This manuscript version is made available under the CC-BY-NC-ND 4.0 license: http://creativecommons.org/licenses/by-nc-nd/4.0/
This author accepted manuscript is made available following 12 month embargo from date of publication (December 2018) in accordance with the publisherâs archiving policyThe subaxial cervical facets are important load-bearing structures, yet little is known about their mechanical response during physiological or traumatic intervertebral motion. Facet loading likely increases when intervertebral motions are superimposed with axial compression forces, increasing the risk of facet fracture. The aim of this study was to measure the mechanical response of the facets when intervertebral axial compression or distraction is superimposed on constrained, non-destructive shear, bending and rotation motions. Twelve C6/C7 motion segments (70âŻÂ±âŻ13âŻyr, nine male) were subjected to constrained quasi-static anterior shear (1âŻmm), axial rotation (4°), flexion (10°), and lateral bending (5°) motions. Each motion was superimposed with three axial conditions: (1) 50âŻN compression; (2) 300âŻN compression (simulating neck muscle contraction); and, (3) 2.5âŻmm distraction. Angular deflections, and principal and shear surface strains, of the bilateral C6 inferior facets were calculated from motion-capture data and rosette strain gauges, respectively. Linear mixed-effects models (αâŻ=âŻ0.05) assessed the effect of axial condition. Minimum principal and maximum shear strains were largest in the compressed condition for all motions except for maximum principal strains during axial rotation. For right axial rotation, maximum principal strains were larger for the contralateral facets, and minimum principal strains were larger for the left facets, regardless of axial condition. Sagittal deflections were largest in the compressed conditions during anterior shear and lateral bending motions, when adjusted for facet side
Quantitative evaluation of facet deflection, stiffness, strain and failure load during simulated cervical spine trauma
© 2018 Elsevier Ltd. This manuscript version is made available under the CC-BY-NC-ND 4.0 license: http://creativecommons.org/licenses/by-nc-nd/4.0/
This author accepted manuscript is made available following 12 month embargo from date of publication (March 2018) in accordance with the publisherâs archiving policyTraumatic cervical facet dislocation (CFD) is often associated with devastating spinal cord injury. Facet fractures commonly occur during CFD, yet quantitative measures of facet deflection, strain, stiffness and failure load have not been reported. The aim of this study was to determine the mechanical response of the subaxial cervical facets when loaded in directions thought to be associated with traumatic bilateral CFD â anterior shear and flexion. Thirty-one functional spinal units (6âŻĂâŻC2/3, C3/4, C4/5, and C6/7, 7âŻĂâŻC5/6) were dissected from fourteen human cadaver cervical spines (mean donor age 69âŻyears, range 48â92; eight male). Loading was applied to the inferior facets of the inferior vertebra to simulate the in vivo inter-facet loading experienced during supraphysiologic anterior shear and flexion motion. Specimens were subjected to three cycles of sub-failure loading (10â100âŻN, 1âŻmm/s) in each direction, before being failed in a randomly assigned direction (10âŻmm/s). Facet deflection, surface strains, stiffness, and failure load were measured. Linear mixed-effects models (αâŻ=âŻ0.05; random effect of cadaver) accounted for variations in specimen geometry and bone density. Specimen-specific parameters were significantly associated with most outcome measures. Facet stiffness and failure load were significantly greater in the simulated flexion loading direction, and deflection and surface strains were higher in anterior shear at the non-destructive analysis point (47âŻN applied load). The sub-failure strains and stiffness responses differed between the upper and lower subaxial cervical regions. Failure occurred through the facet tip during anterior shear loading, while failure through the pedicles was most common in flexion
The structure and function of complex networks
Inspired by empirical studies of networked systems such as the Internet,
social networks, and biological networks, researchers have in recent years
developed a variety of techniques and models to help us understand or predict
the behavior of these systems. Here we review developments in this field,
including such concepts as the small-world effect, degree distributions,
clustering, network correlations, random graph models, models of network growth
and preferential attachment, and dynamical processes taking place on networks.Comment: Review article, 58 pages, 16 figures, 3 tables, 429 references,
published in SIAM Review (2003
GNOSIS: the first instrument to use fibre Bragg gratings for OH suppression
GNOSIS is a prototype astrophotonic instrument that utilizes OH suppression
fibres consisting of fibre Bragg gratings and photonic lanterns to suppress the
103 brightest atmospheric emission doublets between 1.47-1.7 microns. GNOSIS
was commissioned at the 3.9-meter Anglo-Australian Telescope with the IRIS2
spectrograph to demonstrate the potential of OH suppression fibres, but may be
potentially used with any telescope and spectrograph combination. Unlike
previous atmospheric suppression techniques GNOSIS suppresses the lines before
dispersion and in a manner that depends purely on wavelength. We present the
instrument design and report the results of laboratory and on-sky tests from
commissioning. While these tests demonstrated high throughput and excellent
suppression of the skylines by the OH suppression fibres, surprisingly GNOSIS
produced no significant reduction in the interline background and the
sensitivity of GNOSIS and IRIS2 is about the same as IRIS2. It is unclear
whether the lack of reduction in the interline background is due to physical
sources or systematic errors as the observations are detector noise-dominated.
OH suppression fibres could potentially impact ground-based astronomy at the
level of adaptive optics or greater. However, until a clear reduction in the
interline background and the corresponding increasing in sensitivity is
demonstrated optimized OH suppression fibres paired with a fibre-fed
spectrograph will at least provide a real benefits at low resolving powers.Comment: 15 pages, 13 figures, accepted to A
Chandra Observations of the Cl1604 Supercluster at z=0.9: Evidence for an Overdensity of Active Galactic Nuclei
We present the results of Chandra observations of the Cl1604 supercluster at
z~0.9. The system is the largest structure mapped at redshifts approaching
unity, containing at least eight spectroscopically confirmed galaxy clusters
and groups. Using two 50-ksec ACIS-I pointings we examine both the X-ray point
source population and the diffuse emission from individual clusters in the
system. We find a 2.5\sigma excess of point sources detected in the hard band
(2-10 keV) relative to the number of sources found in blank fields observed by
Chandra. No such excess is observed in the soft band (0.5-2 keV). The hard-band
source density is 1.47 times greater than that of a blank field, in agreement
with the previously reported correlation between overdensity amplitude and
cluster redshift. Using a maximum likelihood technique we have matched 112 of
the 161 detected X-ray point sources to optical counterparts and found 15
sources that are associated with the supercluster. All 15 sources have
rest-frame luminosities consistent with emission from active galactic nuclei
(AGN). We find that the supercluster AGN largely avoid the densest regions of
the system and are instead distributed on the outskirts of massive clusters or
within poorer clusters and groups. We have also detected diffuse emission from
two of the eight clusters and groups in the system, clusters Cl1604+4304 and
Cl1604+4314. The systems have bolometric luminosities of 1.43x10^44 and
8.20x10^43 h70^-2 erg s^-1 and gas temperatures of 3.50 (+1.82-1.08) and 1.64
(+0.65-0.45) keV, respectively. Using updated velocity dispersions, we compare
the properties of these systems to the cluster scaling relations followed by
other X-ray and optically selected galaxy clusters at high redshift.Comment: 24 pages, 14 figures, submitted to Ap
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