4,011 research outputs found
Chemical Comparison of Two Ecotypes of Loblolly Pine (Pinus Taeda L.)
Loblolly pine from the continuous range in east Texas was compared with an apparently drought-resistant ecotype, the so-called "Lost pines" or "Bastrop pines." The Bastrop pines are found in a small area of central Texas isolated from the rest of the loblolly range, and in a region receiving considerably less rainfall. Determinations made were: holocellulose, alpha cellulose, ash, specific gravity, and percentages of earlywood and latewood. Nutrient analyses for levels of sodium, potassium, calcium, magnesium, zinc, manganese, and copper were also performed. It was found that the earlywood of the east Texas population had significantly greater levels of holocellulose and alpha cellulose. Regression analyses were performed relating the nutrient values to the chemical components. The appearance of potassium and magnesium in these equations, for the Bastrop pines, may indicate physiological adaptation to the more xeric environment
Abundant and equipotent founder cells establish and maintain acute lymphoblastic leukaemia
High frequencies of blasts in primary acute lymphoblastic leukaemia (ALL) samples have the potential to induce leukaemia and to engraft mice. However, it is unclear how individual ALL cells each contribute to drive leukaemic development in a bulk transplant and the extent to which these blasts vary functionally. We used cellular barcoding as a fate mapping tool to track primograft ALL blasts in vivo. Our results show that high numbers of ALL founder cells contribute at similar frequencies to leukaemic propagation over serial transplants, without any clear evidence of clonal succession. These founder cells also exhibit equal capacity to home and engraft to different organs, although stochastic processes may alter the composition in restrictive niches. Our findings enhance the stochastic stem cell model of ALL by demonstrating equal functional abilities of singular ALL blasts and show that successful treatment strategies must eradicate the entire leukaemic cell population
Properties of pattern formation and selection processes in nonequilibrium systems with external fluctuations
We extend the phase field crystal method for nonequilibrium patterning to
stochastic systems with external source where transient dynamics is essential.
It was shown that at short time scales the system manifests pattern selection
processes. These processes are studied by means of the structure function
dynamics analysis. Nonequilibrium pattern-forming transitions are analyzed by
means of numerical simulations.Comment: 15 poages, 8 figure
Ordering and finite-size effects in the dynamics of one-dimensional transient patterns
We introduce and analyze a general one-dimensional model for the description
of transient patterns which occur in the evolution between two spatially
homogeneous states. This phenomenon occurs, for example, during the
Freedericksz transition in nematic liquid crystals.The dynamics leads to the
emergence of finite domains which are locally periodic and independent of each
other. This picture is substantiated by a finite-size scaling law for the
structure factor. The mechanism of evolution towards the final homogeneous
state is by local roll destruction and associated reduction of local
wavenumber. The scaling law breaks down for systems of size comparable to the
size of the locally periodic domains. For systems of this size or smaller, an
apparent nonlinear selection of a global wavelength holds, giving rise to long
lived periodic configurations which do not occur for large systems. We also
make explicit the unsuitability of a description of transient pattern dynamics
in terms of a few Fourier mode amplitudes, even for small systems with a few
linearly unstable modes.Comment: 18 pages (REVTEX) + 10 postscript figures appende
Extreme clonal diversity and divergence in populations of a selfing hermaphroditic fish.
Flame propagation in random media
We introduce a phase-field model to describe the dynamics of a
self-sustaining propagating combustion front within a medium of randomly
distributed reactants. Numerical simulations of this model show that a flame
front exists for reactant concentration , while its vanishing at
is consistent with mean-field percolation theory. For , we find
that the interface associated with the diffuse combustion zone exhibits kinetic
roughening characteristic of the Kardar-Parisi-Zhang equation.Comment: 4, LR541
The passage of the bomb radiocarbon pulse into the Pacific Ocean
Author Posting. © Arizona Board of Regents on behalf of the University of Arizona, 2010. This article is posted here by permission of Dept. of Geosciences, University of Arizona for personal use, not for redistribution. The definitive version was published in Radiocarbon 52 (2010): 1182-1190.We report and compare radiocarbon observations made on 2 meridional oceanographic sections along 150°W
in the South Pacific in 1991 and 2005. The distributions reflect the progressive penetration of nuclear weapons-produced 14C
into the oceanic thermocline. The changes over the 14 yr between occupations are demonstrably large relative to any possible
drift in our analytical standardization. The computed difference field based on the gridded data in the upper 1600 m of the section
exhibits a significant decrease over time (approaching 40 to 50â° in Î14C) in the upper 200â300 m, consistent with the
decadal post-bomb decline in atmospheric 14C levels. A strong positive anomaly (increase with time), centered on the low
salinity core of the Antarctic Intermediate Water (AAIW), approaches 50â60â° in Î14C, a clear signature of the downstream
evolution of the 14C transient in this water mass. We use this observation to estimate the transit time of AAIW from its âsource
regionâ in the southeast South Pacific and to compute the effective reservoir age of this water mass. The 2 sections show small
but significant changes in the abyssal 14C distributions. Between 1991 and 2005, Î14C has increased by 9â° below 2000 m
north of 55°S. This change is accompanied overall by a modest increase in salinity and dissolved oxygen, as well as a slight
decrease in dissolved silica. Such changes are indicative of greater ventilation. Calculation of âphosphate starâ also indicates
that this may be due to a shift from the Southern Ocean toward North Atlantic Deep Water as the ventilation source of the
abyssal South Pacific.This work was performed under National Science Foundation Grant number OCE-0223434 as well
as a cooperative agreement with NSF (most recently OCE-0228996)
Condensation vs. phase-ordering in the dynamics of first order transitions
The origin of the non commutativity of the limits and in the dynamics of first order transitions is investigated. In the
large-N model, i.e. taken first, the low temperature phase is
characterized by condensation of the large wave length fluctuations rather than
by genuine phase-ordering as when is taken first. A detailed
study of the scaling properties of the structure factor in the large-N model is
carried out for quenches above, at and below T_c. Preasymptotic scaling is
found and crossover phenomena are related to the existence of components in the
order parameter with different scaling properties. Implications for
phase-ordering in realistic systems are discussed.Comment: 15 pages, 13 figures. To be published in Phys. Rev.
Overall time evolution in phase-ordering kinetics
The phenomenology from the time of the quench to the asymptotic behavior in
the phase-ordering kinetics of a system with conserved order parameter is
investigated in the Bray-Humayun model and in the Cahn-Hilliard-Cook model.
From the comparison of the structure factor in the two models the generic
pattern of the overall time evolution, based on the sequence ``early linear -
intermediate mean field - late asymptotic regime'' is extracted. It is found
that the time duration of each of these regimes is strongly dependent on the
wave vector and on the parameters of the quench, such as the amplitude of the
initial fluctuations and the final equilibrium temperature. The rich and
complex crossover phenomenology arising as these parameters are varied can be
accounted for in a simple way through the structure of the solution of the
Bray-Humayun model.Comment: RevTeX, 14 pages, 18 figures, to appear in Phys. Rev.
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