38,474 research outputs found
Evolutionary plant breeding for low input systems
Heritable variation is at the heart of the process of evolution. However, variation is restricted in breeding for uniform crop populations using the pedigree line approach. Pedigree lines are successful in agriculture because synthetic inputs are used to raise fertility and control weeds, pests and diseases.
An alternative method promoted for exploring the value of variation and evolutionary fitness in crops is to create composite cross populations. Composite cross populations are formed by assembling seed stocks with diverse evolutionary origins, recombination of these stocks by hybridization, the bulking of F1 progeny, and subsequent natural election for mass sorting of the progeny in successive natural cropping environments. Composite cross populations can provide dynamic gene pools, which in turn provide a means of conserving germplasm resources: they can also allow selection of heterogeneous crop varieties. The value of composite cross populations in achieving these aims is dependent on the outcome of mass trials by artificial and natural selection acting upon the heterogeneous mixture.
There is evidence to suggest that composite cross populations may be an efficient way of providing heterogeneous crops and of selecting superior pure lines for low input systems characterized by unpredictable stress conditions
Why Not Consider Closed Universes?
We consider structure formation and CMB anisotropies in a closed universe,
both with and without a cosmological constant. The CMB angular power spectrum
and the matter transfer function are presented, along with a discussion of
their relative normalization. This represents the first full numerical
evolution of density perturbations and anisotropies in a spherical geometry. We
extend the likelihood function vs. Omega from the COBE 2-year data to Omega>=1.
For large Omega the presence of a very steep rise in the spectrum towards low
ell allows us to put an upper limit of Omega<=1.5 (95%CL) for primordial
spectra with n<=1. This compares favorably with existing limits on Omega. We
show that there are a range of closed models which are consistent with
observational constraints while being even older than the currently popular
flat models with a cosmological constant. Future constraints from degree scale
CMB data may soon probe this region of parameter space. A derivation of the
perturbed Einstein, fluid and Boltzmann equations for open and closed
geometries is presented in an appendix.Comment: 24 pages, including 13 figures in a uuencoded self-unpacking shell
script. Submitted to Ap
Origami building blocks: generic and special 4-vertices
Four rigid panels connected by hinges that meet at a point form a 4-vertex,
the fundamental building block of origami metamaterials. Here we show how the
geometry of 4-vertices, given by the sector angles of each plate, affects their
folding behavior. For generic vertices, we distinguish three vertex types and
two subtypes. We establish relationships based on the relative sizes of the
sector angles to determine which folds can fully close and the possible
mountain-valley assignments. Next, we consider what occurs when sector angles
or sums thereof are set equal, which results in 16 special vertex types. One of
these, flat-foldable vertices, has been studied extensively, but we show that a
wide variety of qualitatively different folding motions exist for the other 15
special and 3 generic types. Our work establishes a straightforward set of
rules for understanding the folding motion of both generic and special
4-vertices and serves as a roadmap for designing origami metamaterials.Comment: 8 pages, 9 figure
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