9 research outputs found
Unicellular algal growth: A biomechanical approach to cell wall dynamics
We present a model for unicellular algal growth as motivated by several
experiments implicating the importance of calcium ions and ``loosening''
enzymes in morphogenesis. A growing cell at rest in a diffusive calcium
solution is viewed as an elastic shell on short timescales. For a given turgor
pressure, we calculate the stressed shapes of the wall elements whose elastic
properties are determined by Young's modulus and the thickness of the wall. The
local enzyme concentration then determines the rate at which the unstressed
shape of a wall element relaxes toward its stressed shape. The local wall
thickness is calculated from the calcium-mediated addition of material and
thinning due to elongation. We use this model to calculate growth rates for
small perturbations to a circular cell. We find an instability related to
modulations of the wall thickness, leading to growth rates which peak at a
finite wave number.Comment: 5 pages, 2 embedded figure
A Phase-Field Model of Spiral Dendritic Growth
Domains of condensed-phase monolayers of chiral molecules exhibit a variety
of interesting nonequilibrium structures when formed via pressurization. To
model these domain patterns, we add a complex field describing the tilt degree
of freedom to an (anisotropic) complex-phase-field solidification model. The
resulting formalism allows for the inclusion of (in general, non-reflection
symmetric) interactions between the tilt, the solid-liquid interface, and the
bond orientation. Simulations demonstrate the ability of the model to exhibit
spiral dendritic growth.Comment: text plus Four postscript figure file
Measuring and Comparing Party Ideology and Heterogeneity
Estimates of party ideological positions in Western Democracies yield useful party-level information, but lack the ability to provide insight into intraparty politics. In this paper, we generate comparable measures of latent individual policy positions from elite survey data which enable analysis of elite-level party ideology and heterogeneity. This approach has advantages over both expert surveys and approaches based on behavioral data, such as roll call voting and is directly relevant to the study of party cohesion. We generate a measure of elite positions for several European countries using a common space scaling approach and demonstrate its validity as a measure of party ideology. We then apply these data to determine the sources of party heterogeneity, focusing on the role of intraparty competition in electoral systems, nomination rules, and party goals. We find that policy-seeking parties and centralized party nomination rules reduce party heterogeneity. While intraparty competition has no effect, the presence of these electoral rules conditions the effect of district magnitude