Morphologically variable F 2 genotypes derived from hybridization ofcoastal and inland ecotypes of the annual plant Diodia teres were used to identify selection on morphological traits in the natural habitat of each ecotype. These ecotypes occur in very different habitats, and have evolved pronounced morphological differentiation. Selection analysis can suggest whether present patterns of selection can explain morphological differences between ecotypes. F 2 genotypes were characterized morphologically, donally replicated, and transplanted into the habitat of each ecotype. Selection was measured on six morphological traits. Directional and stabilizing selection occurred on many traits; direction and strength of selection varied sharply at different stages of growth, as revealed by a path-analysis approach that divided selection into a set of independent components. Directional selection favored traits of the native population at the coastal habitat, but less so at the inland habitat. Selection was of sufficient strength to create the observed morphological differences between ecotypes in 25-100 generations, given constant selection and sufficient genetic variation. In effects on fitness, most traits were neither independent nor consistently interactive with other traits. Rather, many traits entered into strong but evanescent interactions affecting particular components of fitness. Observed interactions did not support the hypothesis that the morphology of each ecotype was functionally integrated to a high degree.

Genetic variation in photosynthetic traits within populations provides the potential for evolution, but few studies relate phenotypic variation in these traits to variation in fitness. We tested the prediction that a lower photosynthetic rate reduces fecundity and survivorship by comparing wild-type (WT) Amaranthus hybridus family lines to those having a single-gene mutation that confers resistance to atrazine (R) and lowers the rate of photosynthetic carbon assimilation. Wild-type and R family lines with nearly uniform nuclear genomes were used to minimize the confounding effects of other loci. We established experimental populations in agricultural and one-year-old field plots and measured chlorophyll fluorescence, gas exchange, and the fecundity and survivorship of WT and R genotypes for two generations.The R genotype had a lower efficiency of electron transport through photosystem II, which translated into a 20-30% decrease in photosynthetic rate at light levels above 400 mol·m Ϫ2 ·s Ϫ1 . Compared to the WT, the R genotype also had lower water-use efficiency, higher specific leaf area, and greater leaf nitrogen concentration on a mass, but not area, basis. In five of six replicate populations, the R genotype had lower fecundity than the WT in the first generation. Survivorship of seed over winter was similar for the two genotypes, but survivorship of R seedlings during early establishment was lower than the WT in the agricultural field. The consistent pattern of selection against the R genotype during vegetative growth stages suggests that a lower photosynthetic rate reduces fitness. This selection, paired with heritable variation for photosynthetic traits within populations, provides a more complete scenario for the evolution of photosynthetic traits.

Triple point, defined as the junction of metal, dielectric, and vacuum, is the location where electron emission is favored in the presence of a sufficiently strong electric field. In addition to being an electron source, the triple point is generally regarded as the location where flashover is initiated in high voltage insulation, and as the vulnerable spot from which rf breakdown is triggered. In this paper, we focus on the electric field distribution at a triple point of a general geometry, as well as the electron orbits in its immediate vicinity. We calculate the orbit of the first generation electrons, the seed electrons. It is found that, despite the mathematically divergent electric field at the triple point, significant electron yield most likely results from secondary electron emission when the seed electrons strike the dielectric. The analysis gives the voltage scale in which this electron multiplication may occur. It also provides an explanation on why certain dielectric angles are more favorable to electron generation over others, as observed in previous experiments.

Calibrated microwave power and phase measurements are presented for the first recirculating planar magnetron prototype consisting of two coupled six-cavity 1-GHz planar cavity arrays. The results are presented for a solid cathode and two mode-control cathodes (MCCs) with aluminum or velvet electron emitters. The measurements were conducted using a prototype coaxial microwave power extraction scheme. The experimental operating parameters included: pulsed cathode voltages between −250 and −300 kV, voltage pulselengths of 200-600 µs, axial magnetic fields of 0.1-0.32 T, and entrance currents of 1-10 kA. The results showed improved oscillator frequency locking for the MCCs and increases in power and efficiency using the velvet electron emitter. Index Terms-Cavity magnetron, frequency locking, high-power microwaves (HPMs), recirculating planar magnetron (RPM), vacuum electronics.