Specificities of Electromagnetic Field Excitation in a Capacitive HF Discharge. III. Symmetric Discharge Partially Filling the Discharge Chamber

Plasma Physics Reports - Electrodynamic characteristics of a low-pressure (electron collision frequency much lower than the field frequency) capacitive HF discharge maintained by an electromagnetic...     

Adaptation of the Spore Discharge Mechanism in the Basidiomycota

Background

Spore discharge in the majority of the 30,000 described species of Basidiomycota is powered by the rapid motion of a fluid droplet, called Buller''s drop, over the spore surface. In basidiomycete yeasts, and phytopathogenic rusts and smuts, spores are discharged directly into the airflow around the fungal colony. Maximum discharge distances of 1–2 mm have been reported for these fungi. In mushroom-forming species, however, spores are propelled over much shorter ranges. In gilled mushrooms, for example, discharge distances of <0.1 mm ensure that spores do not collide with opposing gill surfaces. The way in which the range of the mechanism is controlled has not been studied previously.

Methodology/Principal Findings

In this study, we report high-speed video analysis of spore discharge in selected basidiomycetes ranging from yeasts to wood-decay fungi with poroid fruiting bodies. Analysis of these video data and mathematical modeling show that discharge distance is determined by both spore size and the size of the Buller''s drop. Furthermore, because the size of Buller''s drop is controlled by spore shape, these experiments suggest that seemingly minor changes in spore morphology exert major effects upon discharge distance.

Conclusions/Significance

This biomechanical analysis of spore discharge mechanisms in mushroom-forming fungi and their relatives is the first of its kind and provides a novel view of the incredible variety of spore morphology that has been catalogued by traditional taxonomists for more than 200 years. Rather than representing non-selected variations in micromorphology, the new experiments show that changes in spore architecture have adaptive significance because they control the distance that the spores are shot through air. For this reason, evolutionary modifications to fruiting body architecture, including changes in gill separation and tube diameter in mushrooms, must be tightly linked to alterations in spore morphology.     

Simulation of the Plasma Afterglow in the Discharge Gap of a Subnanosecond Switch Based on an Open Discharge in Helium

The phenomenon of subnanosecond electrical breakdown in a strong electric field observed in an open discharge in helium at pressures of 6–20 Torr can be used to create ultrafast plasma switches triggering into a conducting state for a time shorter than 1 ns. To evaluate the possible repetition rate of such a subnanosecond switch, it is interesting to study the decay dynamics of the plasma remaining in the discharge gap after ultrafast breakdown. In this paper, a kinetic model based on the particle-in-cell Monte Carlo collision method is used to study the dynamics of the plasma afterglow in the discharge gap of a subnanosecond switch operating with helium at a pressure of 6 Torr. The simulation results show that the radiative, collisional-radiative, and three-body collision recombination mechanisms significantly contribute to the afterglow decay only while the plasma density remains higher than 10¹² cm^?3; the main mechanism of the further plasma decay is diffusion of plasma particles onto the wall. Therefore, the effect of recombination in the plasma bulk is observed only during the first 10–20 μs of the afterglow. Over nearly the same time, plasma electrons become thermalized. The afterglow time can be substantially reduced by applying a positive voltage U_c to the cathode. Since diffusive losses are limited by the ion mobility, the additional ion drift toward the wall significantly accelerates plasma decay. As U_c increases from 0 to +500 V, the characteristic time of plasma decay is reduced from 35 to 10 μs.     

Discharge papilla development in the phycomycete Allomyces

The process of discharge papilla (DP) formation in Allomyces macrogynus was studied by light and electron microscopy. The plug of the DP was first deposited between the plasmalemma and the wall of the zoosporangium (ZS). The wall above the plug subsequently was eroded away. Deposition of a new inner wall layer in the sporangium held the plug in place and thickening of the layer formed a collar around the plug. Further deposition of material after this stage resulted in the characteristic pulley-shape. The plug material appeared homogeneous in electron micrographs but there was evidence of an outer layer. Digestion of the plug at the time of spore release was from within.Abbreviations DP discharge papilla - ZS zoosporangium     

Spore Discharge in the Basidiomycetes: I. Periodicity of Spore Discharge by Trametes gibbosa Fr.

Variations in the rates of spore discharge from this fungushave been examined in controlled laboratory conditions, usingapparatus designed to make a continuous record of the numbersof spores discharged by a strip of hymenium measuring approximately4.5 cm. X o.1 cm. These records have been extended by a directmicroscopic study of the numbers of spores falling through asmall area below the hymenium during successive short periods. Analysis of the continuous record has shown that at any onetime the rate of spore discharge is not uniform over the wholehymenium, and that at any one site it ia not constant. The variationin the rate of discharge in any one site is rhythmical, withintervals of the order of 30 to 90 minutes between maxima. Thesmall linear areas of hymenium which have been examined haveshown nearly the same rhythm-overall, but the phase of thisrhythm has differed at different sites in the area.     

Discharge patterns of cochlear ganglion neurons in the chicken

Summary Physiological recordings were made of the compound action potential from the round window and single neurons in the cochlear ganglion of normal adult chickens (Gallus domesticus). The compound action potential threshold to tone bursts decreased from approximately 42 dB at 0.25 kHz to 30 dB between 1 and 2 kHz and then increased to 51 dB at 4 kHz. Most of the cochlear ganglion cells had characteristic frequencies below 2 kHz and the thresholds of most neurons were roughly 30–35 dB lower than the compound action potential thresholds. At any given characteristic frequency, thresholds varied by as much as 60 dB and units with the highest thresholds tended to have the lowest spontaneous rates. Spontaneous discharge rates ranged from 0 to 200 spikes/s with a mean rate of 86 spikes/s. Interspike interval histograms of spontaneous activity often contained regular peaks with the time interval between peaks approximately equal to 1/(characteristic frequency). Tuning curves were sharply tuned and V-shaped with approximately equal slopes to the curves above and below characteristic frequency. Q_10dB and Q_30dB values for the tuning curves increased with characteristic frequency. Post stimulus time histograms showed sustained firing during the stimulus and were characterized by a slight-to-moderate peak at stimulus onset. Most units showed vigorous phase-locking to tones at characteristic frequency although the degree of phase-locking declined sharply with increasing characteristic frequency. Discharge rate-level functions at characteristic frequency had a mean dynamic range of 42 dB and a mean saturation firing rate of 327 spikes/s. In general, the firing patterns of cochlear ganglion neurons are similar in most respects to those reported in other avians, but differ in several important respects from those seen in mammals.Abbreviations CF characteristic frequency - CAP compound action potential     

Characteristics of X-ray Emission from a Micropinch Discharge at Different Polarities of the Discharge Electrodes

Plasma Physics Reports - The measurement technique and results are presented from studies of the X-ray spectra and X-ray yield from micropinch discharge plasma in a low-inductive vacuum spark...     

Periodicity of Spore Discharge in Daldinia

In Daldinia concentrica spore discharge under natural conditionsis periodic, most spores being discharged during the night andfew in the daytime. An experimental study has been made of dischargeunder controlled conditions of light and temperature. In continuousdarkness periodic discharge was maintained for 12 days, butthen, although spore output continued, it ceased to be periodic.Return to alternating light (12 hrs., 100 f.c.) and darkness(12 hrs.) at once re-established the periodicity. In continuouslight (100 f.c.) periodic discharge ceased after 2 to 3 days,but was immediately re-established in alternating light (12hrs.) and darkness (12 hrs.). When the fungus was placed underconditions of alternating light and darkness each of 6 hours'duration, two peaks of spore-output were soon developed in the24-hour period. The experiments suggest that the natural periodicityis determined by the alternation of day and night.     

The Memory Effect of Microdischarges in the Barrier Discharge in Airflow

Plasma Physics Reports - The paper devoted to the research of the microdischarge dynamics in the dielectric barrier discharge. The discharge between rail electrodes in airflow along the electrodes...     

An Analysis of Spore Discharge in Sordaria

Sordaria fimicola can develop mature perithecia from which sporesare discharged when grown in darkness or in light. Under conditionsof alternating dark and light (12 hrs.: 12 hrs.) each day, sporedischarge is periodic with a low rate during the dark period,succeeded by a gradual rise to a relatively high rate in thelight period followed by a decline before the onset of the nextdark period. There is no trace of an endogenous rhythm. Transferfrom darkness to light always leads to an increase in the rateof discharge, and from light to dark to a decrease. The heightof the peak of discharge rate attained in light following adark period seems to be related to the length of the precedingdark period. Experiments with light of different colours but of roughly thesame energy value show that it is the blue rays that are mainlyeffective. From cultures of filter-paper yeast-extract mediuman orange pigment can be extracted with a maximum absorption,in the visible spectrum, at 470 mµ. It is possible thatthis is important in connexion with the sensitivity of the fungusto blue light.     

On Light-stimulated Spore Discharge in Sordaria

A rough action-spectrum curve (in the range 400–600 mµ)for light-stimulated spore discharge in Sordaria fimicola iscompared with the absorption-spectrum curve of an ethyl-alcoholextract of the fungus. The two curves show some agreement. Amutant lacking black pigment (probably melanin) is shown torespond to light in its spore discharge as does the normal,but evidence is given that it is more sensitive to light. Itseems that the melanin in the perithecial wall of the normalstrain acts as a neutral filter reducing the effectiveness ofthe incident light.     

Light and Spore Discharge in Ascobolus

Spore discharge in Ascobolus crenulatus occurs both in the lightand in the dark. In a 12 h light: 12 h dark daily regime discharge-ratehas peaks in the dark periods, due apparently to light stimulationwith about half a day's interval between stimulus and response. Using a ‘spore clock’ the course of discharge hasbeen followed for a single apothecium on changing from darknessto light. Exposure to light (500 lux) of wave-lengths around400, 440 and 460 mµ immediately causes ’puffing‘,whilst light of longer wave-length (504 and 580 mµ) hasno effect. Change from darkness to white light has no immediateeffect, but there is a delayed stimulation with a marked increasein discharge-rate 10–14 h later. Simultaneous illuminationof an apothecium, which has been in darkness, by blue light(420 mµ, or 440 mµ, 500lux) and yellow light (580mµ, 500 lux) does not result in puffing. The yellow appearsto prevent the blue light from exerting its effects.     

Spatial Structure of the Branching Streamer Channels in a Corona Discharge

The dendritic structure of streamer channels in a corona discharge is described by using fractal theory. It is found that, for a needle-plane discharge, the fractal dimension of the plasma structure is D = 2.16 ± 0.05. The computed spatial distributions of the branching ratios are compared with the available experimental data. The influence of the branching processes on the distribution of chemically active radicals in streamer corona discharges is studied.     

Pulsed Regime of the Diffusive Mode of a Barrier Discharge in Helium

Periodic pulsations of the active current component are revealed experimentally in transversely homogeneous barrier discharges in helium at small values of the parameter Pd (below 500 torr mm) and moderate frequencies of the applied voltage (f < 100 kHz). The frequency of the current pulsations is higher than the frequency of the well-studied pulsations in a transversely inhomogeneous streamer barrier discharge in air by a factor of approximately 100. Numerical calculations show that the physical nature of the observed pulsations can be explained in terms of the negative differential resistance of the cathode fall region, which occupies essentially the entire interelectrode gap in each half-period of the applied voltage.     

Characteristics of the Electrode Plasma of an Electrode Microwave Discharge in Hydrogen

The electrode region of an electrode microwave discharge in hydrogen at pressures of 0.5–4 torr and absorbed powers of up to 12 W is studied using emission spectroscopy and actinometry. It is shown that the gas temperature is at most 700 K and the degree of dissociation does not exceed several percent. Direct electron impact is shown to be the main factor governing all the processes in the electrode region of the discharge, including the excitation of the recorded emission. In particular, the Balmer-series H_α line emission is related to the dissociative electron-impact excitation of hydrogen molecules in the ground state.     

Discharge Measurement in Lower Order Streams

A large number of conventional and less conventional discharge measuring techniques applicable to lower order streams are discussed. The intention is to provide aquatic biologists and water chemists with sufficient alternative methods so that a suitable choice may be made based on factors such as stream characteristics, required accuracy and available funding and man-power. In each case the method is described, details of design and use outlined, and areas of application and possible deficiency noted. In addition the criteria determining method selection are discussed.