Colchicine binding Activity in Extracts of Higher Plants

The binding of [³H] to soluble components in extracts from avariety of higher plant tissues has been assayed by gel filtrationand by the filter disc method. The binding activity, althoughpresent only to a low degree and requiring at least concentrationby ammonium sulphate precipitation before assay, showed someof the characteristics of tubulin, including enhancement bythe presence of vinblastine. Practical difficulties involvedin investigating, and in interpreting the significance of thecolchicine-binding activity of higher plants arise both fromthe inhibiting effect on binding of the plant extracts themselves,and from the fact that lumicolehicine was also bound by thepreparations under certain conditions.     

Developmental threshold, day-degree requirements and voltinism of the aphid predator Episyrphus balteatus (Diptera: Syrphidae)

Episyrphus balteatus is the dominant syrphid predator of aphids in the UK, reestablishment of populations each season is mainly by migration from warmer climes, as overwintering in this country is minimal with an uncertain level of success. The effects of temperature on the developmental rate of E. balteatus were investigated to determine the potential voltinism of this species in the UK. Estimates of the developmental threshold temperature from oviposition to adult emergence varied between 6.0d?C and 7.1d?C with a day-degree (DD) requirement of between 256 and 280 depending on the method of analysis used. Using the DD requirement derived from a weighted linear regression (261 DD above a threshold of 6.8d?C) in combination with the required value from adult emergence to first oviposition, suggests a total requirement of approximately 374 DD above 6.8d?C per generation. These data, together with climate records, indicate that one to three generations are possible each year, depending on the available DD and origin of the population. The advantages and limitations of different approaches and models used to estimate developmental thresholds and DD requirements are discussed.     

Recognizing the forest for the trees: testing temporal patterns of cladogenesis using a null model of stochastic diversification

Computer simulations are developed and employed to examine the expected temporal distributions of nodes under a null model of stochastic lineage bifurcation and extinction. These Markovian models of phylogenetic process were constructed so as to permit direct comparisons against empirical phylogenetic trees generated from molecular or other information available solely from extant species. For replicate simulated phylads with n extant species, cumulative distribution functions (cdf's) of branching times were calculated, and compared (using the Kolmogorov-Smirnov test statistic D) to those from three published empirical trees. Molecular phylogenies for columbine plants and avian cranes showed statistically significant departures from the null expectations, in directions indicating recent and ancient species' radiations, respectively, whereas a molecular phylogeny for the Drosophila virilis species group showed no apparent historical clustering of branching events. Effects of outgroup choice and phylogenetic frame of reference were investigated for the columbines and found to have a predictable influence on the types of conclusions to be drawn from such analyses. To enable other investigators to statistically test for nonrandomness in temporal cladogenetic pattern in empirical trees generated from data on extant species, we present tables of mean cdf's and associated probabilities under the null model for expected branching times in phylads of varying size. The approaches developed in this report complement and extend those of other recent methods for employing null models to assess the statistical significance of pattern in evolutionary trees.      

The distribution and diel vertical migration of Pseudodiaptomus hessei (Mrázek) (Calanoida: Copepoda) in a subtropical lake in southern Africa

The vertical distribution and diel migratory behaviour of Pseudodiaptomus hessei in a freshwater lake is described. All stages showed a pronounced diel migration. During daylight naupliar and copepodite instars were almost exclusively benthic in shallow areas (<10m). Copepodite V and adult stages were predominantly benthic even at 40 m. The diurnal vertical distribution pattern implies age-related differences in photosensitivity and a possible depth-regulatory mechanism, based on the existence of differential photosensitivity, is offered to account for day-depth control. Nocturnal vertical distribution, studied approximately fortnightly during 1970–71, showed no clear seasonal variation. Variations in pattern, most obvious in adult instars, correlated with prevalent lunar conditions. Nocturnal or midnight sinking was evident, particularly in post-naupliar stages. Dusk rise and dawn descent were performed at very low light intensities and paralleled changes in light penetration. Dawn descent was less rapid than the dusk rise. An ephemeral dawn rise was sometimes observed but was not a consistent feature of all stages. An endogenous rhythm of locomotor activity was recorded under laboratory conditions. Activity was low during daylight, increased sharply at dusk and then decreased through the night to reach daytime levels at dawn. The stimulus provided by changes in light intensity and/or endogenous activity changes can account for the dusk and dawn movements of P. hessei but a definitive identification of the direct migratory stimulus is not possible. Attention is called to a closer examination of the role of endogenous activity rhythms in vertical migratory behaviour. The possible adaptive value of vertical migration to P. hessei is examined briefly.     

The Occurrence and Some Properties of a New Filamentous Component of Plant Extracts

The electron microscopic image and some properties of a filamentouscomponent of plant extracts are described. The filaments are3·5 nm in diameter, non-rigid and present a beaded appearancewhen positively stained with uranyl and lead salts. They arepresent in the microsomal supernatant fractions of a varietyof plants, and can be obtained in extracts of fresh tissuesor of acetone powder preparations. Specific tissues, successfullyextracted for filaments, include phloem, xylem and corticalparenchyma. They are not present in extracts of cell wall preparationsor of hair cells of cotton bolls. An outstanding characteristicof these filaments is their great stability to a wide varietyof treatments which includes variation in temperature, ionicenvironment, pH and the presence of urea, thiol reagents orthe detergent Nonidet. Lateral aggregation of the filamentsis evident below pH 3·0 and above pH 8·0. Althoughcellulase was the only enzyme of those tested which digestedthe filaments, it is unlikely, for several reasons, that thefilaments are a form of pure cellulose. Hydroxyproline was presentin all filament fractions after partial purification by variousmethods. The filaments are discussed in relation to known fibrillarcomponents of plant cells.     

Long-term impact of a stand-replacing fire on ecosystem CO2 exchange of a ponderosa pine forest

Ponderosa pine (Pinus ponderosa) forests of the southwestern United States are a mosaic of stands where undisturbed forests are carbon sinks, and stands recovering from wildfires may be sources of carbon to the atmosphere for decades after the fire. However, the relative magnitude of these sinks and sources has never been directly measured in this region, limiting our understanding of the role of fire in regional and US carbon budgets. We used the eddy covariance technique to measure the CO₂ exchange of two forest sites, one burned by fire in 1996, and an unburned forest. The fire was a high‐intensity stand‐replacing burn that killed all trees. Ten years after the fire, the burned site was still a source of CO₂ to the atmosphere [109±6 (SEM) g C m^?2 yr^?1], whereas the unburned site was a sink (?164±23 g C m^?2 yr^?1). The fire reduced total carbon storage and shifted ecosystem carbon allocation from the forest floor and living biomass to necromass. Annual ecosystem respiration was lower at the burned site (480±5 g C m^?2 yr^?1) than at the unburned site (710±54 g C m^?2 yr^?1), but the difference in gross primary production was even larger (372±13 g C m^?2 yr^?1 at the burned site and 858±37 g C m^?2 yr^?1at the unburned site). Water availability controlled carbon flux in the warm season at both sites, and the burned site was a source of carbon in all months, even during the summer, when wet and warm conditions favored respiration more than photosynthesis. Our study shows that carbon losses following stand‐replacing fires in ponderosa pine forests can persist for decades due to slow recovery of the gross primary production. Because fire exclusion is becoming increasingly difficult in dry western forests, a large US forest carbon sink could shift to a decadal‐scale carbon source.     

The behavioural responses of juvenile signal crayfish Pacifastacus leniusculus to stimuli from perch and eels

• 1 Experiments were designed to determine the relative importance of chemical and visual stimuli in eliciting predator avoidance behaviour in juvenile freshwater crayfish Pacifastacus leniusculus (Dana).
• 2 Crayfish placed in visual and/or chemical contact with one of two predators exhibited marked avoidance behaviour, spending less time walking and climbing and more time within shelters.
• 3 The combined effects of both visual and chemical stimuli increased crayfish shelter use and reduced walking and climbing activity to a greater degree than either stimulus when presented alone.
• 4 Crayfish exhibited avoidance behaviour in response to chemical stimuli during periods of light and darkness. Visual detection of predators elicited avoidance behaviour during the day.
• 5 It is suggested that the behavioural response of P. leniusculus to chemical stimuli reduces the likelihood of being detected by visual predators, and that chemical stimuli lower the response threshold for avoidance behaviour in crayfish reacting to visual stimuli. The adaptiviry of using chemical cues to detect predators is emphasized.