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91.
Nedialka G. Markova George G. Markov 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1983,741(1):7-14
Isopicnic centrifugation in Cs2SO4-Ag+ gradients at pH 7.0 reveals that the genome of the marine snail Rapana thomasiana Grosse (Gastropoda) contains an AT-rich satellite fraction comprising 5% of the DNA. Restriction enzyme analysis shows that the satellite DNA is composed of a number of related subsets arranged in tandem arrays. They have evolved from the segmental amplification of an 1460 bp long monomer unit with a complex inner organization. Most probably, the present basic repeat originates from an ancestral 400–500 bp long sequence in which some insertions and/or deletions have occurred. 相似文献
92.
93.
The development of numerical syntaxonomy during its first 20 yr is reviewed. The use of methods of numerical classification and ordination is the dominating feature of the development. National and local phytosociological data banks were established, large data sets handled and many important vegetation monographs were methodically based on multivariate data analysis. Particularly the development in Italy, the Netherlands, Czechoslovakia, and Sweden contributed to new theoretical elements of numerical syntaxonomy. Ordination became a common tool of searching for reticulate synsystematic relations between community types. The most popular ordination techniques have been Principal Components Analysis and Detrended Correspondence Analysis. Hierarchical agglomerative techniques of clustering still prevail in classification, although the divisive strategy of TWINSPAN has also become an effective tool for phytosociological clustering and table sorting. Extensive program packages, also for personal computers have now become standard equipment for many vegetation scientists. 相似文献
94.
Robert G. Knox 《Plant Ecology》1989,83(1-2):129-136
Detrending and non-linear axis rescaling potentially improve the accuracy of gradient recovery in correspondence analyses but also reduce the stability or consistency of solutions. Variation among bootstrapped ordination solutions was compared across methods in analyses of both field and simulated data. Solution accuracy, measured with mean squared errors from Procrustes analysis, was compared using simulated data with known structure.Standard detrending-by-segments combined with non-linear rescaling entailed some cost in solution stability, but could improve the accuracy of solutions for long gradients. Without non-linear rescaling these solutions were usually less stable and less accurate. Although detrending-by-polynomials might be preferable on other grounds, it did not produce more accurate or stable solutions than detrending-by-segments.Abbreviations CA =
correspondence analysis
- DCA =
detrended correspondence analysis
- MSE =
Procrustes mean squared error statistic
- SD =
standard deviation units of species turnover
- SRV =
scaled variance in species ranks 相似文献
95.
Effects of Solar Radiation on Photoorientation,Motility and Pigmentation in a Freshwater Cryptomonas
The effects of solar radiation on motility, photoorientation and pigmentation have been studied in a freshwater Cryptomonas species. The diaphototactic orientation performed by the cells is impaired within about 90 min of solar radiation. Likewise, the percentage of motile cells within the population and the average velocity of the swimming cells decreases within about the same exposure time. This effect is not due to a thermal stress but rather seems to be caused by the solar UV-B component, since decreasing short wavelength UV radiation by means of an artificial ozone filter or UV cut-off filters increased the tolerated exposure time. Solar radiation also bleached the photosynthetic pigments of the cells as shown by absorption difference spectra. 相似文献
96.
Willy R. G. Baeyens Betty Lin Ling Udo A. Th. Brinkman Stephen G. Schulman 《Luminescence》1989,4(1):484-499
An overview is presented of the physicochemical basis of luminescence, and its application to the detection of chemicals (drugs, biomedically important compounds, environmentally active substances) in liquid chromatographic systems. 相似文献
97.
Conformational fluctuations in a globular protein, bovine pancreatic trypsin inhibitor, in the time range between picoseconds and nanoseconds are studied by a Monte Carlo simulation method. Multiple energy minima are derived from sampled conformations by minimizing their energy. They are distributed in clusters in the conformational space. A hierarchical structure is observed in the simulated dynamics. In the time range between 10(-14) and 10(-10) seconds dynamics is well represented by a superposition of vibrational motions within an energy well with transitions among minima within each cluster. Transitions among clusters take place in the time range of nanoseconds or longer. 相似文献
98.
Gordon H. Copp 《Environmental Biology of Fishes》1989,26(1):1-27
Synopsis Fish reproduction in floodplain ecosystems, based on relative abundance and total biomass of 0+ juveniles, was studied using the synchronic approach to typological analysis in conjunction with Point Abundance Sampling by modified electrofishing. In 3 different flood plains of the Upper Rhône River, 1015 point samples yielding 4573 juveniles (0+) from 21 species were collected from 48 ecosystems of various geomorphological origin. The results demonstrate the lotic-to-lentic succession of floodplain ecosystems to be a series of non-sequential reproductive zones, with spawning conditions being reflected by the specific composition and guild structure of the YOY fish assemblages. The habitat diversity and the fish reproductive potential of floodplain ecosystems are strongly influenced by geomorphological origin and by past and present hydrological conditions. The YOY assemblages of autogenically driven ecosystems (usually of anastomose or meander origin) tend to differ both in composition and in quantity from those found in allogenically driven ecosystems (generally of braided origin). Ecosystems of intermediate character, and fish reproduction thereof, occur as the result of either ecosystem rejuvenation or senescence: autogenically driven ecosystems by allogenic mechanisms, or allogenically driven ecosystems by anthropic and/or autogenic mechanisms, respectively. Because of co-occurrence of ecosystems at similar and at different successional status, the flood plain as an entity is seen as stable with respect to fish reproduction. 相似文献
99.
Recent landscape history from a Co. Down lake deposit 总被引:1,自引:1,他引:0
VALERIE A. HALL 《The New phytologist》1990,115(2):377-383
100.
The formation of mor humus in an experimental grassland plot, which has been acidified by long-term fertiliser treatment,
has been studied by comparing the rates of cellulose, soil organic matter and plant litter decay with those in an adjacent
plot with near-neutral pH and mull humus. The decomposition of cellulose filter paper in litter bags of 5 mm, 1-mm and 45-μm
mesh size buried at 3 to 4 cm depth the plots was followed by measuring the weight loss and changes in glucose content over
a 6 month period. Soil pH was either 5.3 or 4.3. Decomposition of native soil organic matter and plant litter in soil from
the same plots were followed using CO2 evolution in laboratory microcosms. Cellulose weight loss at pH 5.3 was greatest from the 5-mm mesh bags and least from the
45-um mesh bags. At pH 4.3 there was little weight loss from bags and no significant differences in weight loss between bags
with different sized mesh. There was, however, a reduction in the glucose content of the hydrolysed and derivatised filter
paper with time. The decomposition rate of native soil organic matter in the low pH soil was increased to that observed in
the less acid soil when the pH of the former was increased from 4.3 to 5.3. The increase in decomposition rate of added plant
litter in the more acid soil as a result of CA(OH)2 addition was only 60% of that observed in the soil with pH 5.3. These data support the hypothesis that the absence of soil
animals and the restricted microbial decomposition in the acidic soil was responsible for mor humus formation. 相似文献