全文获取类型
收费全文 | 2813篇 |
免费 | 278篇 |
国内免费 | 341篇 |
出版年
2023年 | 39篇 |
2022年 | 74篇 |
2021年 | 80篇 |
2020年 | 75篇 |
2019年 | 87篇 |
2018年 | 101篇 |
2017年 | 90篇 |
2016年 | 108篇 |
2015年 | 106篇 |
2014年 | 133篇 |
2013年 | 239篇 |
2012年 | 125篇 |
2011年 | 135篇 |
2010年 | 133篇 |
2009年 | 161篇 |
2008年 | 161篇 |
2007年 | 159篇 |
2006年 | 144篇 |
2005年 | 159篇 |
2004年 | 98篇 |
2003年 | 93篇 |
2002年 | 68篇 |
2001年 | 71篇 |
2000年 | 71篇 |
1999年 | 49篇 |
1998年 | 63篇 |
1997年 | 48篇 |
1996年 | 43篇 |
1995年 | 45篇 |
1994年 | 48篇 |
1993年 | 42篇 |
1992年 | 41篇 |
1991年 | 25篇 |
1990年 | 20篇 |
1989年 | 28篇 |
1988年 | 23篇 |
1987年 | 27篇 |
1986年 | 18篇 |
1985年 | 32篇 |
1984年 | 26篇 |
1983年 | 13篇 |
1982年 | 21篇 |
1981年 | 19篇 |
1980年 | 22篇 |
1979年 | 10篇 |
1978年 | 11篇 |
1977年 | 10篇 |
1976年 | 9篇 |
1975年 | 6篇 |
1971年 | 6篇 |
排序方式: 共有3432条查询结果,搜索用时 421 毫秒
71.
K. K. S. BHAT 《Plant, cell & environment》1981,4(4):297-302
Abstract. The rates of uptake of 32 P-labelled orthophosphate by whole root systems of young apple trees (M.9 rootslocks and Worcester Pearmain seedlings) were measured in solution culture. Using a solution depletion technique, the 32 P-phosphate uptake rates per unit length, surface area or fresh weight of roots were determined as a function of 32 P-phosphate concentration in solution at the root surface over the range 0.25–10 mmol m−3 . The effect of P concentration within various plant parts on the relation between uptake rate and external P concentration was studied using plants differing in internal P levels.
The apparent minimutn P concentration below which P uptake ceased was of the order of 0.25–0.50 mmol m−3 . Fluxes, inflows and unit absorption rates increased approximately proportionately with solution concentration up to 10mmolm−3 . Except perhaps in the case of the low-P M.9 plant, there was no evidence of a diminishing returns type of relationship over the range of solution concentrations examined. The threshold P concentration in solution above which uptake rates cease to increase thus appears to be higher for apples than for other species.
At any given P concentration, fluxes, inflows and unit absorption rates were higher for M.9 than for Worcester and for low-P plants than for high-P plants. The difference between plants of different P status was more marked for M.9 and seems to be more closely related to shoot P levels than to root P. 相似文献
The apparent minimutn P concentration below which P uptake ceased was of the order of 0.25–0.50 mmol m
At any given P concentration, fluxes, inflows and unit absorption rates were higher for M.9 than for Worcester and for low-P plants than for high-P plants. The difference between plants of different P status was more marked for M.9 and seems to be more closely related to shoot P levels than to root P. 相似文献
72.
Paul Nieuwenhuysen Julius Clauwaert 《Journal of biochemical and biophysical methods》1981,5(5):279-286
It is impossible to measure the diffusion coefficient of macromolecules directly and accurately by quasi—elastic light scattering, when aggregates cannot be eliminated from the solutions to be investigated. Nevertheless, a simple method can be applied to overcome this problem in many cases. Aggregates are separated from the monomeric macromolecules by rate-zonal sedimentation in a CsCl density gradient in a transparent centrifugation tube; the monomers are then located by laser light scattering intensity measurements; photon correlation spectroscopy of the scattered light finally yields their diffusion coefficient. The viscosity of aqueous CsCl solutions at different temperatures and concentrations allows a good separation by centrifugation and a low uncertainty in the reduction of the measured diffusion coefficient to standard conditions.The application of the method to eukaryotic large ribosomal subunits is described as an example. 相似文献
73.
Methodology is presented for the determination of growth yield (Y(g)) and maintenance coefficient (m) for carbon utilization of plant cells grown in suspension culture. Estimation of Y(g) and m requires measurements of specific growth rate (micro) and specific rate of substrate uptake (q) at different growth limiting substrate concentrations. Batch culture of tobacco cells did not permit evaluation of Y(g) and m because micro is constant and maximal during most of the growth cycle. In batch culture, the period of declining specific growth rate is extremely brief because of the rapid transition from logarithmic growth to stationary phase. This occurs because the K(m) for growth is relatively small compared to the initial sucrose concentration. Thus, when the substrate level reaches the K(m), the large mass of cells rapidly depletes the remaining substrate. In contrast, semicontinuous culture facilitates the determination of Y(g) and m because various steady-state growth rates can be achieved. Mathematical expressions were developed to determine the effective values of micro and q over the semicontinuous replacement interval. The validity of this approach was verified by conducting simulations using experimentally determined parameters. 相似文献
74.
Effects of temperature on the activity of flucycloxuron on larval stages of Panonychus ulmi (Koch), based on LC50 values, were highly significant (P < 0.001) with temperature coefficients of-1.7 in both the ranges of 15° to 25°C and 20° to 30°C. The slopes of probit regression lines at 15° and 20°C were significantly steeper than those at 25° and 30°C. As a consequence the temperature coefficients based on LC90 values were-4.4 and-2.2, for the 2 temperature ranges. The ovicidal activity of flucycloxuron on P. ulmi was low and was only statistically detectable at 20°C (LC90 of 84 mg a.i./l). In studies with larvae of Aedes aegypti (Linnaeus), Leptinotarsa decemlineata (Say), Plutella xylostella (Linnaeus), Spodeptera exigua (Hübner) and Spodoptera littoralis (Boisduval) probit regression lines were parallel over temperature. The activity of flucycloxuron on these five insect species was not affected by temperature. Based on LC50 values, diflubenzuron showed positive temperature coefficients on P. xylostella of + 2.1 at 15° to 25°C and + 2.5 at 20° to 30°C. For S. littoralis the temperature coefficient was positive (+ 2.4) at 15° to 25°C but negative (-1.9) at the 20° to 30°C range. Temperature coefficients of diflubenzuron were neutral for A. aegypti, L. decemlineata and S. exigua. In the design and analysis of these studies special allowance was made for date effects and variation in natural mortality over temperature. 相似文献
75.
LEO W. D. VAN RAAMSDONK WILLEM A. WIETSMA JAAP N. DE VRIES 《Botanical journal of the Linnean Society. Linnean Society of London》1992,109(2):293-303
VAN RAAMSDONK, L. W. D., WIETSMA, W. A. & DE VRIES, J. N., 1992. Crossing experiments in Allium L. section Cepa . A full diallel was carried out with six diploid species of Allium section Cepa and A. roylei of section Rhizirideum , High isolation barriers were found between the related species A. cepa and A. oschaninii , between A. oschaninii and A. vavilovii , and between A. galanthum and A. pskemense . On the contrary, the species A. cepa and A. roylei , belonging to different sections, show only slight isolation barriers. The Wallace effect, which is the development of internal isolation barriers as such, is likely to have taken place in the evolution of A. oschaninii and A. vavilovii , and possibly also between A. galanthum and A. pskemense . 相似文献
76.
S. Kresovich J. G. K. Williams J. R. McFerson E. J. Routman B. A. Schaal 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1992,85(2-3):190-196
Summary Effective conservation and the use of plant genetic resources are essential for future agricultural progress. Critical to this conservation effort is the development of genetic markers which not only distinguish individuals and accessions but also reflect the inherent variation and genetic relationships among collection holdings. We have examined the applicability of the random amplified polymorphic DNA (RAPD) assay for quick, cost-effective, and reliable use in addressing these needs in relation to collection organization and management. Twenty-five decamer oligonucleotide primers were screened individually with a test array composed of individuals representing a range of genetic relationships in Brassica oleracea L. (vegetable and forage cole crops). Over 140 reproducible, polymorphic fragments were generated for study. Each individual of the test array exhibited a unique molecular genotype and composites specific for accessions and botanical varieties could be established. An analysis of similarity based on amplified DNA fragments reflected the known genetic relationships among the selected entries. These results demonstrated that RAPD markers can be of great value in gene bank management for purposes of identification, measurement of variation, and establishment of genetic similarity at the intraspecific level. 相似文献
77.
Johan C. Sanders M. Francesca Ottaviani Arie van Hoek Antonie J. W. G. Visser Marcus A. Hemminga 《European biophysics journal : EBJ》1992,21(5):305-311
Model membranes with unsaturated lipid chains containing various amounts of M13 coat protein in the -helical form were studied using time-resolved fluorescence and ESR spectroscopy. The lipid-to-protein (L/P) ratios used were > 12 to avoid protein-protein contacts and irreversible aggregation leading to -polymeric coat protein. In the ESR spectra of the 12-SASL probe in dioleoyl phosphatidylcholine (DOPC) bilayers no second protein induced component is observed upon incorporation of M13 coat protein. However, strong effects are detected on the ESR lineshapes upon changing the protein concentration. The ESR lineshapes are simulated by assuming a fixed ratio between the parallel (D) and perpendicular (D) diffusion coefficients of 4, and an order parameter equal to zero. It is found that increasing the protein concentration from L/P to L/P 15 results in a decrease of the rotational diffusion coefficient D from 3.4 × 107 to 1.9 × 107 s–1. In the time-resolved fluorescence experiments with DPH-propionic acid as a probe, it is observed that increasing the M13 coat protein concentration causes an increase of the two fluorescent lifetimes, indicating an increase in bilayer order. Analysis of the time-resolved fluorescence anisotropy decay allows one to quantitatively determine the order parameters P2 and P4, and the rotational diffusion coefficient D of the fluorescent probe. The order parameters P2 and P4 increase from 0.34 to 0.55 and from 0.59 to 0.77, respectively, upon adding M13 coat protein to DOPC bilayers with an L/P ratio of 35. The rotational diffusion coefficient D of the DPH-propionic acid probe decreases on incorporating M13 coat protein, in accordance with the ESR results. It is concluded that M13 coat protein in the -monomeric state is not able to produce a long living lipid boundary shell and consequently an immobilization of the lipids. An overall effect on the lipids is induced, resulting in a reduction in the dynamics and an increase in average lipid order. The hydrophobic region of M13 coat protein is proposed to perfectly match the lipid bilayer, resulting in a relatively small distortion of the bilayer structure of the lipid system. 相似文献
78.
79.
L. P. M. J. Wetsteyn J. C. H. Peeters R. N. M. Duin F. Vegter P. R. M. de Visscher 《Hydrobiologia》1990,195(1):163-177
Phytoplankton primary production, nutrient concentrations and turbidity were monitored at three stations in the Oosterschelde during 1980–1984 as part of an ecosystem study.From comparisons of dissolved nutrient ratios with the nutrient requirements of phytoplankton, and of ambient nutrient concentrations with half-saturation constants for nutrient uptake by natural phytoplankton populations it was concluded that silicate was a limiting nutrient for diatoms after the spring bloom until the end of the summer. Dissolved inorganic nitrogen and phosphate were not considered to be limiting to phytoplankton growth.In general, the phytoplankton growing season started during the first fortnight of April and ended at the end of September. Column production in the whole Oosterschelde varied between 201 and 540 g C m–2 yr–1 and was, on average, 25% higher in the western part than in the eastern part. Basin production in the Oosterschelde varied between 120 and 466 g C m–2 yr–1 and was, on average, 55% higher in the western part than in the eastern part; this difference could be explained by differences in the ratio of euphotic depth to mean depth of the compartments.Estimated carbon-specific growth rates in the eastern part varied between < 0.1 and 3 d–1 and in the western part between < 0.1 and 1 d–1. This difference could be explained by the great differences in depth of the compartments. Carbon-specific growth rates are discussed in relation to phytoplankton loss rates. It is suggested that in the eastern part sedimentation must be an important sink for phytoplankton.Communication no. 473 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands. 相似文献
80.
Thomas Nogrady 《Hydrobiologia》1982,89(3):277-284
The quantitative composition of the rotifer fauna of seven interconnected lakes located in southern California was studied in a preliminary investigation. The lakes are fed by reclaimed sewage, and the water quality increases rapidly from the first lake, barely able to support life, to a series of lovely lakes stocked with fish and utilized for recreation. The Shannon index of rotifer population diversity showed an appropriate increase. Up to 24 – 28 different rotifer taxa in each lake provided a rich community dominated by Brachionids, Keratella and Trichocerca. The diversity and quantity of the rotifer association then declined again in the last lake. Comparison of the lakes using the Bray and Curtis index of similarity and cluster analysis supported the picture of a lake-chain evolution, a biotic development showing a maximum curve. 相似文献