全文获取类型
收费全文 | 2509篇 |
免费 | 210篇 |
国内免费 | 376篇 |
出版年
2024年 | 19篇 |
2023年 | 43篇 |
2022年 | 75篇 |
2021年 | 88篇 |
2020年 | 69篇 |
2019年 | 108篇 |
2018年 | 102篇 |
2017年 | 90篇 |
2016年 | 109篇 |
2015年 | 90篇 |
2014年 | 150篇 |
2013年 | 160篇 |
2012年 | 112篇 |
2011年 | 128篇 |
2010年 | 124篇 |
2009年 | 160篇 |
2008年 | 163篇 |
2007年 | 160篇 |
2006年 | 129篇 |
2005年 | 124篇 |
2004年 | 93篇 |
2003年 | 91篇 |
2002年 | 61篇 |
2001年 | 47篇 |
2000年 | 50篇 |
1999年 | 55篇 |
1998年 | 50篇 |
1997年 | 37篇 |
1996年 | 44篇 |
1995年 | 27篇 |
1994年 | 30篇 |
1993年 | 32篇 |
1992年 | 17篇 |
1991年 | 32篇 |
1990年 | 17篇 |
1989年 | 18篇 |
1988年 | 17篇 |
1987年 | 16篇 |
1986年 | 13篇 |
1985年 | 12篇 |
1984年 | 23篇 |
1983年 | 11篇 |
1982年 | 15篇 |
1981年 | 9篇 |
1980年 | 20篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1971年 | 6篇 |
排序方式: 共有3095条查询结果,搜索用时 31 毫秒
31.
An epi-illuminated microscope configuration for use in fluorescence correlation spectroscopy in bulk solutions has been analyzed. For determining the effective sample dimensions the spatial distribution of the molecule detection efficiency has been computed and conditions for achieving quasi-cylindrical sample shape have been derived. Model experiments on translational diffusion of rhodamine 6G have been carried out using strong focusing of the laser beam, small pinhole size and an avalanche photodiode in single photon counting mode as the detector. A considerable decrease in background light intensity and measurement time has been observed. The background light is 40 times weaker than the fluorescence signal from one molecule of Rh6G, and the correlation function with signal-to-noise ratio of 150 can be collected in 1 second. The effect of the shape of the sample volume on the autocorrelation function has been discussed.
Correspondence to: R. Rigler 相似文献
32.
33.
William J. Etges 《Evolution; international journal of organic evolution》1993,47(3):750-767
The extent of host-specific genetic variation for two life-history traits, egg to adult developmental time and viability, and one morphological trait closely tied to fitness, adult thorax size, was exposed by employing a nested half-sib/full-sib breeding design with Baja and mainland populations of Drosophila mojavensis recently extracted from nature. This study was motivated by the presence of substantial variation in life histories among populations of D. mojavensis that use the fermenting tissues of particular species of columnar cacti for feeding and breeding in the Sonoran Desert. Full-sib progeny from all sire-dam crosses were split into cultures of agria cactus, Stenocereus gummosus, and organ pipe cactus, S. thurberi, to examine patterns of genotype-by-environment interaction for these fitness components. Baja flies expressed shorter egg-to-adult developmental times, higher viabilities, and smaller body sizes than mainland flies consistent with previous studies. Significant sire and dam components of variance were exposed for developmental time and thorax size. Genotype-by-environment interactions were significant at the level of dams for developmental time and nearly significant for viability (P = 0.09). Narrow- and broad-sense heritabilities were influenced by host cactus, sex, and population. No strong pattern of genetic correlation emerged among fitness components suggesting that host-range expansion has not been accompanied by formation of coadapted life histories, yet the ability to estimate genetic correlations and their standard errors was compromised by the unbalanced nature of the data set. Genetic correlations in performance across cacti were slightly positive, evidence for ecological generalism among populations explaining the observed pattern of multiple host cactus use within the species range of D. mojavensis. 相似文献
34.
Edmund D. Brodie 《Evolution; international journal of organic evolution》1993,47(3):844-854
Quantitative genetic models of evolution rely on the genetic variance-covariance matrix to predict the phenotypic response to selection. Both prospective and retrospective studies of phenotypic evolution across generations rely on assumptions about the constancy of patterns of genetic covariance through time. In the absence of robust theoretical predictions about the stability of genetic covariances, this assumption must be tested with empirical comparisons of genetic parameters among populations and species. Genetic variance-covariance matrices were estimated for a suite of antipredator traits in two populations of the northwestern garter snake, Thamnophis ordinoides. The characters studied include color pattern and antipredator behaviors that interact to facilitate escape from predators. Significant heritabilities for all traits were detected in both populations. Genetic correlations and covariances were found among behaviors in both populations and between color pattern and behavior in one of the populations. Phenotypic means differed among populations, but pairwise comparisons revealed no heterogeneity of genetic parameters between the populations. The structure of the genetic variance-covariance matrix has apparently not changed significantly during the divergence of these two populations. 相似文献
35.
D. V. Shaw J. J. Hansen 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1993,87(1-2):170-176
The genetic relationship between vegetative growth at low temperatures and productivity was investigated for strawberries grown in controlled and field environments. Genotypes from 20 biparental crosses were grown in controlled environments with 11°, 14°, and 17 °C days, 11 °C nights, and 11-h daylength to simulate a range of winter growing conditions expected in mediterranean environments. Individual plants were scored for two initial runner traits and eight vegetative growth traits. Significant main effects of temperature and cross were detected for all growth chamber traits, and conservative estimates of the broad sense heritability (h2) for these traits were 0.10–0.28. None of the temperature x cross interaction effects were significant, suggesting that genetic potential for vegetative growth and vigor is expressed similarly at low and optimal growing temperatures. Highly significant genetic correlations were detected between many growth chamber trait pairs, indicating pleiotropic effects for the genes that condition these traits. Complementary field trials were established, and individual plants were scored for traits that describe yield, production pattern, and plant size. Significant negative genetic correlations were detected between traits that describe growth in the chambers and early production in the field trials, but genetic correlations between chamber growth traits and mid-season or total production were significantly positive and occasionally large. Several of the yield and field growth variables were genetically correlated to initial runner plant traits, suggesting that indirect selection using traits scored in the nursery can be used to improve yield and modify production pattern in the field. 相似文献
36.
Gran M. Hgg 《Journal of electromyography and kinesiology》1993,3(4):231-235
The feasibility of action potential velocity (APV) measurements in the upper trapezius muscle with surface electrodes has been investigated. A four-bar electrode array connected to a double differential amplifier system was used. APV was estimated by a polarity correlation algorithm implemented on a PC computer. Six females and six males participated in the investigation. Attempts to get acceptable APV estimates were made in five electrode locations, 5 mm interspaced along the upper rim, beginning in the most distal part. Data were collected while holding out the arm horizontally in the sagittal plane. The results indicate that the method worked in five out of six males while it was difficult to get reliable estimates in the female group. Furthermore, the two most distal electrode locations gave the best results. In these two locations, the average APV for males was 4.8 m s−1,
0.9. The difficulties in the female group were possibly due to small muscle dimensions and subcutaneous fat. Use of the double differential technique seems to be essential; attempts with the single differential technique were fruitless. 相似文献
37.
38.
Trevor Price Dolph Schluter 《Evolution; international journal of organic evolution》1991,45(4):853-861
Life-history traits such as longevity and fecundity often show low heritability. This is usually interpreted in terms of Fisher's fundamental theorem to mean that populations are near evolutionary equilibrium and genetic variance in total fitness is low. We develop the causal relationship between metric traits and life-history traits to show that a life-history trait is expected to have a low heritability whether or not the population is at equilibrium. This is because it is subject to all the environmental variation in the metric traits that affect it plus additional environmental variation. There is no simple prediction regarding levels of additive genetic variance in life-history traits, which may be high at equilibrium. Several other patterns in the inheritance of life-history traits are readily predicted from the causal model. These include the strength of genetic correlations between life-history traits, levels of nonadditive genetic variance, and the inevitability of genotype-environment interaction. 相似文献
39.
Maria J. P. van Dongen Sybren S. Wijmenga Ramón Eritja Fernando Azorín Cornelis W. Hilbers 《Journal of biomolecular NMR》1996,8(2):207-212
Summary A new application of the HMBC experiment is presented that provides a useful means to discriminate between H2 and H8 proton resonances, to assign the base proton resonances to the various residue types and, most importantly, to correlate the H2 and H8 protons for adenine or inosine residues in natural abundance 13C fragments. The utility of this experiment is demonstrated for an unlabeled DNA 20-mer. Thanks to the obtained results, preliminary conclusions could be drawn regarding the molecular conformations of the non-canonical G/I-A base pairs in the hairpin formed by this fragment. 相似文献
40.
A genetic approach to the understanding of tree architecture is to cross trees of contrasting features and to study their
segregating F2 progenies. For this purpose, members of a 3-generation pedigree, combining Populus trichocarpa, P. deltoides, and their F1 and F2 offspring, were grown side by side in a clonally replicated plantation. At 2 and 3 years of growth, tree architecture was
analyzed at the stem, branch, and leaf levels. In all generations, proleptic branches were more numerous, longer, and had
more and larger leaves than sylleptics initiated in the same year. The analysis of variance revealed significant genotypic
effects on growth, branch and leaf biometrics in the F2 family, with broad-sense heritabilities (H2) ranging from 0.50 to 0.80 for most traits. For branch and leaf traits, the H2 values were found to vary among branch types and crown positions. In year 2, the degree of genetic control was stronger for
sylleptics than proleptics and for upper than lower crown positions. These patterns were followed in year 3, except that H2 values were more a function of position within crown, as a consequence of increased competition among trees. The genetic
correlations between branch/leaf morphology and stem growth were also a function of branch type and crown position. Generally,
traits on proleptics or at upper positions were more tightly correlated with height growth, whereas those on sylleptics or
at lower positions, with basal area growth. By year 3, proleptic traits showed increased genetic correlations with both height
and radial growth. The implications of these results for the construction of ideotypes are discussed.
Received: 1 December 1995 相似文献