全文获取类型
收费全文 | 14035篇 |
免费 | 1126篇 |
出版年
2021年 | 163篇 |
2020年 | 122篇 |
2019年 | 156篇 |
2018年 | 222篇 |
2017年 | 166篇 |
2016年 | 337篇 |
2015年 | 557篇 |
2014年 | 553篇 |
2013年 | 816篇 |
2012年 | 991篇 |
2011年 | 947篇 |
2010年 | 664篇 |
2009年 | 521篇 |
2008年 | 782篇 |
2007年 | 854篇 |
2006年 | 855篇 |
2005年 | 820篇 |
2004年 | 800篇 |
2003年 | 733篇 |
2002年 | 744篇 |
2001年 | 164篇 |
2000年 | 113篇 |
1999年 | 156篇 |
1998年 | 197篇 |
1997年 | 136篇 |
1996年 | 115篇 |
1995年 | 140篇 |
1994年 | 122篇 |
1993年 | 118篇 |
1992年 | 138篇 |
1991年 | 97篇 |
1990年 | 79篇 |
1989年 | 101篇 |
1988年 | 88篇 |
1987年 | 81篇 |
1986年 | 83篇 |
1985年 | 97篇 |
1984年 | 128篇 |
1983年 | 102篇 |
1982年 | 114篇 |
1981年 | 109篇 |
1980年 | 100篇 |
1979年 | 64篇 |
1978年 | 69篇 |
1977年 | 62篇 |
1976年 | 58篇 |
1975年 | 50篇 |
1974年 | 57篇 |
1972年 | 43篇 |
1971年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
Summary H-Y (male-specific) antigen has been detected on the plasma membranes of both caput and caudal ram spermatozoa using both immunoperoxidase and immunofluorescence labelling techniques. In these spermatozoa the distribution of H-Y antigen appears to be confined to both the posterior region of the head and the mid-piece region of the flagellum. In addition, caput spermatozoa also exhibit intense immunoperoxidase staining of the cytoplasmic droplet which is situated on the flagellum at the base of the head. Western blot analyses of purified plasma membranes from the flagella of caudal spermatozoa have revealed the presence of a malespecific protein with an estimated molecular weight of 25,000–27,000. 相似文献
32.
Barbara S. Parris 《Brittonia》2001,53(2):270-283
Four major austral continental distribution patterns are evident in pteridophytes. Twenty-two species are completely circum-Antarctic.
Another 39 species are partially circum-Antarctic, occurring in Australasia (Australia and New Zealand) and Africa (including
Madagascar) but not South America, while 29 are in Africa and South America but not Australasia, and 13 are in South America
and Australasia but not Africa. Two hypotheses are considered as explanations for the patterns: continental drift following
the breakup of Gondwana and long-distance dispersal. Fossil evidence indicates that the majority of pteridophyte families
involved appeared after the southern continents had drifted apart, so long-distance dispersal is likely to explain the distribution
of species in these families on now widely separated continents. For those families extant before the break-up, there is no
indication in the fossil record that the species involved were present in Gondwana. Aspects of the ecology of the species
that are partly or completely circum-Antarctic indicate that long-distance dispersal, rather than continental drift, is a
likely explanation for the patterns. 相似文献
33.
Barbara Tillmann Pierre Jolic?ur Masami Ishihara Nathalie Gosselin Olivier Bertrand Yves Rossetti Isabelle Peretz 《PloS one》2010,5(4)
Congenital amusia is a neurogenetic disorder of music processing that is currently ascribed to a deficit in pitch processing. A recent study challenges this view and claims the disorder might arise as a consequence of a general spatial-processing deficit. Here, we assessed spatial processing abilities in two independent samples of individuals with congenital amusia by using line bisection tasks (Experiment 1) and a mental rotation task (Experiment 2). Both amusics and controls showed the classical spatial effects on bisection performance and on mental rotation performance, and amusics and controls did not differ from each other. These results indicate that the neurocognitive impairment of congenital amusia does not affect the processing of space. 相似文献
34.
Janet E. Clements Barbara B. Anderson 《Biochimica et Biophysica Acta (BBA)/General Subjects》1980,632(2):159-163
The red-cell enzymes, glutathione reductase (FAD-dependent) and pyridoxine (pyridoxamine) phosphate oxidase (FMN-dependent), were studied in control subjects. The wide range the glutathione reductase activity correlated inversely with the percentage stimulation by FAD added in vitro, and with pyridoxine (pyridoxamine) phosphate oxidase activity. Both enzymes were stimulated after ingestion of riboflavin. The results support the suggestion that the rate of metabolism of riboflavin in the red cell controls the activity of both enzymes, and the rate of red-cell metabolism of vitamin B-6. 相似文献
35.
36.
Robert A. Holland Felix Eigenbrod Paul R. Armsworth Barbara J. Anderson Chris D. Thomas Kevin J. Gaston 《Biodiversity and Conservation》2011,20(14):3285-3294
A growing literature aims to identify areas of congruence in the provision of multiple ecosystem goods and services. However,
little attention has been paid to the effect that temporal variation in the provision of such services may have on understanding
of these relationships. Due to a lack of temporally and spatially replicated monitoring surveys, such relationships are often
assessed using data from disparate time periods. Utilising temporally replicated data for indices of freshwater quality and
agricultural production we demonstrate that through time the biophysical values of ecosystem services may vary in a spatially
non-uniform way. This can lead to differing conclusions being reached about the strength of relationships between services,
which in turn has implications for the prioritisation of areas for management of multiple services. We present this first
analysis to illustrate the effect that the use of such temporally disparate datasets may have, and to highlight the need for
further research to assess under what circumstances temporal variation of this sort will have the greatest impact. 相似文献
37.
38.
Edward F. Rossomando Jane Hadjimichael Barbara Varnum-Finney David R. Soil 《Differentiation; research in biological diversity》1987,35(2):88-93
A conjugate of hippuryllysine (HP) and adenylic acid was synthesized and purified. The structure of the conjugate, hippuryllysyl(N-epsilon-5'-phospho)adenosine (HLAMP) was established using 31P nuclear magnetic resonance, UV spectroscopy, acid/base lability, and enzyme digestion with AMP deaminase, alkaline phosphatase, 5'-nucleotidase, and a phosphoamidase activity recently identified in Dictyostelium discoideum. The results indicate that HLAMP contains a phosphoamide bond between the phosphate of AMP and the epsilon amino group of HL. Employing a microdroplet assay to assess chemotactic activity, HLAMP was found to be a potent chemoattractant of 7-h developing amoebae of D. discoideum. Other conjugates, including lysine-AMP (LAMP), tuftsin-AMP (TAMP) and avidin-AMP (AVAMP), as well as the degradation products of HLAMP (HL, AMP, and lysine) exhibited no chemotactic activity. The molecular structure of HLAMP is compared to that of other known chemoattractants of the cellular slime molds, and possible chemotactic receptors for HLAMP are considered. 相似文献
39.
Butow Barbara; Wynne David; Sukenik Assaf; Hadas Ora; Tel-Or Elisha 《Journal of plankton research》1998,20(2):355-369
Lipid peroxidation in Peridinium samples taken from two differentdepths in Lake Kinneret fluctuated throughout the spring withan overall increasing trend. Samples from 0.5 and 5 m showeda similar peroxidation pattern, which was maximal after thefall off in algal biomass. The rapid decline in Peridinium biomasscoincided with ambient lake temperatures of 2123C. Fattyacid composition profiles were similar at both depths, althoughafter the peak of the bloom, a significant increase in polyunsaturatedfatty acids and oleic acid was only found at 0.5 m, togetherwith a decrease in the percentage of polyunsaturated fatty acids.These effects were related to ambient light stress rather thana result of lipid peroxidation. Lake samples taken at differentperiods of the bloom and incubated at various temperatures showeddifferential peroxidation. Higher temperatures caused increasedlipid peroxidation, but this appeared to be dependent on thesampling period. Samples withdrawn from the lake at the beginningof the bloom showed little peroxidation after a 5 day incubationat 14C, room temperature (25C) or ambient lake temperature(16C) compared to mid-bloom samples in which there was a significantincrease in peroxidation when they were incubated at room temperature(25C) or ambient lake temperature (22C). Incubation at 14Cinhibited peroxidation; however, samples from mid-bloom againshowed enhanced peroxidation compared with those from the beginningof the bloom. These in situ results suggested a relationshipbetween temperature, another environmental variable during thebloom and lipid peroxidation in Peridinium. As total dissolvedinorganic carbon (DIC) concentrations fall significantly duringthe progress of the bloom and represent an important sourceof environmental stress, laboratory experiments were establishedto investigate the synergistic effect of temperature and carbonnutrition on lipid peroxidation in Peridinium cultures. Increasedtemperature alone caused a slight increase in lipid peroxidation,but this was greatly augmented by carbon limitation. Althoughcarbon limitation induced increased catalase activity, at highertemperatures activity declined after 48 h, allowing for thesubstantial increase in lipid peroxidation. 相似文献
40.