全文获取类型
收费全文 | 72814篇 |
免费 | 4730篇 |
国内免费 | 7篇 |
出版年
2023年 | 495篇 |
2022年 | 598篇 |
2021年 | 1278篇 |
2020年 | 1042篇 |
2019年 | 1196篇 |
2018年 | 2196篇 |
2017年 | 2033篇 |
2016年 | 2679篇 |
2015年 | 3500篇 |
2014年 | 3604篇 |
2013年 | 4829篇 |
2012年 | 5620篇 |
2011年 | 5139篇 |
2010年 | 3319篇 |
2009年 | 2674篇 |
2008年 | 3903篇 |
2007年 | 3653篇 |
2006年 | 3494篇 |
2005年 | 2973篇 |
2004年 | 2881篇 |
2003年 | 2587篇 |
2002年 | 2371篇 |
2001年 | 1653篇 |
2000年 | 1608篇 |
1999年 | 1268篇 |
1998年 | 627篇 |
1997年 | 468篇 |
1996年 | 449篇 |
1995年 | 427篇 |
1994年 | 321篇 |
1993年 | 313篇 |
1992年 | 634篇 |
1991年 | 544篇 |
1990年 | 490篇 |
1989年 | 483篇 |
1988年 | 464篇 |
1987年 | 425篇 |
1986年 | 393篇 |
1985年 | 388篇 |
1984年 | 393篇 |
1983年 | 271篇 |
1982年 | 273篇 |
1981年 | 198篇 |
1980年 | 199篇 |
1979年 | 248篇 |
1978年 | 234篇 |
1975年 | 218篇 |
1974年 | 239篇 |
1973年 | 239篇 |
1972年 | 207篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
M. Nováková Beat M. Riederer Vladmír Viklický Pavel Dráber 《Histochemistry and cell biology》1997,107(4):337-344
A panel of monoclonal antibodies specific of α-tubulin (TU-01, TU-09) and β-tubulin (TU-06, TU-13) subunits was used to study
the location of N-terminal structural domains of tubulin in adult mouse brain. The specificity of antibodies was confirmed
b immunoblotting experiments. Immunohistochemical staining of vibratome sections from cerebral cortex, cerebellum, hippocampus,
and corpus callosum showed that antibodies TU-01, TU-09, and TU13 reacted with neuronal and glial cells and their processes,
whereas the TU-06 antibody stained only the perikarya. Dendrites and axons were either unstained or their staining was very
weak. As the TU-06 epitope is located on the N-terminal structural domain of β-tubulin, the observed staining pattern cannot
be interpreted as evidence of a distinct subcellular localization of β-tubulin isotypes or known post-translational modifications.
The limited distribution of the epitope could, rather, reflect differences between the conformations of tubulin molecules
in microtubules of somata and neurites or, alternatively, a specific masking of the corresponding region on the N-terminal
domain of β-tubulin by interacting protein(s) in dendrites and axons.
Accepted: 11 November 1996 相似文献
912.
Characterization of SKM1, a Saccharomyces cerevisiae gene encoding a novel Ste20/PAK-like protein kinase 总被引:1,自引:0,他引:1
Humberto Martín Alfonso Mendoza Jose M. Rodríguez-Pachón María Molina & César Nombela 《Molecular microbiology》1997,23(3):431-444
Ste20/PAK serine/threonine protein kinases have been suggested as playing essential roles in cell signalling and morphogenesis as potential targets of Cdc42 and Rac GTPases. We have isolated and characterized the Saccharomyces cerevisiae SKM1 gene, which codes for a novel member of this family of protein kinases. The amino acid sequence analysis of Skm1p revealed the presence of a PH domain and a putative p21-binding domain near its amino terminus, suggesting its involvement in cellular signalling or cytoskeletal functions. However, deletion of SKM1 produced no detectable phenotype under standard laboratory conditions. Moreover, disruption of each of the two other S. cerevisiae Ste20/PAK-like kinase-encoding genes, STE20 and CLA4 , in skm1 backgrounds, showed that Skm1p is not redundant with Ste20p or Cla4p. Interestingly, overexpression of SKM1 led to morphological alterations, indicating a possible role for this protein in morphogenetic control. Furthermore, overproduction of Skm1p lacking its N-terminus caused growth arrest. This effect was also seen when similarly truncated versions of Ste20p or Cla4p were overexpressed. We further observed that overproduction of this C-terminal fragment of Skm1p complements the mating defect of a ste20 mutant strain. These results suggest that the N-terminal domains of S. cerevisiae Ste20/PAK-like protein kinases share a negative regulatory function and play a role in substrate specificity. 相似文献
913.
A new, non-radioactive and cheap colorimetric method for determination of activity of sialyltransferases of various specifities using natural substrates based on 2-thiobarbituric acid assay is presented. The assay was tested with three different sialyltransferases (a-2,3 and a-2,6) and compared with the radioactive assay. 相似文献
914.
P. van't Veer I. E. Lobbezoo J. M. Martín-Moreno E. Guallar J. Gómez-Aracena A. F. Kardinaal L. Kohlmeier B. C. Martin J. J. Strain M. Thamm P. van Zoonen B. A. Baumann J. K. Huttunen F. J. Kok 《BMJ (Clinical research ed.)》1997,315(7100):81-85
OBJECTIVE: To examine any possible links between exposure to DDE (1,1-dichloro-2,2-bis (p-chlorophenyl)ethylene), the persistent metabolite of the pesticide dicophane (DDT), and breast cancer. DESIGN: Multicentre study of exposure to DDE by measurement of adipose tissue aspirated from the buttocks. Laboratory measurements were conducted in a single laboratory. Additional data on risk factors for breast cancer were obtained by standard questionnaires. SETTING: Centres in Germany, the Netherlands, Northern Ireland, Switzerland, and Spain. SUBJECTS: 265 postmenopausal women with breast cancer and 341 controls matched for age and centre. MAIN OUTCOME MEASURE: Adipose DDE concentrations. RESULTS: Women with breast cancer had adipose DDE concentrations 9.2% lower than control women. No increased risk of breast cancer was found at higher concentrations. The odds ratio of breast cancer, adjusted for age and centre, for the highest versus the lowest fourth of DDE distribution was 0.73 (95% confidence interval 0.44 to 1.21) and decreased to 0.48 (0.25 to 0.95; P for trend = 0.02) after adjustment for body mass index, age at first birth, and current alcohol drinking. Adjustment for other risk factors did not materially affect these estimates. CONCLUSIONS: The lower DDE concentrations observed among the women with breast cancer may be secondary to disease inception. This study does not support the hypothesis that DDE increases risk of breast cancer in postmenopausal women in Europe. 相似文献
915.
Ivette Perfecto John Vandermeer Paul Hanson Victor Cartín 《Biodiversity and Conservation》1997,6(7):935-945
The coffee (Coffea arabica) agro-ecosystem in the Central Valley of Costa Rica was formerly characterized by a high vegetational diversity. This complex system has been undergoing a major transformation to capital-intensive monocultural plantations where all shade trees are eliminated. In this study we examined the pattern of arthropod biodiversity loss associated with this transformation. Canopy arthropods were sampled in three coffee farms: a traditional plantation with many species of shade trees, a moderately shaded plantation with only Erythrina poeppigeana and coffee, and a coffee monoculture. An insecticidal fogging technique was used to sample both canopy and coffee arthropods. Data are presented on three major taxonomic groups: Coleoptera, non-formicid Hymenoptera, and Formicidae. Data demonstrate that the transformation of the coffee agro-ecosystem results in a significant loss of biological diversity of both canopy arthropods as well as arthropods living in coffee bushes. Percentage of species overlap was very small for all comparisons. Furthermore, species' richness on a per tree basis was found to be within the same order of magnitude as that reported for trees in tropical forests. If results presented here are generalizable, this means that conservation efforts to preserve biological diversity should also include traditional agro-ecosystems as conservation units. 相似文献
916.
917.
918.
Hydrological cycle and interannual variability of the aquatic community in a temporary saline lake (Fuente de Piedra,Southern Spain) 总被引:4,自引:2,他引:2
García Carlos M. García-Ruiz Roberto Rendón Manuel Niell F. Xavier Lucena Juan 《Hydrobiologia》1997,345(2-3):131-141
Fuente de Piedra is a shallow, temporary saline lake whoseseasonal behavior is strongly dependent on the annual hydrologicalbudget. In this study, we outline the characteristics of Fuente dePiedra Lake for two years that had different hydrological budgets.The high precipitations in 1989–90 caused the lake not to dry asusual, and decreased both salinity and the amplitude of changes.There were also differences in nutrient dynamics, with generallylower concentrations of soluble reactive phosphorus and ammonium,whereas in the more humid year nitrate showed a distinct maximum inwinter. Winter bloom chlorophyll a concentrations were alsomuch higher in 1989–90 (>600 g l-1) but there wasalso a winter productive phase that was presumably poorly coupledwith consumption processes that predominate in spring. Planktonicassemblages were different between years. Highly halotolerantphytoplankton species (Dunaliella salina and D. viridis) became scarcer, and especially two previouslyunrecorded diatoms (Cyclotella sp. and Chaetoceros sp.)became dominant in the bloom time in the wet year. The speciesrichness of the zooplankton increased in the wet year, with newspecies appearing that were not collected during 1987–88(Branchinectella media, Daphnia mediterranea, Macrothrix sp.,Arctodiaptomus salinus, Cyclops sp., Sigara sp...).There was also a much higher development of macrophytes (Ruppiadrepanensis, Althenia orientalis, Lamprothamnium papulosum)and bird populations, especially flamingoes (Phoenicopterusruber).Important interannual variations in this sort of system pointto the need for long term studies to observe the whole range ofstates that define the lake as an entity. 相似文献
919.
A new genus of Platycopioida is described from a boxcore sample taken at a depth of 534 m in the ArcticBarents Sea. This is the deepest record ofPlatycopioida so far. Sarsicopia gen. n. is thesistergroup of a taxon comprising Platycopia and Nanocopia; the sistergroup ofthese is Antrisocopia. Sarsicopia gen. n.is the only platycopioid to retain 2 inner setae onthe second endopod segment P2–P4, and 8 setae in thethird endopod segment of P2. The male antennnule isremarkable in having a geniculation located betweenancestral segments XX and XXI. It is suggested thatthis flexure zone was already present in thegroundpattern of Copepoda. Platycopia and Nanocopia have secondarily lost thisgeniculation. 相似文献
920.
Several aspects of community organization wereanalyzed comparatively in a small side-arm of theParaná River (Correntoso) and a shallowfloodplain lake (El Tigre) (31° 41 S and60° 42 W), in relation to the hydrology of thesystem. Taxonomic and morphological composition inthe river differed from that in the lake: the riverhad lower species richness (151 vs 218),different contributions of some Classes to totalspecies number (higher Cyano-, Zygo- andDiatomophyceae vs higher Chlorophyceae), anddiffent proportions of nannoplanktonic algae (67.5%vs 80.7%) and netplanktonic filamentousspecies (18.2% vs 4.2%). Phytoplanktonbiomass, higher in the lake than in the river due tothe retention time, was mostly dominated bynannoplankton and netplankton. Loticphytoplankton was dominated by typical fluvialspecies of Diatomophyceae (R-strategists). Riverconditions seem to maintain a subclimacticcommunity, which was little impacted by the flushingof populations from floodplain lakes. Water levelwas the main factor controlling phytoplanktonbiomass, species diversity (H), evenness (E) andcommunity change rate () in the river. Inthe lake, phytoplankton had an autogenicsuccessional sequence during the isolation phase (C-to S-strategists) and other responses todisturbance, mainly during the flood(R-strategists). Frequent changes in phytoplanktoncomposition, biomass, H, E and , revealed aenvironmental instability in the lake, which may beexplained by interactions of external factors(hydrology and climatology) and those of internalorigin, such as nutrients and grazing. 相似文献