全文获取类型
收费全文 | 63047篇 |
免费 | 5756篇 |
国内免费 | 39篇 |
出版年
2022年 | 399篇 |
2021年 | 751篇 |
2020年 | 472篇 |
2019年 | 584篇 |
2018年 | 774篇 |
2017年 | 715篇 |
2016年 | 1254篇 |
2015年 | 2069篇 |
2014年 | 2378篇 |
2013年 | 3108篇 |
2012年 | 4030篇 |
2011年 | 4028篇 |
2010年 | 2666篇 |
2009年 | 2448篇 |
2008年 | 3493篇 |
2007年 | 3700篇 |
2006年 | 3560篇 |
2005年 | 3308篇 |
2004年 | 3335篇 |
2003年 | 3236篇 |
2002年 | 3021篇 |
2001年 | 652篇 |
2000年 | 453篇 |
1999年 | 725篇 |
1998年 | 891篇 |
1997年 | 643篇 |
1996年 | 593篇 |
1995年 | 575篇 |
1994年 | 546篇 |
1993年 | 599篇 |
1992年 | 550篇 |
1991年 | 461篇 |
1990年 | 466篇 |
1989年 | 446篇 |
1988年 | 446篇 |
1987年 | 441篇 |
1986年 | 442篇 |
1985年 | 500篇 |
1984年 | 616篇 |
1983年 | 556篇 |
1982年 | 672篇 |
1981年 | 724篇 |
1980年 | 681篇 |
1979年 | 415篇 |
1978年 | 462篇 |
1977年 | 452篇 |
1976年 | 391篇 |
1975年 | 346篇 |
1974年 | 406篇 |
1973年 | 355篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Identifying the drivers of community structure and dynamics is a major pursuit in ecology. Emphasis is typically placed on the importance of local scale interactions when attempting to explain these fundamental ecological patterns. However, regional scale phenomena are also important predictors. The importance of regional scale context should be more evident in assemblages where multiple species are close to their range margins. Here, we test the importance of regional scale context using data from a temperate forest plot that contains two species groups – one near its northern range limit and one near its southern range limit. We show the proximity of species to their southern or northern range margins is linked to local scale co-occurrence, similarity in gene expression responses to a key environmental driver, demographic performance and inter-specific variation in conspecific negative density dependence. In sum, many of the key local scale patterns and processes of interest to community ecologists are linked to biogeographic context that is frequently ignored. 相似文献
992.
Robert K. Bright Michael H. Shearer Ronald C. Kennedy 《Cancer immunology, immunotherapy : CII》1993,37(1):31-39
Baculovirus-derived recombinant simian virus 40 (SV40) large tumor antigen (SV40 T-Ag) was used to immunize inbred strains of mice to compare the humoral immune responses. Specifically we examined the epitope specificities and idiotype (Id) expression on anti-(SV40 T-Ag) responses induced in BALB/c and C57BL/6 inbred strains of mice. The predominant SV40 T-Ag epitopes recognized by the anti-(SV40 T-Ag) responses appeared to differ between these two inbred strains, this being based on the ability of sera to inhibit the binding of several murine monoclonal antibodies specific for SV40 T-Ag. In addition, anti-(SV40 T-Ag) responses produced in C57BL/6 mice failed to express a previously described cross-reactive Id expressed in the anti-(SV40 T-Ag) response in BALB/c mice. This cross-reactive Id is detected by a mouse monoclonal anti-Id, designated 58D, which has been shown to represent a potential focal point for manipulating the humoral immune response to SV40-induced tumors in BALB/c mice. Together, these data indicate that the functional duality of the humoral immune response, as assessed by epitope recognition and Id expression, differs between these two inbred strains of mice when immunized with a recombinant SV40 T-Ag. 相似文献
993.
Noriko Arase-Fukushi Hisashi Arase Bingyan Wang Mari Hirano Kazumasa Ogasawara Robert A. Good Kazunori Ono 《Microbiology and immunology》1993,37(11):883-894
Allo-chimerism and clonal elimination of self antigen (Ag) (Ia + Mls-1a) reactive Vβ6+ T cells were analyzed and compared between allogeneic bone marrow (BM) chimeras reconstituted with BM cells which had been treated with anti-Thy-1 monoclonal antibody (mAb) plus complement (C) (T– chimeras) and BM chimeras which had been reconstituted with BM cells pretreated with anti-Thy-1 mAb alone (T+ chimeras). When lethally irradiated AKR (Mls-1a) mice were reconstituted with BM cells from B10 or B10 H-2 congenic mice, both T+ and T– chimeras were entirely free of signs of graft-versus-host reaction (GVHR). However, complete replacement of the AKR lymphoid tissues by donor BM cells was accomplished at an early stage in T+ chimeras but not in T– chimeras. On the other hand, clonal elimination of Vβ6+ T cells reactive to the recipient Ag (Mls-1a) was abolished in T+ chimeras but successfully induced in T– chimeras. The Vβ6+ T cells not eliminated in T+ chimeras showed depressed responses against Mls-1a antigens. The findings herein demonstrate that T cells which contaminate a BM inoculum survive in recipient mice after treatment with anti-Thy-1 mAb without C in vitro followed by BMT. The surviving T cells have been estimated to represent fewer than 0.5% of the BM cells inoculated. These cells appear to accelerate the full replacement of recipient lymphoid tissues by donor cells. Furthermore, the T cells which survive in the marrow inoculum influence eventually the development of a tolerant state in the T cell repertoire of the donor. 相似文献
994.
Molecular cloning and sequencing of infC, the gene encoding translation initiation factor IF3, from four enterobacterial species 总被引:3,自引:0,他引:3
Dionysios Liveris John J. Schwartz Robert Geertman Ira Schwartz 《FEMS microbiology letters》1993,112(2):211-216
Abstract Translation initiation factor IF3 plays a crucial role in initiation of protein synthesis in bacteria. In order to elucidate the IF3 structural elements required for these functions, the evolutionary conservation of IF3 and its gene, infC , was investigated. Homologous infC sequences from Salmonella typhimurium, Klebsiella pneumoniae, Serratia marcescens and Proteus vulgaris were amplified by the polymerase chain reaction and sequenced. Analysis of these sequences, as well as that from Bacillus stearothermophilus , revealed several regions (e.g. residues 62–73 and 173–177) of absolute sequence conservation, suggesting an important role for these regions in IF3 function. 相似文献
995.
996.
Daniel E. Gomez Jacqueline L. Hartzler Robert H. Corbitt Alexander M. Nason Unnur P. Thorgeirsson 《In vitro cellular & developmental biology. Animal》1993,29(6):451-455
Summary We describe a fast and reproducible method that can be used as a final step in obtaining pure populations of liver endothelial
cells. This method employs endothelial cell specific lectin covalently bound to magnetic polystyrene beads (Dynabeads). Evonymus
europaeus agglutinin (EEA)-coated Dynabeads were used to purify monkey liver endothelium from Percoll gradient separated nonparenchymal
cells. EEA-coated beads were also successfully used to purify monkey aortic endothelial cells. The endothelial cells grew
to confluence as a cobblestonelike monolayer, expressed Factor VIII related antigen, and incorporated acetylated-low density
lipoprotein. The magnetic beads seemed not to modify the normal properties of the isolated endothelium, thus facilitating
their use in experimental studies. This immunomagnetic separation technique may be applicable for purification of endothelial
cells from a wide variety of tissue sources. 相似文献
997.
Johanna Plendl Laura Hartwell Robert Auerbach 《In vitro cellular & developmental biology. Animal》1993,29(1):25-31
Summary Endothelial cells of the NMRI mouse strain express a cell surface glycoprotein recognized by the lectinDolichos biflorus agglutinin (DBA). This study documents a marked organ-specific increase in DBA-specific lectin binding of myocardium-derived
endothelial cells (MEC) of the NMRI/GSF mouse during in vitro cultivation. An up to 20-fold increase in DBA binding sites
is observed in long-term culture, an increase not found in other NMRI-derived endothelial cell lines (e.g., brain, aorta).
The increase appears restricted to DBA in that binding with other lectins (PNA, WGA) was unaltered. NMRI MEC cultures maintain
typical endothelial cell attributes such as cobblestone morphology on confluence, expression of endothelial cell-specific
surface markers, and production of angiotensin-converting enzyme. Cultures routinely become aneuploid within 4 passages, several
passages before upregulation of the DBA binding site(s). Myocardial endothelial cells sorted to obtain DBAhi and DBAlo cell populations generally maintained their sorted phenotype for 3 to 4 passages. Limiting dilution cloning resulted in clones
varying in DBA expression. Clones for DBAhi expression maintained their DBA affinity for at least 10 passages (>30 doublings), whereas DBAlo clones gave rise to varying numbers of DBAhi cells within 2 to 4 passages. We hypothesize that the change in DBA affinity accompanies in vitro aging, that the change
is independent of alterations in karyotype, and that the increase in DBA affinity may reflect a change in one or more other
endothelial cell properties. Additional studies will be necessary to determine whether the in vitro changes are correlated
with specific functional alterations and whether they accurately reflect progressive changes of MEC in vivo. 相似文献
998.
Alan M. Goldberg John M. Frazier David Brusick Michael S. Dickens Oliver Flint Stephen D. Gettings Richard N. Hill Robert L. Lipnick Kevin J. Renskers June A. Bradlaw Robert A. Scala Bellina Veronesi Sidney Green Neil L. Wilcox Rodger D. Curren 《In vitro cellular & developmental biology. Animal》1993,29(9):688-692
Summary The development and application of in vitro alternatives designed to reduce or replace the use of animals, or to lessen the
distress and discomfort of laboratory animals, is a rapidly developing trend in toxicology. However, at present there is no
formal administrative process to organize, coordinate, or evaluate validation activities. A framework capable of fostering
the validation of new methods is essential for the effective transfer of new technologic developments from the research laboratory
into practical use. This committee has identified four essential validation resources: chemical bank(s), cell and tissue banks,
a data bank, and reference laboratories. The creation of a Scientific Advisory Board composed of experts in the various aspects
and endpoints of toxicity testing, and representing the academic, industrial, and regulatory communities, is recommended.
Test validation acceptance is contingent on broad buy-in by disparate groups in the scientific community—academics, industry,
and government. This is best achieved by early and frequent communication among parties and agreement on common goals. It
is hoped that the creation of a validation infrastructure composed of the elements described in this report will facilitate
scientific acceptance and utilization of alternative methodologies and speed implementation of replacement, reduction, and
refinement alternatives in toxicity testing. 相似文献
999.
1000.
Maria Ermakova Russell Woodford Zachary Taylor Robert T. Furbank Srinivas Belide Susanne von Caemmerer 《Plant biotechnology journal》2023,21(6):1206-1216
Sorghum is one of the most important crops providing food and feed in many of the world's harsher environments. Sorghum utilizes the C4 pathway of photosynthesis in which a biochemical carbon-concentrating mechanism results in high CO2 assimilation rates. Overexpressing the Rieske FeS subunit of the Cytochrome b6f complex was previously shown to increase the rate of photosynthetic electron transport and stimulate CO2 assimilation in the model C4 plant Setaria viridis. To test whether productivity of C4 crops could be improved by Rieske overexpression, we created transgenic Sorghum bicolor Tx430 plants with increased Rieske content. The transgenic plants showed no marked changes in abundances of other photosynthetic proteins or chlorophyll content. The steady-state rates of electron transport and CO2 assimilation did not differ between the plants with increased Rieske abundance and control plants, suggesting that Cytochrome b6f is not the only factor limiting electron transport in sorghum at high light and high CO2. However, faster responses of non-photochemical quenching as well as an elevated quantum yield of Photosystem II and an increased CO2 assimilation rate were observed from the plants overexpressing Rieske during the photosynthetic induction, a process of activation of photosynthesis upon the dark–light transition. As a consequence, sorghum with increased Rieske content produced more biomass and grain when grown in glasshouse conditions. Our results indicate that increasing Rieske content has potential to boost productivity of sorghum and other C4 crops by improving the efficiency of light utilization and conversion to biomass through the faster induction of photosynthesis. 相似文献