全文获取类型
收费全文 | 61995篇 |
免费 | 5084篇 |
国内免费 | 38篇 |
出版年
2023年 | 305篇 |
2022年 | 667篇 |
2021年 | 1195篇 |
2020年 | 844篇 |
2019年 | 950篇 |
2018年 | 1332篇 |
2017年 | 1130篇 |
2016年 | 1854篇 |
2015年 | 2678篇 |
2014年 | 2672篇 |
2013年 | 3360篇 |
2012年 | 3980篇 |
2011年 | 3694篇 |
2010年 | 2311篇 |
2009年 | 2161篇 |
2008年 | 2708篇 |
2007年 | 2672篇 |
2006年 | 2498篇 |
2005年 | 2746篇 |
2004年 | 2715篇 |
2003年 | 2303篇 |
2002年 | 1890篇 |
2001年 | 1537篇 |
2000年 | 1440篇 |
1999年 | 1313篇 |
1998年 | 641篇 |
1997年 | 622篇 |
1996年 | 652篇 |
1995年 | 494篇 |
1994年 | 515篇 |
1993年 | 476篇 |
1992年 | 970篇 |
1991年 | 867篇 |
1990年 | 796篇 |
1989年 | 791篇 |
1988年 | 783篇 |
1987年 | 710篇 |
1986年 | 672篇 |
1985年 | 656篇 |
1984年 | 613篇 |
1983年 | 467篇 |
1982年 | 372篇 |
1981年 | 364篇 |
1980年 | 341篇 |
1979年 | 453篇 |
1978年 | 373篇 |
1977年 | 312篇 |
1975年 | 309篇 |
1974年 | 312篇 |
1973年 | 318篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
111.
Rat-liver DNA alkylation by diethylnitrosamine (DEN), dimethylnitrosamine (DMN) and ethyl methanesulphonate (EMS) was studied in an attempt to relate chromosome-damaging effects of these agents (the formation of micronuclei in hepatocytes; see preceding paper) to specific alkylation patterns. No correlation was observed between the induction of micronuclei and liver DNA N-alkylation, measured as 3- and 7-alkyl-purines. O6-Alkylguanine is probably not involved in micronucleus induction because it is lost from DNA too rapidly to explain the much more persistent clastogenic effects. In contrast, both the initial amounts of alkylphosphotriesters and the persistencies of these products roughly paralleled the respective effects on micronucleus induction. The possible involvement of alkylphosphotriesters or other O-alkylation products of comparable stabilities is discussed. Results with DMN suggest that part of the primary DNA methylation damage is converted into a secondary (DNA) lesion and that both the primary and secondary lesion(s) contribute to the process of micronucleus formation. 相似文献
112.
The Importance of Hydrodynamics for Protected and Endangered Biodiversity of Lowland Rivers 总被引:3,自引:3,他引:0
R. J. W. de Nooij W. C. E. P. Verberk H. J. R. Lenders R. S. E. W. Leuven P. H. Nienhuis 《Hydrobiologia》2006,565(1):153-162
This paper examines the relationship between protected and endangered riverine species (target species) and hydrodynamics
in river-floodplain ecosystems, combining ecological and policy-legal aspects of biodiversity conservation in river management.
The importance of different hydrodynamic conditions along a lateral gradient was quantified for various taxonomic groups.
Our results show that (i) target species require ecotopes along the entire hydrodynamic gradient; (ii) different parts of
the hydrodynamic gradient are important to different species, belonging to different taxonomic groups; (iii) in particular
low-dynamic parts are important for many species and (iv) species differ in their specificity for hydrodynamic conditions.
Many species of higher plants, fish and butterflies have a narrow range for hydrodynamics and many species of birds and mammals
use ecotopes along the entire gradient. Even when focussing only on target species, the entire natural hydrodynamic gradient
is important. This means that the riverine species assemblage as a whole can benefit from measures focussing on target species
only. River reconstruction and management should aim at re-establishing the entire hydrodynamic gradient, increasing the spatial
heterogeneity of hydrodynamic conditions. 相似文献
113.
114.
115.
A mesoscale model of DNA is presented (3SPN.1), extending the scheme previously developed by our group. Each nucleotide is mapped onto three interaction sites. Solvent is accounted for implicitly through a medium-effective dielectric constant and electrostatic interactions are treated at the level of Debye-Hückel theory. The force field includes a weak, solvent-induced attraction, which helps mediate the renaturation of DNA. Model parameterization is accomplished through replica exchange molecular dynamics simulations of short oligonucleotide sequences over a range of composition and chain length. The model describes the melting temperature of DNA as a function of composition as well as ionic strength, and is consistent with heat capacity profiles from experiments. The dependence of persistence length on ionic strength is also captured by the force field. The proposed model is used to examine the renaturation of DNA. It is found that a typical renaturation event occurs through a nucleation step, whereby an interplay between repulsive electrostatic interactions and colloidal-like attractions allows the system to undergo a series of rearrangements before complete molecular reassociation occurs. 相似文献
116.
Thirty components were identified in Lavandula latifolia essential oil (spike oil). One of the compounds, espliegol (δ-terpineol), is a new natural product. 相似文献
117.
118.
E. L. García Diez J. L. Labajo Salazar F. de Pablo Dávila 《International journal of biometeorology》1993,37(4):194-199
The present paper offers data on the effect of certain meteorological parameters on the outbreak of forest fires in Galicia (Spain). In a day-to-day analysis, the values of the stability of the air column and the saturation deficit at the lower levels are related to the number of fires occurring in the following 24 h. 相似文献
119.
S D Kroll J Chen M De Vivo D J Carty A Buku R T Premont R Iyengar 《The Journal of biological chemistry》1992,267(32):23183-23188
120.
Johannes Schwerk Mario K?ster Hansj?rg Hauser Manfred Rohde Marcus Fulde Mathias W. Hornef Tobias May 《PloS one》2013,8(8)
Cell lines derived from the small intestine that reflect authentic properties of the originating intestinal epithelium are of high value for studies on mucosal immunology and host microbial homeostasis. A novel immortalization procedure was applied to generate continuously proliferating cell lines from murine E19 embryonic small intestinal tissue. The obtained cell lines form a tight and polarized epithelial cell layer, display characteristic tight junction, microvilli and surface protein expression and generate increasing transepithelial electrical resistance during in vitro culture. Significant up-regulation of Cxcl2 and Cxcl5 chemokine expression upon exposure to defined microbial innate immune stimuli and endogenous cytokines is observed. Cell lines were also generated from a transgenic interferon reporter (Mx2-Luciferase) mouse, allowing reporter technology-based quantification of the cellular response to type I and III interferon. Thus, the newly created cell lines mimic properties of the natural epithelium and can be used for diverse studies including testing of the absorption of drug candidates. The reproducibility of the method to create such cell lines from wild type and transgenic mice provides a new tool to study molecular and cellular processes of the epithelial barrier. 相似文献