全文获取类型
收费全文 | 53540篇 |
免费 | 5392篇 |
国内免费 | 32篇 |
出版年
2021年 | 634篇 |
2020年 | 496篇 |
2019年 | 596篇 |
2018年 | 774篇 |
2017年 | 667篇 |
2016年 | 1235篇 |
2015年 | 2074篇 |
2014年 | 2191篇 |
2013年 | 2864篇 |
2012年 | 3447篇 |
2011年 | 3499篇 |
2010年 | 2218篇 |
2009年 | 1951篇 |
2008年 | 2888篇 |
2007年 | 2957篇 |
2006年 | 2672篇 |
2005年 | 2720篇 |
2004年 | 2647篇 |
2003年 | 2521篇 |
2002年 | 2433篇 |
2001年 | 792篇 |
2000年 | 710篇 |
1999年 | 798篇 |
1998年 | 714篇 |
1997年 | 540篇 |
1996年 | 501篇 |
1995年 | 510篇 |
1994年 | 488篇 |
1993年 | 480篇 |
1992年 | 629篇 |
1991年 | 558篇 |
1990年 | 483篇 |
1989年 | 483篇 |
1988年 | 494篇 |
1987年 | 460篇 |
1986年 | 434篇 |
1985年 | 525篇 |
1984年 | 519篇 |
1983年 | 406篇 |
1982年 | 491篇 |
1981年 | 393篇 |
1980年 | 370篇 |
1979年 | 375篇 |
1978年 | 373篇 |
1977年 | 333篇 |
1976年 | 330篇 |
1975年 | 306篇 |
1974年 | 305篇 |
1973年 | 345篇 |
1972年 | 270篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Sven Hoefman David van der Ha Nico Boon Peter Vandamme Paul De Vos Kim Heylen 《Antonie van Leeuwenhoek》2014,105(2):353-366
The growth of twelve methanotrophic strains within the genus Methylomonas, including the type strains of Methylomonas methanica and Methylomonas koyamae, was evaluated with 40 different variations of standard diluted nitrate mineral salts medium in 96-well microtiter plates. Unique profiles of growth preference were observed for each strain, showing a strong strain dependency for optimal growth conditions, especially with regards to the preferred concentration and nature of the nitrogen source. Based on the miniaturized screening results, a customized medium was designed for each strain, allowing the improvement of the growth of several strains in a batch setup, either by a reduction of the lag phase or by faster biomass accumulation. As such, the maintenance of fastidious strains could be facilitated while the growth of fast-growing Methylomonas strains could be further improved. Methylomonas sp. R-45378 displayed a 50 % increase in cell dry weight when grown in its customized medium and showed the lowest observed nitrogen and oxygen requirement of all tested strains. We demonstrate that the presented miniaturized approach for medium optimization is a simple tool allowing the quick generation of strain-specific growth preference data that can be applied downstream of an isolation campaign. This approach can also be applied as a first step in the search for strains with biotechnological potential, to facilitate cultivation of fastidious strains or to steer future isolation campaigns. 相似文献
2.
3.
4.
5.
A new genetic polymorphism of an unidentified plasma protein (PLP1) in pigs was described by using a method of two-dimensional gel electrophoresis and protein staining. Two codominant alleles, with frequencies of 0.83 and 0.17, were found in the Swedish Yorkshire breed. The PLP1 marker was typed in a three-generation pedigree and tested for linkage against a set of 128 markers. The PLP1 locus showed significant LOD score values with three different microsatellite markers (S0092, DAGK and S005), previously assigned to chromosome 5. 相似文献
6.
Paul J Curley 《BMJ (Clinical research ed.)》1994,309(6952):486-487
7.
Phylogenetic analysis and secondary structure of the Bacillus subtilis bacteriophage RNA required for DNA packaging 总被引:4,自引:0,他引:4
S Bailey J Wichitwechkarn D Johnson B E Reilly D L Anderson J W Bodley 《The Journal of biological chemistry》1990,265(36):22365-22370
An unusual RNA molecule encoded by the Bacillus subtilis bacteriophage phi 29 is a structural component of the viral prohead and is required for the ATP-dependent packaging of DNA. Here we report a model of secondary structure for this prohead RNA developed from a phylogenetic analysis of the primary sequences of prohead RNAs of related phages. Twenty-nine phages related to phi 29 were found to produce prohead RNAs. These RNAs were analyzed by their ability to replace phi 29 RNA in in vitro phage assembly, by Northern blot hybridization with a probe complementary to phi 29 RNA, and by partial and complete sequence analyses. These analyses revealed four quite different sequences ranging in length from 161 to 174 residues. The secondary structure deduced from these sequences, in agreement with earlier observations, indicated that prohead RNA is organized into two domains. The larger 5'-domain (Domain I) is composed of 113-117 residues and contains four helices. Three of these helices appear to be organized into a central stem that is interrupted by two unpaired loops and the fourth helix and loop. The smaller 3'-domain (Domain II) is composed of 40-44 residues and consists of two helices. Domains I and II are separated by 8-13 unpaired residues. Nuclease cleavage occurs readily in this single-stranded joining region, and this cleavage allows the subsequent separation of the two RNA domains. The separated Domain I is fully active in DNA packaging in vitro. The functional significance and biological role of Domain II are unknown. The phylogenetic secondary structure model provides a basis for further analysis of the role of this RNA in bacteriophage morphogenesis. 相似文献
8.
Paul Linhardt 《Biodegradation》1997,8(3):201-210
Case histories and proposed mechanisms formicrobiologically influenced corrosion of metals andalloys by metal depositing microorganisms arereviewed. Mechanisms with indirect participation ofthese microorganisms, usually iron- and manganeseoxidizing species, are distinguished from anothermechanism which accounts specifically for theelectrochemical properties of deposits containingoxides and hydroxides of Mn in higher oxidationstates. The possible influence of such deposits whichwere formed microbiologically is evaluated. Theevaluation is based on the principles ofelectrochemical corrosion of metals and on theelectrochemical properties of Mn3+/4+- compounds.After briefly reviewing the microbiologicalMn-oxidation, experimental evidence for the predictedcorrosion by such deposits is provided and a model formicrobiologically influenced corrosion by manganeseoxidizing microorganisms is proposed for stainlesssteel. Possible consequences of the model andpractical aspects of such a corrosion are discussed. 相似文献
9.
Maxence Gérard Baptiste Martinet Manuel Dehon Pierre Rasmont Paul H. Williams Denis Michez 《Systematic Entomology》2020,45(4):849-856
Since the beginning of taxonomy, species have been described based on morphology, but the advent of using semio-chemicals and genetics has led to the discovery of cryptic species (i.e. morphologically similar species). When a new cryptic species is described, earlier type specimens have to be re-evaluated, although this process can be challenging as only nondestructive methods ought to be used in order to preserve the integrity of the type specimens. Methods should allow comparison with recently collected specimens clustered based on chemical, ethological and/or genetic traits with old specimens (i.e. type specimens) where only morphological traits are available. Here we develop a method based on geometric morphometric analyses of wing shape for a taxonomically challenging group of bumblebees, the subgenus Alpinobombus Skorikov. We consider nine monophyletic taxa (including several cryptic species) to assess the accuracy of this method to discriminate the taxa based on their wing shape and then to attribute type specimens using a leave-one-out cross-validation procedure. We show that, for these bees, wing shape is taxon-specific, except for two sister taxa for which the species status is still debated. Moreover, for most of the taxa, type specimens were correctly attributed with high posterior probabilities of attribution, except for a few type specimens corresponding to the same two sister taxa where taxa delimitation based on wing shape was previously the subject of discussion. Our study highlights the potential of geometric morphometric analyses to help in the re-attribution of type specimens when the existence of cryptic species is revealed. 相似文献
10.
Paul M. Severns 《Journal of Insect Conservation》2008,12(6):651-661
Fender’s blue butterfly is an endangered species restricted to fragmented, grassland remnants that are becoming increasingly
dominated by tall, invasive grasses in western Oregon, USA. I performed a removal experiment to assess the impacts of structural
degradation accompanying the invasion of Arrhenatherum elatius, tall oat grass, on butterfly fitness and fitness related behaviors. Clipping of A. elatius to native grass sward height resulted in 2.5–5 times as many eggs laid per leaf of host plant. Both male and female butterflies
basked more frequently in areas removed of A. elatius inflorescences and upon encountering the treatment edge butterflies had a high rate of return into a large area removed of
the grass inflorescences. Although butterfly behavior appeared to be affected by the change in sward height on the treatment
edge, there was no evidence for the edge causing a disproportionate egg load. Invasion and dominance by A. elatius appeared to diminish host plant apparency which may result in overloading of eggs on conspicuous host plants, increased incidence
of emigration, and a decrease in the likelihood of colonization because female butterflies appeared indifferent to larval
resources beneath A. elatius inflorescences. Dominance of natural shortgrass prairies by tall stature grasses like A. elatius may be an insidious form of habitat degradation for grassland Lepidoptera worldwide, but it may go largely unnoticed because
larval and adult resources can persist under the unnaturally tall grass canopy. 相似文献