全文获取类型
收费全文 | 20021篇 |
免费 | 1647篇 |
国内免费 | 5篇 |
专业分类
21673篇 |
出版年
2023年 | 89篇 |
2022年 | 232篇 |
2021年 | 432篇 |
2020年 | 233篇 |
2019年 | 306篇 |
2018年 | 423篇 |
2017年 | 353篇 |
2016年 | 636篇 |
2015年 | 1030篇 |
2014年 | 1169篇 |
2013年 | 1409篇 |
2012年 | 1747篇 |
2011年 | 1649篇 |
2010年 | 1062篇 |
2009年 | 936篇 |
2008年 | 1264篇 |
2007年 | 1292篇 |
2006年 | 1105篇 |
2005年 | 1060篇 |
2004年 | 1034篇 |
2003年 | 937篇 |
2002年 | 892篇 |
2001年 | 203篇 |
2000年 | 144篇 |
1999年 | 153篇 |
1998年 | 233篇 |
1997年 | 149篇 |
1996年 | 136篇 |
1995年 | 125篇 |
1994年 | 112篇 |
1993年 | 102篇 |
1992年 | 82篇 |
1991年 | 65篇 |
1990年 | 69篇 |
1989年 | 60篇 |
1988年 | 50篇 |
1987年 | 56篇 |
1986年 | 42篇 |
1985年 | 46篇 |
1984年 | 43篇 |
1983年 | 62篇 |
1982年 | 37篇 |
1981年 | 32篇 |
1980年 | 36篇 |
1979年 | 28篇 |
1978年 | 42篇 |
1977年 | 35篇 |
1976年 | 32篇 |
1975年 | 25篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
Niraj Shrestha Pallavi Chaturvedi Xiaoyun Zhu Michael J. Dee Varghese George Christopher Janney Jack O. Egan Bai Liu Mark Foster Lynne Marsala Pamela Wong Celia C. Cubitt Jennifer A. Foltz Jennifer Tran Timothy Schappe Karin Hsiao Gilles M. Leclerc Lijing You Christian Echeverri Catherine Spanoudis Ana Carvalho Leah Kanakaraj Crystal Gilkes Nicole Encalada Lin Kong Meng Wang Byron Fang Zheng Wang Jin-an Jiao Gabriela J. Muniz Emily K. Jeng Nicole Valdivieso Liying Li Richard Deth Melissa M. Berrien-Elliott Todd A. Fehniger Peter R. Rhode Hing C. Wong 《Aging cell》2023,22(5):e13806
123.
Organization of the genes necessary for hydrogenase expression in Rhodobacter capsulatus. Sequence analysis and identification of two hyp regulatory mutants 总被引:11,自引:0,他引:11
Annette Colbeau Pierre Richaud † Bertrand Toussaint F. Javier Caballero ‡ Christine Elster Christian Delphin Russell L. Smith § Jacqueline Chabert Paulette M. Vignais 《Molecular microbiology》1993,8(1):15-29
A 25kbp DNA fragment from the chromosome of Rhodobacter capsulatus B10 carrying hydrogenase (hup) determinants was completely sequenced. Coding regions corresponding to 20 open reading frames were identified. The R. capsulatus hydrogenase-specific gene (hup and hyp) products bear significant structural identity to hydrogenase gene products from Escherichia coli (13), from Rhizobium liguminosarum (16), from Azotobacter vinelandii (10) and from Alcaligenes eutrophus (11). The sequential arrangement of the R. capsulatus genes is: hupR2-hupU-hypF -hupS-hupL-hupM-hupD -hupF -hupG -hupH -huoJ -hupK -hypA-hypB-hupR1-hypC -hypD -hypE -ORF19 -ORF20 , all contiguous and transcribed from the same DNA strand. The last two potential genes do not encode products that are related to identified hydrogenase-specific gene products in other species. The sequence of the 12 R. capsulatus genes underlined above is presented. The mutation site in two of the Hup? mutants used in this study, RS13 and RCC12, was identified in the hypF gene (deletion of one G) and in the hypD qene (deletion of 54 bp), respectively. The hypF gene product shares 45% identity with the product of hydA from E. coli and the product of hypF from R. leguminosarum. Those products present at their N-terminus a Cys arrangement typical of zinc-finger proteins. The G deletion in the C-terminal region of hypF in the RS13 mutant 相似文献
124.
125.
Lars J. S. Knutsen Jesper Lau Malcolm J. Sheardown Christian Thomsen 《Bioorganic & medicinal chemistry letters》1993,3(12):2661-2666
The synthesis and SAR of a series of novel derivatives of N-aminoadenosine is described, along with their in vitro effects in biochemical assays. The rat brain A1 adenosine receptor binding of these compounds is very dependent upon the purine 2-substituent. The novel agonist, 2-chloro-N-[4-(phenylthio)-1-piperidinyl]adenosine, exhibits a Li value for A1 receptor binding of <1 nM. 相似文献
126.
Christian Boedeker Michael J. Wynne Giuseppe C. Zuccarello 《Journal of phycology》2023,59(6):1284-1298
The continental coasts and remote islands in the high-latitude Southern Hemisphere, including the subantarctic region, are characterized by many endemic species, high abundance of taxa, and intermediate levels of biodiversity. The macroalgal flora of these locations has received relatively little attention. Filamentous green algae are prolific in the intertidal of southern islands, but the taxonomy, distribution, and evolutionary history of these taxa are yet to be fully explored, mostly due to the difficulty of access to some of these locations. In this study, we examined specimens of the order Cladophorales from various locations in the high-latitude Southern Hemisphere including the subantarctic (the Auckland Islands, Bounty Islands, Campbell Island, Macquarie Island, and Kerguelen Islands), as well as mainland New Zealand, the Chatham Islands, Chile, and Tasmania. The analyses of the rDNA sequences of the samples revealed the existence of two new clades in a phylogeny of the Cladophoraceae. One of these clades is described as the novel genus Vandenhoekia gen. nov., which contains three species that are branched or unbranched. The amended genus Rama is reinstated to accommodate the other clade, and contains four species, including the Northern Hemisphere “Cladophora rupestris.” In Rama both branched and unbranched morphologies are found. It is remarkable that gross morphology is not a predictor for generic affiliations in these algae. This study illustrates that much can still be learned about diversity in the Cladophorales and highlights the importance of new collections, especially in novel locations. 相似文献
127.
Bernhard Koppenhoefer Andreas Nothdurft Joanna Pierrot-Sanders Patrick Piras Cristina Popescu Christian Roussel Matthias Stiebler Ulrich Trettin 《Chirality》1993,5(4):213-219
In order to cope with the increasing number of publications on the separation of enantiomers by chromatography on a chiral stationary phase, the graphical molecular database CHIRBASE was created. In the present state, the database package covers information (structural, bibiographic, and chromatographic data) on liquid-, supercritical fluid-, and gas chromatography; other methods will follow. CHIRBASE, running on the MDL software Chembase®, meets the requirements of contemporary information management in the chemical and pharmaceutical industry. (Detailed information including a demo-version of each part of CHIRBASE can be obtained from the authors on request.) © 1993 Wiley-Liss, Inc. 相似文献
128.
Christian Brochmann Pamela S. Soltis Douglas E. Soltis 《American journal of botany》1992,79(6):673-688
Draba (Brassicaceae) is well known for its taxonomic complexity in arctic and alpine floras, and the polyploids in particular present vexing taxonomic problems. It has been suggested that polyploids in Draba may have formed recurrently from different populations of the parental species (polytopy), and it is also possible that a given taxonomic species may actually comprise several polyploid races, each originating from different progenitor species (polyphyly). To unravel the taxonomic complexity of polyploid Draba in the Nordic area, we investigated three of the most morphologically variable species and their possible progenitors using enzyme electrophoresis and restriction site analysis of chloroplast DNA (cpDNA) and nuclear ribosomal RNA genes (rDNA): D. norvegica (6x), D. lactea (6x), and D. corymbosa (16x). Electrophoretic analyses of progeny showed high levels of fixed heterozygosity in all three polyploids, demonstrating that all are genetic alloploids. Electrophoretic and rDNA data indicate that polytopic and/or polyphyletic origins have contributed to the complexity of these polyploids. However, a lack of cpDNA variation among the species limited the usefulness of this molecule for analysis of polyploid origins. The considerable electrophoretic variation observed in D. norvegica necessitates a minimum of three and probably 13 independent origins. Electrophoretic and rDNA data suggest that D. lactea and D. corymbosa are polyphyletic polyploids. Crossing data also support that D. corymbosa is polyphyletic. Given the widespread geographic distributions of these species and their possible progenitors, and that the populations analyzed represent only a small fraction of their geographic distributions, it is likely that these species have formed numerous times in different areas. As more molecular analyses of polyploids are completed, the data continue to suggest that multiple origins of polyploids are the rule rather than the exception. 相似文献
129.
Christian Damgaard 《Oikos》2004,107(2):225-230
Two-species plant competition experiments are often drawn on in an attempt to understand and possibly to predict the formation of plant community structures. Frequently, such competition experiments use a non-random spatial design but is analysed in models where a random spatial design is implicitly assumed. Here, the effect of spatial covariance in plant competition experiments on the inferred ecological conclusions is demonstrated. It is shown that spatial covariance, which is dependent on the competition kernel and the experimental design, may affect the estimation of parameters of interest. Based on the same set of experimental data, the predicted outcome of competition between two Arabidopsis thaliana genotypes can vary between (almost certain) coexistence and (almost certain) exclusion of one genotype, depending on the assumptions made about the spatial scale of competition between the plants . The predicted outcome of competition depended strongly on the mean interaction distance of the competition kernel, whereas the functional shape of the competition kernel was less important. It is suggested that plants in manipulated competition experiments should be placed randomly, such that the design of the competition experiment is in agreement with the model used in the analysis. 相似文献
130.
Thomas F Floyd James M Clark Robert Gelfand John A Detre Sarah Ratcliffe Dimitri Guvakov Christian J Lambertsen Roderic G Eckenhoff 《Journal of applied physiology》2003,95(6):2453-2461
Breathing 100% O2 at 1 atmosphere absolute (ATA) is known to be associated with a decrease in cerebral blood flow (CBF). It is also accompanied by a fall in arterial Pco2 leading to uncertainty as to whether the cerebral vasoconstriction is totally or only in part caused by arterial hypocapnia. We tested the hypothesis that the increase in arterial Po2 while O2 was breathed at 1.0 ATA decreases CBF independently of a concurrent fall in arterial Pco2. CBF was measured in seven healthy men aged 21-62 yr by using noninvasive continuous arterial spin-labeled-perfusion MRI. The tracer in this technique, magnetically labeled protons in blood, has a half-life of seconds, allowing repetitive measurements over short time frames without contamination. CBF and arterial blood gases were measured while breathing air, 100% O2, and 4 and 6% CO2 in air and O2 backgrounds. Arterial Po2 increased from 91.7 +/- 6.8 Torr in air to 576.7 +/- 18.9 Torr in O2. Arterial Pco2 fell from 43.3 +/- 1.8 Torr in air to 40.2 +/- 3.3 Torr in O2. CBF-arterial Pco2 response curves for the air and hyperoxic runs were nearly parallel and separated by a distance representing a 28.7-32.6% decrement in CBF. Regression analysis confirmed the independent cerebral vasoconstrictive effect of increased arterial Po2. The present results also demonstrate that the magnitude of this effect at 1.0 ATA is greater than previously measured. 相似文献