全文获取类型
收费全文 | 13453篇 |
免费 | 1269篇 |
国内免费 | 5篇 |
专业分类
14727篇 |
出版年
2023年 | 52篇 |
2022年 | 123篇 |
2021年 | 207篇 |
2020年 | 138篇 |
2019年 | 170篇 |
2018年 | 227篇 |
2017年 | 192篇 |
2016年 | 366篇 |
2015年 | 545篇 |
2014年 | 638篇 |
2013年 | 760篇 |
2012年 | 980篇 |
2011年 | 969篇 |
2010年 | 611篇 |
2009年 | 556篇 |
2008年 | 786篇 |
2007年 | 798篇 |
2006年 | 719篇 |
2005年 | 716篇 |
2004年 | 719篇 |
2003年 | 658篇 |
2002年 | 637篇 |
2001年 | 177篇 |
2000年 | 150篇 |
1999年 | 180篇 |
1998年 | 220篇 |
1997年 | 143篇 |
1996年 | 120篇 |
1995年 | 118篇 |
1994年 | 123篇 |
1993年 | 117篇 |
1992年 | 115篇 |
1991年 | 104篇 |
1990年 | 87篇 |
1989年 | 96篇 |
1988年 | 83篇 |
1987年 | 75篇 |
1986年 | 78篇 |
1985年 | 84篇 |
1984年 | 59篇 |
1983年 | 77篇 |
1982年 | 92篇 |
1981年 | 62篇 |
1980年 | 53篇 |
1979年 | 66篇 |
1978年 | 64篇 |
1976年 | 45篇 |
1975年 | 45篇 |
1974年 | 52篇 |
1973年 | 49篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
Marianne E. Walsh Charles M. Collins Thomas F. Jenkins Alan D. Hewitt Jeff Stark Karen Myers 《Soil & Sediment Contamination》2003,12(5):631-645
Fort Greely, Alaska has an extensive complex of weapon training and testing areas located on lands withdrawn from the public domain under the Military Lands Withdrawal Act (PL106-65). The Army has pledged to implement a program to identify possible munitions contamination. Because of the large size (344,165,000 m2) of the high hazard impact areas, characterization of these constituents will be difficult. We used an authoritative sampling design to find locations most likely to contain explosives-residues on three impact areas. We focused our sampling on surface soils and collected multi-increment and discrete samples at locations of known firing events and from areas on the range that had craters, pieces of munitions, targets, or a designation as a firing point. In the two impact areas used primarily by the Army, RDX was the most frequently detected explosive. In the impact area that was also used by the Air Force, TNT was the most frequently detected explosive. Where detected, the explosives concentrations generally were low (<0.05 mg/kg) except in soils near low-order detonations, where the explosive-filler was in contact with the soil surface. These low-order detonations potentially can serve as localized sources for groundwater contamination if positioned in recharge areas. 相似文献
3.
4.
5.
Sign language studies of cross-fostered chimpanzees measure the effect of special rearing conditions on the development of
very young chimpanzees. Cross-fostered chimpanzees, like human children, develop gradually in a process that takes many years.
Here we discuss details of the procedure, the overlap between human and chimpanzee infants in the contents of the first 50-item
vocabularies, and the ways in which the signs of the chimpanzees exhibit the fuzziness of natural language categories. We
also compare the cross-fostering approach with more traditional modular approaches to the study of language-like behavior
in nonhuman animals.
Project Washoe was originally supported by grants MH-12154 from the National Institute of Mental Health and GB-7432 from the
National Science Foundation. We gratefully acknowledge this support and the support that later sign language studies of chimpanzees
have received since then from NIH, NSF, the National Geographic Society, the Grant Foundation, the Spencer Foundation, the
University of Nevada, and the UNR Foundation. 相似文献
6.
Use of Metabolic Inhibitors to Estimate Protozooplankton Grazing and Bacterial Production in a Monomictic Eutrophic Lake with an Anaerobic Hypolimnion 总被引:15,自引:8,他引:7 下载免费PDF全文
Inhibitors of eucaryotes (cycloheximide and amphotericin B) and procaryotes (penicillin and chloramphenicol) were used to estimate bacterivory and bacterial production in a eutrophic lake. Bacterial production appeared to be slightly greater than protozoan grazing in the aerobic waters of Lake Oglethorpe. Use of penicillin and cycloheximide yielded inconsistent results in anaerobic water and in aerobic water when bacterial production was low. Production measured by inhibiting eucaryotes with cycloheximide did not always agree with [3H]thymidine estimates or differential filtration methods. Laboratory experiments showed that several common freshwater protozoans continued to swim and ingest bacterium-size latex beads in the presence of the eucaryote inhibitor. Penicillin also affected grazing rates of some ciliates. We recommend that caution and a corroborating method be used when estimating ecologically important parameters with specific inhibitors. 相似文献
7.
A method for identifying cysteine-containing peptides in proteins is presented using 2-bromoacetamido-4-nitrophenol (BNP) to introduce an easily detectable probe. The formation of a covalent bond between the protein sulfhydryl group and the acetamido moiety of BNP introduces a chromophore with an absorbance maximum at 410 nm. The modified protein can then be cleaved with appropriate proteases and the resulting peptides separated by chromatographic methods. Monitoring the effluent at a single wavelength (405 nm) provides a rapid and simple method of detecting and isolating only those peptides which contain cysteine residue(s). The nitrophenol derivative is stable under conditions required for protease cleavage. The reagent is therefore useful for locating cysteine-containing peptides in protein digests and can be used to explore the accessibility of different cysteines under a variety of conditions. The ease of modification, specificity of reaction, product stability, and simple detection of modified peptides make BNP ideal for investigation of cysteine residues. 相似文献
8.
The substantia nigra plays a pivotal role in the relay of output from the striatum. One neural pathway from substantia nigra projects GABAergic fibers to the caudal mesencephalic tegmentum, terminating in the vicinity of the pedunculopontine nucleus (PPN). To evaluate the functional importance of this projection in the mediation of stereotyped behaviors of striatal and nigral origin, we microinjected low doses of the GABA agonist, muscimol, bilaterally into the vicinity of the PPN. This muscimol treatment resulted in a total blockade of all stereotyped behaviors normally elicited by systemic apomorphine or by intranigral muscimol. Blockade was not observed in animals microinjected with muscimol into the dorsal reticular formation, 1 mm above the level of the PPN. Our results indicate that the nigrotegmental projection may play a crucial role in the expression of stereotyped and dyskinetic behaviors of basal ganglia origin. 相似文献
9.
10.
A neonate with aniridia was found to have a one band paracentric inversion of the short arm of chromosome 4. This was initially difficult to interpret on high resolution banding. The inversion was present in three generations of the family. 相似文献