全文获取类型
收费全文 | 6321篇 |
免费 | 642篇 |
国内免费 | 2篇 |
出版年
2023年 | 21篇 |
2022年 | 60篇 |
2021年 | 138篇 |
2020年 | 88篇 |
2019年 | 109篇 |
2018年 | 96篇 |
2017年 | 99篇 |
2016年 | 194篇 |
2015年 | 291篇 |
2014年 | 315篇 |
2013年 | 354篇 |
2012年 | 516篇 |
2011年 | 533篇 |
2010年 | 331篇 |
2009年 | 239篇 |
2008年 | 400篇 |
2007年 | 398篇 |
2006年 | 312篇 |
2005年 | 343篇 |
2004年 | 318篇 |
2003年 | 282篇 |
2002年 | 288篇 |
2001年 | 75篇 |
2000年 | 80篇 |
1999年 | 90篇 |
1998年 | 76篇 |
1997年 | 53篇 |
1996年 | 59篇 |
1995年 | 45篇 |
1994年 | 38篇 |
1993年 | 31篇 |
1992年 | 37篇 |
1991年 | 43篇 |
1990年 | 43篇 |
1989年 | 42篇 |
1988年 | 25篇 |
1987年 | 27篇 |
1986年 | 30篇 |
1984年 | 28篇 |
1983年 | 30篇 |
1982年 | 22篇 |
1981年 | 23篇 |
1980年 | 22篇 |
1979年 | 22篇 |
1978年 | 19篇 |
1976年 | 18篇 |
1974年 | 27篇 |
1973年 | 18篇 |
1971年 | 19篇 |
1969年 | 23篇 |
排序方式: 共有6965条查询结果,搜索用时 31 毫秒
51.
Many sperm flagella contain a central pair of singlet tubules surrounded by nine doublet tubules. Insect sperm flagella, however, usually contain in addition to the typical 9+2 arrangement of tubules, an additional row of nine singlet tubules which are called accessory tubules. Each of these accessory tubules in the Colorado potato beetle (Leptinotarsa decemlineata (Say)) contains a single central prototubule surrounded by an array of 15 or 16 peripheral prototubules. The number of prototubules in the accessory tubules of Leptinotarsa decemlineata was determined with an electron microscope and image details were reinforced by the rotation techniques of Markham. 相似文献
52.
N. A. Simmons 《BMJ (Clinical research ed.)》1969,3(5672):692.2-696
Potentiation of colistin by sulphamethoxazole and sulphamethizole was demonstrated with 19 out of 20 strains of Pseudomonas aeruginosa. This enhancement was bactericidal as well as bacteriostatic. Synergy between trimethoprim and sulphamethoxazole was also demonstrated with four strains of Ps. aeruginosa, but even when the two drugs were combined high concentrations of trimethoprim were still required to produce a bactericidal effect. Combinations of sulphamethoxazole and gentamicin appeared to be synergistic when the bacteriostatic effect was measured, but the combined bactericidal effect was indifference. The bactericidal and bacteriostatic effect of combinations of carbenicillin with sulphamethoxazole was also indifference. 相似文献
53.
54.
55.
Physical Assay and Growth Cycle Studies of a Defective Adeno-Satellite Virus 总被引:29,自引:19,他引:10
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Wade P. Parks Joseph L. Melnick Robert Rongey Heather D. Mayor 《Journal of virology》1967,1(1):171-180
Electron microscopic particle counting of the defective adeno-satellite virus (ASV), by use of pseudoreplication and negative staining with phosphotungstic acid, was shown to be a reproducible quantitative assay procedure. Particles of satellite type 4 that were counted in fluids from infected cultures had the same morphology as particles that banded at a buoyant density of 1.43 g/cc in cesium chloride. Other satellite virus serotypes examined in the same manner had a buoyant density of 1.37 to 1.38 g/cc. A comparison of satellite titers obtained by complement fixation and by particle counting demonstrated that an increase in satellite particles resulted in a corresponding increase in CF titers; however, electron microscopy was at least 10 times more sensitive than complement fixation for detecting satellite virus. Growth cycle studies of satellite virus in cells co-infected with adenovirus, as assayed by particle counting, indicated that the kinetics of satellite virus production closely followed the kinetics of its helper adenovirus production, with an eclipse period of 12 to 16 hr. The eclipse period of the satellite remained the same when cultures were preinfected with satellite 24 hr prior to adenovirus inoculation. However, when cultures were infected with adenovirus 12 hr before satellite virus, the eclipse period of the satellite was shortened to between 4 and 6 hr. Thus, satellite virus replication seems dependent upon a relatively late event in the adenovirus replication cycle. When cells were co-infected with adenovirus and its defective satellite, the yield of adenovirus was markedly reduced from that obtained in cells singly infected with adenovirus. 相似文献
56.
The immunochemistry of Salmonella chemotype VI O-antigens. The structure of oligosaccharides from Salmonella group U (o 43) lipopolysaccharides 总被引:8,自引:2,他引:6
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
1. A series of oligosaccharides was isolated from Salmonella milwaukee lipopolysaccharide by partial acid hydrolysis. 2. Structural studies on these oligosaccharides indicated that the O-specific side chain of this lipopolysaccharide has a repeating pentasaccharide unit that is probably alpha-d-galactosyl-(1-->3)-beta-d-galactosyl- (1-->3)-N-acetylgalactosaminyl-(1-->3)-N-acetyl- d-glucosaminyl-(1-->4)-l-fucose. 3. Another oligosaccharide, which is not structurally related to the repeating pentasaccharide unit, has also been isolated and it is indistinguishable from an oligosaccharide obtained from Salmonella ;rough' (R) lipopolysaccharides. The isolation of this and similar core oligosaccharides from all chemotype VI lipopolysaccharides supports the view that Salmonella S-lipopolysaccharides have a common core that is probably identical with RII lipopolysaccharide. 相似文献
57.
Heather J. Lawlor Jen A. McComb Michael A. Borowitzka 《Journal of applied phycology》1989,1(2):105-112
The effects of light quality and irradiance, and supply of organic carbon and vitamins on the growth of two forms of Ecklonia radiata in tissue culture were examined. A callus of unpigmented cells developed over the cut surface of newly excised explants of
stipe. This growth was best in the dark but stopped after 10 weeks. Pigmented, mainly filamentous clumps of cells developed
from explants after several weeks in culture. These required light for growth, with growth being enhanced by increasing photon
flux density up to 30 μmol photon m-2 s-1, with the active spectral component being red light (> 600 nm). The addition to the medium of a range of organic carbon sources
or vitamins did not stimulate growth of either culture type in the dark.
author for correspondence 相似文献
58.
Noctuidonema guyaneme is an interesting ectoparasite of adult Lepidoptera that feeds on hosts from at least five families with its long stylet. Noctuidonema guyanense spends its entire life on the adult moth and is sustained as it is passed from moth to moth during host mating. Overlapping host generations are essential for parasite survival. This nematode occurs throughout tropical and subtropical America and is transported by at least one of its hosts, Spodoptera frugiperda, during migration to northern sites in the United States each spring. Noctuidonema guyanense debilitates its hosts. Research conducted to help determine the biological control importance of this nematode is reviewed. Two additional species, N. daptria and N. dibolia, are now known for Noctuidonema. 相似文献
59.
C. A. Hainfeld S. L. Boatright-Horowitz S. S. Boatright-Horowitz A. Megela Simmons 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1996,179(1):75-87
- Male bullfrogs at two different natural calling sites were presented with playbacks of synthetic advertisement calls differing in phase spectra. Sounds were presented in a ABA design to analyze the ability of the animals to perceive changes in repeated series of stimuli.
- The number of individual croaks in an answering call significantly increased over repeated presentations of two of the three stimulus phase types in condition A1. There were significantly fewer croaks to the third stimulus. These data suggest that two stimuli were perceived in a similar manner.
- Latency of calling to stimuli presented in conditions A and B changed in response to shifts in phase spectrum at a low density calling site. These differences were significant when comparing latency to playbacks where shifts in the phase spectrum changed the temporal fine-structure and waveform periodicity of the stimulus.
- The increase in number of croaks and decrease in response latency across condition A1 and the increase in latency in condition B suggest that discrimination may take the form of stimulus-specific sensitization. In this context, sensitization might reflect an increase in arousal due to repeated presentation of a salient stimulus.
- The operation of a hypothetical ‘mating call detector,’ based on linear summation of temporal responses from the eighth nerve, provides output similar to the behavioral results.
60.
James K. Friel Claude Mercer Wayne L. Andrews Brian R. Simmons Simon E. Jackson 《Biological trace element research》1996,54(2):135-142
Contamination in a trace element laboratory can come from a variety of sources, including laboratory gloves. Therefore, vinyl
and latex gloves were obtained from as many manufacturers as would supply gloves. These gloves were either prepared for acid-washing
and subsequent soaking in an acid solution, or immersed in an acid solution for a duration of either 1 min or 1 h. Incubation
washes were analyzed for a variety of trace elements by flame atomic abosrption spectroscopy (AAS) or inductively coupled
mass spectrometry (ICP-MS). Results indicated that only three brands of vinyl gloves were acceptable for use in a trace element
laboratory, whereas others had contamination of different elements. Latex gloves contained such high levels of biologically
important elements that they were not considered suitable for routine trace element work. Vinyl gloves of choice should be
routinely acid-washed before use in a trace element laboratory. 相似文献