全文获取类型
收费全文 | 27876篇 |
免费 | 97篇 |
国内免费 | 118篇 |
专业分类
系统科学 | 127篇 |
丛书文集 | 451篇 |
教育与普及 | 37篇 |
理论与方法论 | 97篇 |
现状及发展 | 12173篇 |
研究方法 | 1292篇 |
综合类 | 13442篇 |
自然研究 | 472篇 |
出版年
2013年 | 222篇 |
2012年 | 449篇 |
2011年 | 926篇 |
2010年 | 185篇 |
2008年 | 511篇 |
2007年 | 624篇 |
2006年 | 553篇 |
2005年 | 591篇 |
2004年 | 633篇 |
2003年 | 511篇 |
2002年 | 540篇 |
2001年 | 866篇 |
2000年 | 825篇 |
1999年 | 589篇 |
1992年 | 557篇 |
1991年 | 388篇 |
1990年 | 422篇 |
1989年 | 407篇 |
1988年 | 405篇 |
1987年 | 414篇 |
1986年 | 454篇 |
1985年 | 580篇 |
1984年 | 403篇 |
1983年 | 335篇 |
1982年 | 295篇 |
1981年 | 320篇 |
1980年 | 370篇 |
1979年 | 869篇 |
1978年 | 679篇 |
1977年 | 654篇 |
1976年 | 522篇 |
1975年 | 516篇 |
1974年 | 771篇 |
1973年 | 640篇 |
1972年 | 708篇 |
1971年 | 763篇 |
1970年 | 1071篇 |
1969年 | 842篇 |
1968年 | 723篇 |
1967年 | 748篇 |
1966年 | 712篇 |
1965年 | 529篇 |
1964年 | 180篇 |
1959年 | 267篇 |
1958年 | 525篇 |
1957年 | 361篇 |
1956年 | 279篇 |
1955年 | 255篇 |
1954年 | 297篇 |
1948年 | 186篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
231.
232.
Luiz A. Rocha Joseph D. DiBattista Tane H. Sinclair-Taylor Michael L. Berumen 《Journal of Natural History》2020,54(15-16):1019-1023
ABSTRACT Here we document three cases of mimicry in coral reef fishes not previously reported in the literature involving two groupers (Epinephelus leucogrammicus and Plectropomus marisrubri) and a soapfish (Diploprion drachi) as mimics, and two wrasses (Larabicus quadrilineatus and Cheilinus quinquecinctus) and a blenny (Meiacanthus nigrolineatus) as models. All three cases are of aggressive mimicry, with a predatory species mimicking a harmless one, and in one of the cases, the mimicry is also Müllerian, where both the predator and harmless species are unpalatable. 相似文献
233.
On-chip natural assembly of silicon photonic bandgap crystals. 总被引:20,自引:0,他引:20
Photonic bandgap crystals can reflect light for any direction of propagation in specific wavelength ranges. This property, which can be used to confine, manipulate and guide photons, should allow the creation of all-optical integrated circuits. To achieve this goal, conventional semiconductor nanofabrication techniques have been adapted to make photonic crystals. A potentially simpler and cheaper approach for creating three-dimensional periodic structures is the natural assembly of colloidal microspheres. However, this approach yields irregular, polycrystalline photonic crystals that are difficult to incorporate into a device. More importantly, it leads to many structural defects that can destroy the photonic bandgap. Here we show that by assembling a thin layer of colloidal spheres on a silicon substrate, we can obtain planar, single-crystalline silicon photonic crystals that have defect densities sufficiently low that the bandgap survives. As expected from theory, we observe unity reflectance in two crystalline directions of our photonic crystals around a wavelength of 1.3 micrometres. We also show that additional fabrication steps, intentional doping and patterning, can be performed, so demonstrating the potential for specific device applications. 相似文献
234.
Water capture by a desert beetle. 总被引:15,自引:0,他引:15
Some beetles in the Namib Desert collect drinking water from fog-laden wind on their backs. We show here that these large droplets form by virtue of the insect's bumpy surface, which consists of alternating hydrophobic, wax-coated and hydrophilic, non-waxy regions. The design of this fog-collecting structure can be reproduced cheaply on a commercial scale and may find application in water-trapping tent and building coverings, for example, or in water condensers and engines. 相似文献
235.
236.
237.
238.
Neuronal connectivity is fundamental to information processing in the brain. Therefore, understanding the mechanisms of sensory processing requires uncovering how connection patterns between neurons relate to their function. On a coarse scale, long-range projections can preferentially link cortical regions with similar responses to sensory stimuli. But on the local scale, where dendrites and axons overlap substantially, the functional specificity of connections remains unknown. Here we determine synaptic connectivity between nearby layer 2/3 pyramidal neurons in vitro, the response properties of which were first characterized in mouse visual cortex in vivo. We found that connection probability was related to the similarity of visually driven neuronal activity. Neurons with the same preference for oriented stimuli connected at twice the rate of neurons with orthogonal orientation preferences. Neurons responding similarly to naturalistic stimuli formed connections at much higher rates than those with uncorrelated responses. Bidirectional synaptic connections were found more frequently between neuronal pairs with strongly correlated visual responses. Our results reveal the degree of functional specificity of local synaptic connections in the visual cortex, and point to the existence of fine-scale subnetworks dedicated to processing related sensory information. 相似文献
239.
Rasmussen SG DeVree BT Zou Y Kruse AC Chung KY Kobilka TS Thian FS Chae PS Pardon E Calinski D Mathiesen JM Shah ST Lyons JA Caffrey M Gellman SH Steyaert J Skiniotis G Weis WI Sunahara RK Kobilka BK 《Nature》2011,477(7366):549-555
G protein-coupled receptors (GPCRs) are responsible for the majority of cellular responses to hormones and neurotransmitters as well as the senses of sight, olfaction and taste. The paradigm of GPCR signalling is the activation of a heterotrimeric GTP binding protein (G protein) by an agonist-occupied receptor. The β(2) adrenergic receptor (β(2)AR) activation of Gs, the stimulatory G protein for adenylyl cyclase, has long been a model system for GPCR signalling. Here we present the crystal structure of the active state ternary complex composed of agonist-occupied monomeric β(2)AR and nucleotide-free Gs heterotrimer. The principal interactions between the β(2)AR and Gs involve the amino- and carboxy-terminal α-helices of Gs, with conformational changes propagating to the nucleotide-binding pocket. The largest conformational changes in the β(2)AR include a 14 ? outward movement at the cytoplasmic end of transmembrane segment 6 (TM6) and an α-helical extension of the cytoplasmic end of TM5. The most surprising observation is a major displacement of the α-helical domain of Gαs relative to the Ras-like GTPase domain. This crystal structure represents the first high-resolution view of transmembrane signalling by a GPCR. 相似文献
240.