全文获取类型
收费全文 | 11855篇 |
免费 | 24篇 |
国内免费 | 42篇 |
专业分类
系统科学 | 35篇 |
丛书文集 | 80篇 |
教育与普及 | 30篇 |
理论与方法论 | 42篇 |
现状及发展 | 5124篇 |
研究方法 | 565篇 |
综合类 | 5889篇 |
自然研究 | 156篇 |
出版年
2013年 | 96篇 |
2012年 | 191篇 |
2011年 | 330篇 |
2010年 | 85篇 |
2009年 | 59篇 |
2008年 | 191篇 |
2007年 | 222篇 |
2006年 | 204篇 |
2005年 | 225篇 |
2004年 | 208篇 |
2003年 | 208篇 |
2002年 | 198篇 |
2001年 | 379篇 |
2000年 | 364篇 |
1999年 | 263篇 |
1992年 | 232篇 |
1991年 | 180篇 |
1990年 | 204篇 |
1989年 | 190篇 |
1988年 | 201篇 |
1987年 | 205篇 |
1986年 | 160篇 |
1985年 | 248篇 |
1984年 | 170篇 |
1983年 | 149篇 |
1982年 | 166篇 |
1981年 | 135篇 |
1980年 | 170篇 |
1979年 | 385篇 |
1978年 | 291篇 |
1977年 | 287篇 |
1976年 | 249篇 |
1975年 | 287篇 |
1974年 | 309篇 |
1973年 | 308篇 |
1972年 | 344篇 |
1971年 | 344篇 |
1970年 | 428篇 |
1969年 | 361篇 |
1968年 | 381篇 |
1967年 | 351篇 |
1966年 | 319篇 |
1965年 | 203篇 |
1959年 | 107篇 |
1958年 | 204篇 |
1957年 | 137篇 |
1956年 | 122篇 |
1955年 | 105篇 |
1954年 | 81篇 |
1948年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
922.
923.
924.
925.
Michael A. B. Deakin 《Archive for History of Exact Sciences》1985,33(4):307-319
Euler invented integral transforms in the context of second order differential equations. He used them in a fragment published in 1763 and in a chapter of Institutiones Calculi Integralis (1769). In introducing them he made use of earlier work in which a concept akin to the integral transform is implicit. It would, however, be reading too much into that earlier work to see it as contributing to the theory of the integral transform. Other work sometimes cited in this context in fact has different concerns. 相似文献
926.
Summary A communication signal in an African freshwater electric fish, its pulse-like Electric Organ Discharge (EOD), was investigated in order to determine whether inter- or intraindividual variability of the EOD waveform provides a putative cue for communication, in addition to the electric cues already identified. In contrast to an individual's highly stereotyped EODs showing extremely low variation (fig. 1), variability between individuals was considerable (fig. 2). The dependence of an individual's EOD duration on temperature was weak with a Q10 of close to 1.5. In none of four quantitative EOD waveform measures can a sexual dimorphism be discerned (table).Gnathonemus petersii very likely relies on mechanisms other than discrimination of intraspecific EOD waveforms for mate recognition. 相似文献
927.
928.
Production of 'hybrid' antibiotics by genetic engineering 总被引:30,自引:0,他引:30
D A Hopwood F Malpartida H M Kieser H Ikeda J Duncan I Fujii B A Rudd H G Floss S Omura 《Nature》1985,314(6012):642-644
The recent development of molecular cloning systems in Streptomyces has made possible the isolation of biosynthetic genes for some of the many antibiotics produced by members of this important genus of bacteria. Such clones can now be used to test the idea that novel antibiotics could arise through the transfer of biosynthetic genes between streptomycetes producing different antibiotics. The likelihood of a 'hybrid' compound being produced must depend on the substrate specificities of the biosynthetic enzymes, about which little is known. In attempts to demonstrate hybrid antibiotic production, we therefore began with strains producing different members of the same chemical class of compounds in order to maximize the chance of success. Here we report the production of novel compounds by gene transfer between strains producing the isochromanequinone antibiotics actinorhodin, granaticin and medermycin. These experiments were made possible by the recent cloning of the whole set of genes for the biosynthetic pathway of actinorhodin from Streptomyces coelicolor A3(2) (ref. 8). We believe that this represents the first report of the production of hybrid antibiotics by genetic engineering. 相似文献
929.
930.
Autophagy and lysosomal proteolysis in the liver 总被引:1,自引:0,他引:1
B Grinde 《Experientia》1985,41(9):1089-1095
Autophagy is defined as any process whereby cellular macromolecules destined for degradation gain access to the lysosomes. A review is presented on the physiological significance, mechanisms and regulation of autophagy in hepatocytes, concentrating on the issue of regulation. The article ends by discussing techniques available for future research. 相似文献