全文获取类型
收费全文 | 20008篇 |
免费 | 40篇 |
国内免费 | 68篇 |
专业分类
系统科学 | 69篇 |
丛书文集 | 250篇 |
教育与普及 | 66篇 |
理论与方法论 | 72篇 |
现状及发展 | 8775篇 |
研究方法 | 877篇 |
综合类 | 9718篇 |
自然研究 | 289篇 |
出版年
2013年 | 123篇 |
2012年 | 276篇 |
2011年 | 555篇 |
2010年 | 119篇 |
2008年 | 316篇 |
2007年 | 386篇 |
2006年 | 359篇 |
2005年 | 401篇 |
2004年 | 370篇 |
2003年 | 345篇 |
2002年 | 367篇 |
2001年 | 646篇 |
2000年 | 636篇 |
1999年 | 432篇 |
1994年 | 298篇 |
1992年 | 399篇 |
1991年 | 329篇 |
1990年 | 354篇 |
1989年 | 327篇 |
1988年 | 322篇 |
1987年 | 327篇 |
1986年 | 352篇 |
1985年 | 466篇 |
1984年 | 328篇 |
1983年 | 288篇 |
1982年 | 221篇 |
1981年 | 240篇 |
1980年 | 283篇 |
1979年 | 672篇 |
1978年 | 505篇 |
1977年 | 500篇 |
1976年 | 398篇 |
1975年 | 397篇 |
1974年 | 521篇 |
1973年 | 439篇 |
1972年 | 483篇 |
1971年 | 538篇 |
1970年 | 700篇 |
1969年 | 560篇 |
1968年 | 564篇 |
1967年 | 515篇 |
1966年 | 492篇 |
1965年 | 351篇 |
1959年 | 186篇 |
1958年 | 327篇 |
1957年 | 244篇 |
1956年 | 216篇 |
1955年 | 176篇 |
1954年 | 188篇 |
1948年 | 152篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Zusammenfassung Die Acetylcholinesterase der Erythrozytenmembranen wurde mit 1, 5-Difluor-2, 4-dinitrobenzol stärker inaktiviert als mit 1-Fluor-2, 4-dinitrobenzol. Nach Difluordinitrobenzol-Behandlung war die restliche enzymatische Aktivität hitze- und harnstoffunempfindlich. Die Inaktivierung des Fermentes war konzentrations-, zeit- und temperaturabhängig. 相似文献
992.
993.
994.
Forces involved ithe conformational stability of nucleic acids 总被引:4,自引:0,他引:4
995.
996.
997.
lambda Repressor and cro--components of an efficient molecular switch 总被引:48,自引:0,他引:48
In a lysogen, most genes of phage lambda are repressed; in response to a transient induction signal, they are efficiently switched on. The switch, which consists in part of a tripartite operator to which two regulatory proteins bind, depends not only on DNA--protein interactions, but also on effects transmitted from one DNA-bound protein to another. lambda Exemplifies a strategy that facilitates efficient switching between two physiological states in response to a transient signal. 相似文献
998.
Apart from echolocation and the pursuit of prey by bats, the function of ultrasound in animal communication is poorly understood. This is mainly because of the broad range of responses that it can evoke and the widely varied contexts in which it is used (for example, in rodents of the Muridae family it may indicate distress in infants or a sexual or predatory encounter in adults). Here we find that a purely ultrasonic signal is produced in the wild by a rodent of the Sciuridae family, Richardson's ground squirrel, and show that its function is to warn conspecifics of impending danger. To our knowledge, ultrasonic alarm calls have not previously been detected in any animal group, despite their twin advantages of being highly directional and inaudible to key predators. 相似文献
999.
Although molecular data have revealed the vast scope of microbial diversity, two fundamental questions remain unanswered even for well-defined natural microbial communities: how many bacterial types co-exist, and are such types naturally organized into phylogenetically discrete units of potential ecological significance? It has been argued that without such information, the environmental function, population biology and biogeography of microorganisms cannot be rigorously explored. Here we address these questions by comprehensive sampling of two large 16S ribosomal RNA clone libraries from a coastal bacterioplankton community. We show that compensation for artefacts generated by common library construction techniques reveals fine-scale patterns of community composition. At least 516 ribotypes (unique rRNA sequences) were detected in the sample and, by statistical extrapolation, at least 1,633 co-existing ribotypes in the sampled population. More than 50% of the ribotypes fall into discrete clusters containing less than 1% sequence divergence. This pattern cannot be accounted for by interoperon variation, indicating a large predominance of closely related taxa in this community. We propose that such microdiverse clusters arise by selective sweeps and persist because competitive mechanisms are too weak to purge diversity from within them. 相似文献
1000.
Migratory animals capable of navigating to a specific destination, and of compensating for an artificial displacement into unfamiliar territory, are thought to have a compass for maintaining their direction of travel and a map sense that enables them to know their location relative to their destination. Compasses are based on environmental cues such as the stars, the Sun, skylight polarization and magnetism, but little is known about the sensory mechanism responsible for the map sense. Here we show that the green sea-turtle (Chelonia mydas) has a map that is at least partly based on geomagnetic cues. 相似文献