全文获取类型
收费全文 | 6735篇 |
免费 | 34篇 |
国内免费 | 21篇 |
专业分类
系统科学 | 60篇 |
丛书文集 | 18篇 |
教育与普及 | 4篇 |
理论与方法论 | 23篇 |
现状及发展 | 3325篇 |
研究方法 | 331篇 |
综合类 | 2930篇 |
自然研究 | 99篇 |
出版年
2017年 | 45篇 |
2016年 | 48篇 |
2013年 | 75篇 |
2012年 | 144篇 |
2011年 | 195篇 |
2010年 | 48篇 |
2008年 | 125篇 |
2007年 | 147篇 |
2006年 | 150篇 |
2005年 | 148篇 |
2004年 | 154篇 |
2003年 | 121篇 |
2002年 | 130篇 |
2001年 | 211篇 |
2000年 | 163篇 |
1999年 | 137篇 |
1992年 | 127篇 |
1991年 | 89篇 |
1990年 | 84篇 |
1989年 | 72篇 |
1988年 | 71篇 |
1987年 | 84篇 |
1986年 | 66篇 |
1985年 | 107篇 |
1984年 | 116篇 |
1983年 | 93篇 |
1982年 | 79篇 |
1981年 | 75篇 |
1980年 | 75篇 |
1979年 | 207篇 |
1978年 | 167篇 |
1977年 | 147篇 |
1976年 | 151篇 |
1975年 | 152篇 |
1974年 | 181篇 |
1973年 | 162篇 |
1972年 | 161篇 |
1971年 | 184篇 |
1970年 | 250篇 |
1969年 | 189篇 |
1968年 | 215篇 |
1967年 | 187篇 |
1966年 | 183篇 |
1965年 | 151篇 |
1964年 | 85篇 |
1959年 | 52篇 |
1958年 | 80篇 |
1957年 | 57篇 |
1956年 | 48篇 |
1954年 | 50篇 |
排序方式: 共有6790条查询结果,搜索用时 93 毫秒
21.
The induction of secondary seed dormancy by oxygen deficiency in a barnyard grassEchinochloa crus-galli 总被引:2,自引:0,他引:2
At 25°C, secondary dormancy was induced in seeds ofE. crus-galli exposed for 100 days to oxygen deficiency. By contrast, hypoxia did not induce dormancy at 15°C or prevent dormancy termination at 7°C. Secondary dormancy was terminated after 2 months stratification at 7°C. Oxygen deficiency may increase the proportion of dormant seeds in the soil, and affect the dynamics of the barnyard-grass soil seed bank.We thank Miss Jitka Králová for her excellent technical assistance. 相似文献
22.
Secular variation in carbon isotope ratios from Upper Proterozoic successions of Svalbard and East Greenland 总被引:12,自引:0,他引:12
Analyses of stratigraphically continuous suites of samples from Upper Proterozoic sedimentary successions of East Greenland, Spitsbergen and Nordaustlandet (Svalbard) provide an approximation to the secular variation in carbon isotope ratios during a geologically and biologically important period of change from around 900 million years ago to the beginning of the Cambrian period. Late Riphean carbonates and organic material show a stratigraphically useful pattern of enrichment in 13C relative to Phanerozoic or earlier Proterozoic samples. Isotopic compositions of isolated samples from other localities are consistent with a worldwide extended interval of enhanced organic burial and consequent net survival of oxidized material, probably O2, just before the initial radiation of metazoans. 相似文献
23.
6-Trichloromethyl-9-methylpurine (1) rearranges to 6-dichloromethyl-9-methyl-8-oxopurine (2) in aqueous mild acidic solution. The rearrangement is rationalized in terms of a reaction involving protonation, covalent hydration, prototropic equilibrium and/or a hydride transfer. An alternative mechanism involving a "positive' halogen compound and hypochlorous acid as an intermediary is also proposed. Compound 1 condenses with 4,5-diaminopyrimidine to give the purine-pyrimidine Schiff base pair 4. 相似文献
24.
Summary InPyrrhocoris apterus, a low temperature, 15°C, prevented the termination of diapause by long days and, unexpectedly, also the induction of diapause by short days. Both responses were enabled at a higher temperature, 26°C. In contrast to current concepts, it was proved that the summation of photoperiodic signals was temperature-dependent, since the morphogenetic development was prevented by starvation. 相似文献
25.
Pineal gland changes of rats exposed to heat 总被引:1,自引:0,他引:1
26.
I. Grattan-Guinness 《Archive for History of Exact Sciences》1970,6(5):372-400
Summary In this paper I discuss the development of mathematical analysis during the second and third decades of the nineteenth century; and in particular I assert that the well-known correspondence of new ideas to be found in the writings of Bolzano and Cauchy is not a coincidence, but that Cauchy had read one particular paper of Bolzano and drew on its results without acknowledgement. The reasons for this conjecture involve not only the texts in question but also the state of development of mathematical analysis itself, Cauchy both as personality and as mathematician, and the rivalries which were prevalent in Paris at that time. 相似文献
27.
28.
MDMethodinSolvingMultipleCriteriaDecisionProblemsArisingfromInvestmentSystemsDUWenzhong;CIJianhua(EconomicInformationCenterof... 相似文献
29.
H. Tsuchiya M. Sato M. Iinuma J. Yokoyama M. Ohyama T. Tanaka I. Takase I. Namikawa 《Cellular and molecular life sciences : CMLS》1994,50(9):846-849
Phytoalexins, defensive compounds produced by plants against microbial infections, were purified fromSophora exigua (Leguminosae) and their growth inhibitory effects on oral cariogenic bacteria were determined in vitro. Among three isolated compounds, 5,7,2,4-tetrahydroxy-8-lavandulylflavanone completely inhibited the growth of oral bacteria including primary cariogenic mutans streptococci, other oral streptococci, actinomycetes, and lactobacilli, at concentrations of 1.56 to 6.25 g/ml. 相似文献
30.
Highly active metabolites have been detected in the hemolymph of the lepidopteranSpodoptera exigua infected with the mycopathogen,Beauveria bassiana. A combination of phenyl sepharose and CM ion exchange chromatography was utilized to extract the active metabolites from infected hemolymph samples. The active in vivo metabolites, having a molecular mass greater than 10 KDa, were thermolabile and were inactivated by proteinase K. These metabolites were characterized by their ability to disrupt metamorphosis, killing treated larvae at the wandering or pupal stage. Additionally, injection ofS. exigua larvae with active samples caused a reduction in the number of filopodial-producing hemocytes. The biological activities and biochemical properties suggest that novel compounds are produced duringB. bassiana mycosis. 相似文献