全文获取类型
收费全文 | 13599篇 |
免费 | 998篇 |
国内免费 | 8篇 |
出版年
2021年 | 133篇 |
2020年 | 104篇 |
2019年 | 133篇 |
2018年 | 169篇 |
2017年 | 178篇 |
2016年 | 239篇 |
2015年 | 399篇 |
2014年 | 413篇 |
2013年 | 602篇 |
2012年 | 747篇 |
2011年 | 663篇 |
2010年 | 516篇 |
2009年 | 431篇 |
2008年 | 616篇 |
2007年 | 603篇 |
2006年 | 601篇 |
2005年 | 604篇 |
2004年 | 606篇 |
2003年 | 622篇 |
2002年 | 583篇 |
2001年 | 198篇 |
2000年 | 157篇 |
1999年 | 168篇 |
1998年 | 194篇 |
1997年 | 176篇 |
1996年 | 165篇 |
1995年 | 167篇 |
1994年 | 139篇 |
1993年 | 176篇 |
1992年 | 136篇 |
1991年 | 161篇 |
1990年 | 149篇 |
1989年 | 128篇 |
1988年 | 137篇 |
1987年 | 120篇 |
1986年 | 109篇 |
1985年 | 125篇 |
1984年 | 183篇 |
1983年 | 152篇 |
1982年 | 146篇 |
1981年 | 161篇 |
1980年 | 134篇 |
1979年 | 119篇 |
1978年 | 141篇 |
1977年 | 109篇 |
1976年 | 94篇 |
1975年 | 103篇 |
1974年 | 90篇 |
1973年 | 79篇 |
1970年 | 61篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Jan Batelka Jakub Prokop Hans Pohl Ming Bai Weiwei Zhang Rolf G. Beutel 《Systematic Entomology》2019,44(2):396-407
Extremely miniaturized longipedes insects (body length c. 0.3 mm) embedded in two pieces of Cretaceous amber from Myanmar are described and interpreted. Using inverted fluorescence and light microscopy for detailed analysis of microstructures, the inclusions were identified as primary larvae of the beetle family Ripiphoridae, subfamily Ripidiinae. While the structure of thoracic and abdominal segments including appendages corresponds well with the groundplan known in recent members of Ripidiinae, a curved prosternal ridge with prominent spines (each c. 5 μm), the reduced condition of stemmata and antennae and the lack of sharp mandibles are unique features within the entire family, apparently apomorphies of the longipedes larvae. A sinuate prosternal edge with a dense row of spines (prosternoctenidium) might be homologous with ‘head ctenidia’ in some previously described miniaturized conicocephalate larvae, but further investigation is needed. The morphological differences between the head of longipedes larvae and extant Ripidiinae are interpreted as adaptations to different groups of hosts and life strategies. Palaeoethology of the longipedes larvae is briefly discussed. In addition, the systematic placement of conicocephalate larvae from Canadian, Myanmar and Russian Cretaceous ambers, already interpreted by various authors as primary instars within Coleopterida (assigned to either Strepsiptera or to the coleopteran Tenebrionoidea: Ripiphoridae), is discussed. 相似文献
12.
Martin T. R. Kuiper Marijke Holtrop Hans de Vries 《Molecular & general genetics : MGG》1988,213(2-3):519-528
13.
14.
Various laboratory techniques have been utilized worldwide for measuring lead-210 in sub-recent deposits through its grand-daughter product polonium-210. Isotope dilution alpha spectrometry proved a suitable tool for absolute determination of lead-210 for the dating of aquatic deposits. Moreover, isotope dilution alpha spectrometry along with speciation experiments can be used to resolve depositional anomalies arising from supported lead-210/Ra-226 disequilibrium levels and unsupported lead-210 mobile fractions. Isotope dilution alpha spectrometry of sub-recent sediment and peat deposits has been critically evaluated for more than ten years. Our results show that type, size and composition of deposits analyzed as well as radiochemical procedures used, together with alpha counting techniques, are important factors influencing lead-210 determinations and tailing corrections using its granddaughter product polonium-210. Optimization of these parameters is of prime importance to achieve economic and accurate analyses, especially at low lead-210 concentrations and small sample sizes. 相似文献
15.
16.
17.
How often will elections end in landslides? What is the probability for a head-to-head race? Analyzing ballot results from several large countries rather anomalous and yet unexplained distributions have been observed. We identify tactical voting as the driving ingredient for the anomalies and introduce a model to study its effect on plurality elections, characterized by the relative strength of the feedback from polls and the pairwise interaction between individuals in the society. With this model it becomes possible to explain the polarization of votes between two candidates, understand the small margin of victories frequently observed for different elections, and analyze the polls'' impact in American, Canadian, and Brazilian ballots. Moreover, the model reproduces, quantitatively, the distribution of votes obtained in the Brazilian mayor elections with two, three, and four candidates. 相似文献
18.
The emission maximum of the single tryptophan residue of melittin was measured in the presence of phosphatidylethanolamine liposomes and Escherichia coli cytoplasmic membranes. In both cases, the fluorescence maximum was shifted to shorter wavelengths indicating a transfer of the indole ring to an apolar environment. E. coli membranes were labelled in position 2 of their phospholipids with [14C]oleic acid. These membranes were used for measuring the activity of an endogenous phospholipase A2. A slow hydrolysis is observed, which can be accelerated by adding melittin. The extent of the stimulation depends on the molar ratio of melittin to membrane phospholipid. Under suitable conditions, the initial rate of hydrolysis is six to seven times higher in the presence than in the absence of melittin. The action of the phospholipase A2 from bee venom is also stimulated by melittin. An identical stimulation was observed with either E. coli membranes or pure phosphatidylethanolamine liposomes as substrate. 相似文献
19.
Siegfried Huneck Klaus Schreiber Hans Dieter Grimmecke 《Journal of Plant Growth Regulation》1984,3(1-3):75-84
Seventy-two Schiffs bases, 44 corresponding secondary amines, and 12 N-acetylated compounds were tested on their growth activity. Eighty-one compounds were active as growth inhibitors in at least one of three bioassays. 相似文献
20.