全文获取类型
收费全文 | 4074篇 |
免费 | 231篇 |
国内免费 | 610篇 |
出版年
2023年 | 68篇 |
2022年 | 84篇 |
2021年 | 114篇 |
2020年 | 145篇 |
2019年 | 147篇 |
2018年 | 164篇 |
2017年 | 156篇 |
2016年 | 150篇 |
2015年 | 141篇 |
2014年 | 230篇 |
2013年 | 229篇 |
2012年 | 172篇 |
2011年 | 210篇 |
2010年 | 161篇 |
2009年 | 210篇 |
2008年 | 254篇 |
2007年 | 248篇 |
2006年 | 246篇 |
2005年 | 228篇 |
2004年 | 164篇 |
2003年 | 121篇 |
2002年 | 168篇 |
2001年 | 112篇 |
2000年 | 92篇 |
1999年 | 101篇 |
1998年 | 83篇 |
1997年 | 82篇 |
1996年 | 64篇 |
1995年 | 66篇 |
1994年 | 81篇 |
1993年 | 58篇 |
1992年 | 56篇 |
1991年 | 35篇 |
1990年 | 39篇 |
1989年 | 29篇 |
1988年 | 30篇 |
1987年 | 15篇 |
1986年 | 24篇 |
1985年 | 12篇 |
1984年 | 19篇 |
1983年 | 10篇 |
1982年 | 29篇 |
1981年 | 10篇 |
1980年 | 11篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 9篇 |
1976年 | 3篇 |
1974年 | 4篇 |
1973年 | 4篇 |
排序方式: 共有4915条查询结果,搜索用时 15 毫秒
1.
K. Ferrazzoli Devienne M.S. Gonalves Raddi R. Gomes Coelho W. Vilegas 《Phytomedicine》2005,12(5):378-381
Three naturally occurring isocoumarins (paepalantine, paepalantine 9-O-beta-D-glucopyranoside and paepalantine 9-O-beta-D-allopyranosyl(1 --> 6) glucopyranoside) and two semi-synthetic analogues, 9,10-acylated compound and 9-OH-10-methylated compound, structurally similar to paepalantine, were evaluated for antimicrobial activity using a spectrophotometric microdilution technique. The paepalantine was active against S. aureus, S. epidermidis, and E. faecalis while the other four compounds proved ineffective against all microorganisms tested at concentrations of 500 microg/ml. Variations in phenolic substitution at OH-9 and/or OH-10 in the paepalantine molecule resulted in compounds without antimicrobial activity. A combination of structural features, two phenolic groups as cathecolic system, forms an oxygenated system arrangement that may reflect the potentially antimicrobial properties of paepalantine. 相似文献
2.
N. Jepsen 《Journal of fish biology》2003,63(1):260-262
Radio-tagged pikeperch Stizostedion lucioperca (55–74 cm L T ) were recaptured in a reservoir by anglers 52–55 months after tagging. A total of four fish were recaptured during the summer of 2001. These recaptures are remarkable because of the long period between tagging and recapture and because only one (of 15 potential) tagged pikeperch had been caught in the long period since tagging. 相似文献
3.
Chafik Maazouzi Gérard Masson Maria Soledad Izquierdo Jean-Claude Pihan 《Journal of thermal biology》2008
The effects of the 2003 European heat wave on a freshwater plankton assemblage and its fatty acid (FA) composition were investigated. Composition and FA profiles of four size categories of planktonic organisms collected in 2003 were compared to those of the colder year 2002. 相似文献
4.
The ability of a number of nitrogen-containing compounds that simultaneously carry the adamantane and monoterpene moieties to inhibit Tdp1, an important enzyme of the DNA repair system, is studied. Inhibition of this enzyme has the potential to overcome chemotherapeutic resistance of some tumor types. Compound (+)-3c synthesized from 1-aminoadamantane and (+)-myrtenal, and compound 4a produced from 2-aminoadamantane and citronellal were found to be most potent as they inhibited Tdp1 with IC50 values of 6 and 3.5 µM, respectively. These compounds proved to have low cytotoxicity in colon HCT-116 and lung A-549 human tumor cell lines (CC50 > 50 µM). It was demonstrated that compound 4a at 10 µM enhanced cytotoxicity of topotecan, a topoisomerase 1 poison in clinical use, against HCT-116 more than fivefold and to a lesser extent of 1.5 increase in potency for A-549. 相似文献
5.
多种天敌控制多种害虫的模糊数学模型 总被引:2,自引:0,他引:2
从生态经济学的原理出发,利用模糊集理论,探讨如何评价天敌控制多种害虫的为害作用,提出了多种天敌控制多种害虫的模糊数学模型。同时又得到了天敌对害虫的控制能力C,影响率G,影响强度H,及权数ω(x)的计算公式,以及在天敌作用下安全度的讨论与分析。 相似文献
6.
The variation of the essential oils composition of 10 Tunisian Mentha x rotundifolia (L.) Huds. Populations and their antioxidant activity were assessed. Essential oils showed high percentages of oxygenated monoterpenes and sesquiterpene hydrocarbons. Rotundifolone, p-menthane-1,2,3-triol, β-caryophyllene and germacrene D were identified as main compounds at the species level. A variation in the essential oil composition was observed according to the populations and ecological factors. The populations 7, 8, 9 and 10 from the upper semi-arid bioclimatic zone and situated at high altitudes, exhibited the highest amount of rotundifolone. The populations 1, 2, 3, 4 and 5 from the lower humid showed a rotundifolone/β-caryophyllene/germacrene D chemotype. The population 6, situated at the lowest altitude, was characterized by the highest amount of p-menthane-1,2,3-triol. The level of antioxidant activity of the populations was linked to their chemical composition difference. The highest scavenging activity and the best ability to reduce ferric ions were recorded for the population 10. The most important capacity to inhibit β-carotene bleaching was revealed for the population 8. For all populations, the antioxidant activities were substantial but lower than antioxidant standards used (Trolox and BHT).The populations (7, 8, 9 and 10) from the upper semi-arid showed the best yields of essential oils and the highest contents of rotundifolone. Chemotypes within these populations could be selected for improvement programs. 相似文献
7.
8.
We analyzed the pattern of correlations among fitness components, herbivory, and resin characteristics in a natural all-aged
stand of ponderosa pine, to infer the strength and mechanism of natural selection on plant chemistry. Male and female cone
production were monitored yearly for 15 years, and levels of herbivory for 9 years in 165 trees. Resin flow rate and monoterpene
composition were determined for these same trees. Multiple regression of fitness components on resin characteristics showed
significant associations consistent with directional selection for increased resin flow rates and increased proportions of
α- and β-pinene, myrcene and terpinolene. However, negative correlations among monoterpene fractions of the resin constrained
the overall selection. Selective herbivory by aphids approached statistical significance and monoterpenes showed some (non-significant)
effect as deterrents against deer browse. Resin characteristics were not correlated with attack by cone insects or porcupines.
However, the association between resin characteristics and fitness is significantly different from that predicted by the path
coefficients involving herbivores. Therefore the hypothesis that these herbivores mediate selection on the resin is not supported
by the observed pattern of correlations among resin characteristics, herbivory, growth and fecundity. In this population,
most of the association between resin characteristics and fitness appears to be mediated by some other factor independent
of attack by herbivore species present.
Received: 18 March 1996/Accepted: 18 July 1996 相似文献
9.
《Bioorganic & medicinal chemistry》2020,28(24):115820
This review discusses all pyridine alkaloids with CNS activity, their therapeutic potential, and the interesting array of sources whence they originate. 相似文献
10.
Yoshifumi Tanaka 《Ocean Development & International Law》2013,44(2):129-149
Attention is increasingly being given to genetic resources in the deep seabed beyond the limits of national jurisdiction owing to their considerable potential scientific and economic value. At the same time, there are concerns that the increased demand for these genetic resources may result in their unsustainable collection or even in the extinction of species in the deep seabed. At present there is no specific legal framework governing these resources in international law. Thus, this article explores the relevant rules of international law applicable to the conservation and sustainable use of genetic resources in the deep seabed. 相似文献