全文获取类型
收费全文 | 1814篇 |
免费 | 196篇 |
国内免费 | 2篇 |
出版年
2021年 | 17篇 |
2019年 | 15篇 |
2017年 | 15篇 |
2016年 | 24篇 |
2015年 | 54篇 |
2014年 | 68篇 |
2013年 | 74篇 |
2012年 | 87篇 |
2011年 | 86篇 |
2010年 | 52篇 |
2009年 | 50篇 |
2008年 | 63篇 |
2007年 | 60篇 |
2006年 | 59篇 |
2005年 | 72篇 |
2004年 | 65篇 |
2003年 | 60篇 |
2002年 | 81篇 |
2001年 | 68篇 |
2000年 | 56篇 |
1999年 | 46篇 |
1998年 | 22篇 |
1997年 | 26篇 |
1996年 | 18篇 |
1995年 | 20篇 |
1994年 | 23篇 |
1993年 | 32篇 |
1992年 | 33篇 |
1991年 | 41篇 |
1990年 | 30篇 |
1989年 | 33篇 |
1988年 | 26篇 |
1987年 | 33篇 |
1986年 | 24篇 |
1985年 | 24篇 |
1984年 | 20篇 |
1983年 | 22篇 |
1981年 | 14篇 |
1980年 | 20篇 |
1978年 | 18篇 |
1977年 | 19篇 |
1976年 | 14篇 |
1975年 | 14篇 |
1974年 | 24篇 |
1973年 | 16篇 |
1972年 | 16篇 |
1971年 | 16篇 |
1970年 | 22篇 |
1968年 | 15篇 |
1967年 | 22篇 |
排序方式: 共有2012条查询结果,搜索用时 15 毫秒
81.
82.
JK Sugden 《Biotechnic & histochemistry》2004,79(2):71-90
An overview of the basic principles of photochemistry is presented to facilitate discussion of fluorescence, quenching and quantum yields. These topics in turn provide the foundation for an account of fluorescence spectroscopy and its application to microscopy. A brief overview of light microscopy and the application of fluorescence microscopy is given. The influences of molecular features, such as aromatic character and substitution patterns, on color and fluorescence are described. The concept of color fading is considered with particular reference to its effect on microscopic preparations. A survey of representative fluorescent probes is provided, and their sensitivity, application, and limitations are described. The phototoxicity of fluorescent molecules is discussed using biomembranes and DNA as examples of targets of toxicity. Photodynamic therapy, a relatively new clinical application of phototoxicity, is described. Both anticancer and antimicrobial applications are noted, and an assessment is given of the current ideas on the ideal physicochemical properties of the sensitizing agents for such applications. 相似文献
83.
Tippu S Sheriff Pamela Carr Michael B Hursthouse Mark E Light 《Inorganica chimica acta》2004,357(9):2494-2502
The mononuclear complexes (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)] and (Bu4N)2[Mn(Cl4Cat)3] (H2Cat=1,2-dihydroxybenzene) have been synthesised and characterised by X-ray diffraction. This work provides a direct, independent, synthesis of these complexes and an interesting example of how solvent effects can promote the formation of either a manganese(III) or manganese(IV) complex of the same ligand. The characterisation of (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)] supports previous work that manganese(III) is extremely reluctant to form tris (catecholato) complexes due to the short `bite distance' of catecholate oxygen atoms (2.79 Å) which are unable to span the elongated coordination axes of the Jahn-Teller distorted Mn(III) ion and explains the 2:1 and 3:1 tetrachlorocatechol:manganese ratios in the Mn(III) and Mn(IV) complexes, respectively. Hydrogen peroxide production using dioxygen and hydroxylamine as substrates in acetonitrile/water mixtures, under ambient conditions, can be demonstrated with both complexes, suggesting that neither labile coordination sites nor the oxidation state of the manganese are important to the catalytic system. Turn over frequencies (TOF, moles of H2O2 per moles of manganese per hour) of ∼10 000 h−1 are obtained and this compares very favourably with the commercial production of hydrogen peroxide by the autoxidation of 2-ethylanthrahydroquinone (AO process). 相似文献
84.
85.
The histological effect on and stress response of post juvenile Clarias gariepinus exposed to Qua Iboe crude oil and rig wash were investigated. Fish weighing 60–90 g and measuring 16–18 cm were exposed for 7–28 days to 8.00 ml?1 Qua Iboe crude oil and 0.0018 ml–1 rig wash, both being 0.1 of the 96 hr LC50. Blood samples of C. gariepinus were collected every seven days and evaluated for stress by measuring cortisol concentration. The gills and liver were studied and scored for Gill Alteration Index (GAI) and Hepatic Alteration Index (HAI), respectively. There was an increase in cortisol level up to the 7th and 14th day among the group exposed to Qua Iboe crude oil, with a decrease on the 21st and 28th day. The rig wash group increased in cortisol level up to the 7th day and decreased slightly on the 14th day, after which the trend became irregular. The toxic effects of the Qua Iboe crude oil and rig wash were time dependent, as shown by the histopathological alteration index (HAI) of gill and liver. After 28 days of exposure, the gills had irreparable damage due to high frequency of cellular necrosis and degeneration, whereas the liver had from moderate to severe damage due to the high frequency of cellular degeneration and inflammation. Qua Iboe crude oil and rig wash are both toxic to C. gariepinus, therefore their indiscriminate discharge to the environment must be discouraged. 相似文献
86.
Nonalcoholic fatty liver disease and measures of early brain health in middle‐aged adults: The CARDIA study 下载免费PDF全文
87.
The effect of nitrogen availability and water conditions on competition between a facultative CAM plant and an invasive grass 下载免费PDF全文
Kailiang Yu Paolo D'Odorico David E. Carr Ashden Personius Scott L. Collins 《Ecology and evolution》2017,7(19):7739-7749
Abstract Plants with crassulacean acid metabolism (CAM) are increasing their abundance in drylands worldwide. The drivers and mechanisms underlying the increased dominance of CAM plants and CAM expression (i.e., nocturnal carboxylation) in facultative CAM plants, however, remain poorly understood. We investigated how nutrient and water availability affected competition between Mesembryanthemum crystallinum (a model facultative CAM species) and the invasive C3 grass Bromus mollis that co‐occur in California's coastal grasslands. Specifically we investigated the extent to which water stress, nutrients, and competition affect nocturnal carboxylation in M. crystallinum. High nutrient and low water conditions favored M. crystallinum over B. mollis, in contrast to high water conditions. While low water conditions induced nocturnal carboxylation in 9‐week‐old individuals of M. crystallinum, in these low water treatments, a 66% reduction in nutrient applied over the entire experiment did not further enhance nocturnal carboxylation. In high water conditions M. crystallinum both alone and in association with B. mollis did not perform nocturnal carboxylation, regardless of the nutrient levels. Thus, nocturnal carboxylation in M. crystallinum was restricted by strong competition with B. mollis in high water conditions. This study provides empirical evidence of the competitive advantage of facultative CAM plants over grasses in drought conditions and of the restricted ability of M. crystallinum to use their photosynthetic plasticity (i.e., ability to switch to CAM behavior) to compete with grasses in well‐watered conditions. We suggest that a high drought tolerance could explain the increased dominance of facultative CAM plants in a future environment with increased drought and nitrogen deposition, while the potential of facultative CAM plants such as M. crystallinum to expand to wet environments is expected to be limited. 相似文献
88.
Isabelle A Carré 《Trends in plant science》2002,7(1):4-6
The EARLY FLOWERING 3 (ELF3) gene of Arabidopsis regulates plant morphology, flowering time and circadian rhythms. ELF3 was proposed to function as a modulator of light signal transduction downstream of phytochromes, and, perhaps, other photoreceptors. Recent work indicates that ELF3 encodes a novel nuclear protein that is expressed rhythmically and interacts with phytochrome B. How ELF3 mediates the circadian gating of light responses and regulates light input to the clock is the subject of discussion. 相似文献
89.
90.