全文获取类型
收费全文 | 4573篇 |
免费 | 357篇 |
国内免费 | 5篇 |
专业分类
4935篇 |
出版年
2024年 | 5篇 |
2023年 | 26篇 |
2022年 | 89篇 |
2021年 | 128篇 |
2020年 | 110篇 |
2019年 | 221篇 |
2018年 | 185篇 |
2017年 | 113篇 |
2016年 | 170篇 |
2015年 | 239篇 |
2014年 | 239篇 |
2013年 | 309篇 |
2012年 | 414篇 |
2011年 | 398篇 |
2010年 | 213篇 |
2009年 | 193篇 |
2008年 | 271篇 |
2007年 | 246篇 |
2006年 | 252篇 |
2005年 | 200篇 |
2004年 | 173篇 |
2003年 | 156篇 |
2002年 | 133篇 |
2001年 | 35篇 |
2000年 | 36篇 |
1999年 | 46篇 |
1998年 | 51篇 |
1997年 | 32篇 |
1996年 | 23篇 |
1995年 | 34篇 |
1994年 | 12篇 |
1993年 | 16篇 |
1992年 | 19篇 |
1991年 | 11篇 |
1990年 | 10篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 13篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1978年 | 7篇 |
1977年 | 9篇 |
1976年 | 5篇 |
1974年 | 4篇 |
1973年 | 8篇 |
1971年 | 4篇 |
排序方式: 共有4935条查询结果,搜索用时 15 毫秒
1.
2.
3.
The experimental study of the relationship between biodiversity and ecosystem function has mainly addressed the effect of species and number of functional groups. In theory, this approach has mainly focused on how extinction affects function, whereas dispersal limitation of ecosystem function has been rarely discussed. A handful of seed introduction experiments, as well as numerous observations of the effects of long‐distance dispersal of alien species, indicate that ecosystem function may be strongly determined by dispersal limitation at the local, regional and/or global scales. We suggest that it is time to replace biodiversity manipulation experiments, based on random draw of species, with those addressing realistic scenarios of either extinction or dispersal. Experiments disentangling the dispersal limitation of ecosystem function should have to take into account the probability of arrival. The latter is defined as the probability that a propagule of a particular species will arrive at a particular community. Arrival probability depends on the dispersal ability and the number of propagules of a species, the distance a species needs to travel, and the permeability of the matrix landscape. Current databases, in particular those in northwestern and central Europe now enable robust estimation of arrival probability in plant communities. We suggest a general hypothesis claiming that dispersal limitation according to arrival probability will have ecosystem‐level effects different from those arising due to random arrival. This hypothesis may be rendered more region‐, landscape‐ or ecosystem‐specific by estimating arrival probabilities for different background conditions. 相似文献
4.
5.
6.
7.
Véronique Lavoie Anne‐Elen Kernaleguen Guy Charron Nada Farhat Mariève Cossette Aida M. Mamarbachi Bruce G. Allen Eric Rhéaume Jean‐Claude Tardif 《Obesity (Silver Spring, Md.)》2011,19(4):722-728
Adiponectin is an adipokine whose plasma levels are inversely correlated to metabolic syndrome components. Adiponectin protects against atherosclerosis and decreases risks in myocardial infarction. Endothelial progenitor cells (EPCs) are a heterogeneous population of circulating cells involved in vascular repair and neovascularization. EPCs number is reduced in patients with cardiovascular disease. We hypothesize that the positive effects of adiponectin against atherosclerosis are explained in part by its interactions with EPCs. Cells were obtained from healthy volunteers' blood by mononuclear cell isolation and plating on collagen‐coated dishes. Three sub‐populations of EPCs were identified and characterized using flow cytometry. EPCs' expression of adiponectin receptors, AdipoR1, and AdipoR2 was evaluated by quantitative PCR. The effects of recombinant adiponectin on EPCs' susceptibility to apoptosis were assessed. Finally, expression of neutrophil elastase by EPCs and activity of this enzyme on adiponectin processing were assessed. Quantitative PCR analysis of EPCs mRNAs showed that AdipoR1 mRNA is expressed at higher levels than AdipoR2. Expression of AdipoR1 protein was confirmed by western blot. Adiponectin significantly increased survival of two sub‐populations of EPCs in conditions of serum deprivation. Such effect could not be demonstrated in the third EPCs sub‐population. We also demonstrated that EPCs, particularly one sub‐population, express neutrophil elastase. Neutrophil elastase activity was confirmed in EPCs' conditioned media. Adiponectin protects some EPCs sub‐populations against apoptosis and therefore could modulate EPCs ability to induce repair of vascular damage. Neutrophil elastase activity of EPCs could locally modulate adiponectin activity by its involvement in the generation of the globular form of adiponectin. 相似文献
8.
John A. Nyakatura Vivian R. Allen Jonas Laustr?er Amir Andikfar Marek Danczak Hans-Jürgen Ullrich Werner Hufenbach Thomas Martens Martin S. Fischer 《PloS one》2015,10(9)
Orobates pabsti, a basal diadectid from the lower Permian, is a key fossil for the understanding of early amniote evolution. Quantitative analysis of anatomical information suffers from fragmentation of fossil bones, plastic deformation due to diagenetic processes and fragile preservation within surrounding rock matrix, preventing further biomechanical investigation. Here we describe the steps taken to digitally reconstruct MNG 10181, the holotype specimen of Orobates pabsti, and subsequently use the digital reconstruction to assess body mass, position of the centre of mass in individual segments as well as the whole animal, and study joint mobility in the shoulder and hip joints. The shape of most fossil bone fragments could be recovered from micro-focus computed tomography scans. This also revealed structures that were hitherto hidden within the rock matrix. However, parts of the axial skeleton had to be modelled using relevant isolated bones from the same locality as templates. Based on the digital fossil, mass of MNG 10181 was estimated using a model of body shape that was varied within a plausible range to account for uncertainties of the dimension. In the mean estimate model the specimen had an estimated mass of circa 4 kg. Varying of the mass distribution amongst body segments further revealed that Orobates carried most of its weight on the hind limbs. Mostly unrestricted joint morphology further suggested that MNG 10181 was able to effectively generate propulsion with the pelvic limbs. The digital reconstruction is made available for future biomechanical studies. 相似文献
9.
10.