全文获取类型
收费全文 | 3695篇 |
免费 | 356篇 |
专业分类
4051篇 |
出版年
2024年 | 10篇 |
2023年 | 40篇 |
2022年 | 90篇 |
2021年 | 170篇 |
2020年 | 65篇 |
2019年 | 109篇 |
2018年 | 117篇 |
2017年 | 96篇 |
2016年 | 188篇 |
2015年 | 250篇 |
2014年 | 242篇 |
2013年 | 263篇 |
2012年 | 370篇 |
2011年 | 308篇 |
2010年 | 180篇 |
2009年 | 159篇 |
2008年 | 214篇 |
2007年 | 208篇 |
2006年 | 177篇 |
2005年 | 162篇 |
2004年 | 171篇 |
2003年 | 116篇 |
2002年 | 134篇 |
2001年 | 14篇 |
2000年 | 18篇 |
1999年 | 16篇 |
1998年 | 28篇 |
1997年 | 16篇 |
1996年 | 14篇 |
1995年 | 8篇 |
1994年 | 16篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1974年 | 4篇 |
1970年 | 3篇 |
1967年 | 2篇 |
1902年 | 1篇 |
1900年 | 2篇 |
1899年 | 1篇 |
排序方式: 共有4051条查询结果,搜索用时 0 毫秒
11.
Studies carried out in cultured cells have implicated modifiers of epigenetic reprogramming in the regulation of telomere
length, reporting elongation in cells that were null for DNA methyltransferase DNA methyltransferase 1 (Dnmt1), both de novo
DNA methyltransferases, Dnmt3a and Dnmt3b or various histone methyltransferases. To investigate this further, we assayed telomere
length in whole embryos or adult tissue from mice carrying mutations in four different modifiers of epigenetic reprogramming:
Dnmt1, DNA methyltransferase 3-like, structural maintenance of chromosomes hinge domain containing 1, and forkhead box O3a. Terminal restriction fragment analysis was used to compare telomere length in homozygous mutants, heterozygous mutants and
wild-type littermates. Contrary to expectation, we did not detect overall lengthening in the mutants, raising questions about
the role of epigenetic processes in telomere length in vivo. 相似文献
12.
Background
A survey was conducted amongst 1,017 Hong Kong residents ages 18 and over to determine their knowledge of fungal nail infections (onychomycosis) and the psychosocial impact of the disease on the relationships, social lives and careers of sufferers.Methods
The Fungal Nail Perception Survey was conducted by email and online between May 29th and June 10th, 2013. Participants were shown three photographs of people with and without onychomycosis of the toenails. Respondents were asked ten questions (repeated for each picture) to ascertain their perceptions of the people in the pictures. Questions were related to perceptions around the ability of sufferers and non-sufferers to form relationships with others, social activities of sufferers and non-sufferers, perceptions of the effect of the disease on the potential for career success, and awareness of fungal nail disease and health. The sub-population who themselves suffered from onychomycosis were asked about self-perception as well as their perception of others with onychomycosis.Results
Compared with non-sufferers, survey respondents perceived those with onychomycosis as less likely to be able to form good relationships. They also indicated that they would be more likely to exclude sufferers than non-sufferers from social activities and that they would be more likely to feel uncomfortable when sitting or standing beside an infected person than beside an uninfected person. Respondents perceived people with onychomycosis to be less able to perform well in their chosen career than with someone without onychomycosis. Interestingly, those respondents who themselves were infected felt socially excluded, upset and embarrassed by their infection.Conclusions
Onychomycosis may lead to stigmatization and social exclusion. Misconceptions of onychomycosis are high and education about the disease needs to be improved. Early recognition and treatment of the disease is essential to avoid complications and improve treatment outcomes, which would lead to reduced psychosocial impact on those with fungal nail infections.13.
14.
This article outlines the subsector of the information and communication technology (ICT) industry concerned with reducing the economy's environmental impact, dubbed ICT‐enabled low carbon technologies (ICTeLCTs). The article is based on a study funded by United Kingdom (UK) Trade and Investment, a division of the UK Department for Business, Innovation and Skills. ICTeLCTs can be segmented into specialist and generalist operators. Specialists focus on one or two ICT applications to monitor or reduce environmental issues, while generalists supply products and services enabling a firm or a private household to reduce the environmental impact of its activities. The subsector can be further segmented into green ICT, energy management, building management, carbon accounting, waste management, intelligent transport systems (ITSs), and water management. The main factors driving ICTeLCTs include legislation, voluntary environmental standards, corporate social responsibility (CSR) activities, customer demand, and competitive market factors. Policy makers should continue to drive the growth of ICTeLCTs with the introduction and refinement of environmental legislation regulating energy use and markets. 相似文献
15.
Roberto Gamarra Emma Ortúñez Pablo Galán Cela Virginia Guadaño 《Plant Systematics and Evolution》2012,298(3):597-607
Seed micromorphology of 24 taxa of the genera Anacamptis and Orchis was examined under light and scanning electron microscopy. Seed qualitative characters appear very useful at the supraspecific
level in the subtribe Orchidinae. Based on our observations, the sculpturing of the periclinal walls of the medial testa cells,
the seed shape and several features of the anticlinal walls of the apical cells showed variability between the studied taxa,
with special relevance of the first character. According to this factor, we found one type of seed for Anacamptis and two for Orchis. The taxonomic value of seed coat characters is compared with the last taxonomic proposals for the genera Anacamptis and Orchis. 相似文献
16.
17.
Group living can incur both benefits and costs, mediated by different mechanisms. In many gregarious caterpillars, collective use of a network of silk trails is thought to improve foraging. Grouping, i.e., close contact with conspecifics, has been postulated to have both positive (thermoregulation and predator defense) and negative (competition and pathogen transmission) effects. The present experiment distinguishes between silk produced by group members and grouping per se in their effects on growth and development of both early and late larval stadia of the forest tent caterpillar [Malacosoma disstria Hübner (Lepidoptera: Lasiocampidae)] in a laboratory context. For both developmental stadia tested, pre‐established silk trails decreased latency to food finding and hence increased food consumption and growth rate. For younger larvae, pre‐established silk also decreased investment in silk production. Grouping young caterpillars accelerated development at the expense of growth, possibly as a mechanism to avoid intraspecific competition in later larval stadia. In older caterpillars, grouping decreased meal duration, suggesting that competition can indeed occur towards the end of larval development, even in the presence of surplus food. This led to a decrease in growth without any effect on instar duration. The benefits of exogenous silk thus decreased during larval development, whereas the costs associated with crowding increased. Ontogenetic shifts in grouping are common in many taxa: the present study is among the first to provide empirical evidence for an adaptive explanation of observed ontogenetic changes in aggregative behavior. 相似文献
18.
I Bruce M Akhlaq GC Bloomfield E Budd B Cox B Cuenoud P Finan P Gedeck J Hatto JF Hayler D Head T Keller L Kirman C Leblanc DL Grand C McCarthy D O'Connor C Owen MS Oza G Pilgrim NE Press L Sviridenko L Whitehead 《Bioorganic & medicinal chemistry letters》2012,22(17):5445-5450
Using a parallel synthesis approach to target a non-conserved region of the PI3K catalytic domain a pan-PI3K inhibitor 1 was elaborated to provide alpha, delta and gamma isoform selective Class I PI3K inhibitors 21, 24, 26 and 27. The compounds had good cellular activity and were selective against protein kinases and other members of the PI3K superfamily including mTOR and DNA-PK. 相似文献
19.
Ginette S Hoare Emma J Birks Christopher Bowles Nandor Marczin Magdi H Yacoub 《Journal of applied physiology》2006,101(5):1466-1473
BACKGROUND: vascular endothelial cell activation and dysfunction are observed in patients with severe heart failure and may contribute to systemic manifestations of this syndrome. It remains unknown whether inflammatory activation of these cells occurs in these patients because of increased circulating proinflammatory mediators. Aim: to determine whether the serum from patients with heart failure possesses a net proinflammatory bioactivity to active proinflammatory pathways in cultured endothelial cells. METHODS: serum was obtained from stable patients with end-stage heart failure undergoing elective cardiac transplantation (Tx) and severely decompensated patients with heart failure requiring emergency left ventricular assist device (LVAD) implantation. Net proinflammatory bioactivity of serum was investigated by monitoring IkappaBalpha degradation and E-selectin expression in cultured human pulmonary artery endothelial cells (HPAEC) following incubation with serum samples. Serum cytokine concentrations were measured by ELISA and neutralizing antibodies were used to determine the role of specific factors in the observed bioactivity. RESULT: serum from both patient groups induced HPAEC IkappaBalpha degradation. Low basal HPAEC E-selectin expression significantly increased following treatment with Tx but not LVAD serum. Serum tumor necrosis factor-alpha (TNF-alpha) and IL-10 concentrations were higher in patients with LVAD than those with Tx, and soluble TNF-alpha receptor expression was high in both groups. Neither TNF-alpha nor IL-10 blocking experiments altered either bioassay result. CONCLUSION: activation of a specific profile of pro- and anti-inflammatory mediators is associated with heart failure resulting in HPAEC nuclear factor (NF)-kappaB activation. However, E-selectin expression is further regulated by unidentified factors. TNF-alpha is upregulated but appears to play no part in NFkappaB activation in these patients. These findings could have important therapeutic implications. 相似文献
20.