首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1584篇
  免费   178篇
  国内免费   1篇
  1763篇
  2023年   10篇
  2022年   9篇
  2021年   24篇
  2020年   24篇
  2019年   22篇
  2018年   36篇
  2017年   20篇
  2016年   51篇
  2015年   56篇
  2014年   85篇
  2013年   98篇
  2012年   139篇
  2011年   105篇
  2010年   58篇
  2009年   80篇
  2008年   73篇
  2007年   69篇
  2006年   57篇
  2005年   49篇
  2004年   58篇
  2003年   48篇
  2002年   44篇
  2001年   53篇
  2000年   38篇
  1999年   37篇
  1998年   28篇
  1997年   33篇
  1996年   21篇
  1995年   16篇
  1994年   16篇
  1993年   16篇
  1992年   45篇
  1991年   28篇
  1990年   24篇
  1989年   17篇
  1988年   22篇
  1987年   18篇
  1986年   20篇
  1985年   15篇
  1984年   11篇
  1983年   12篇
  1982年   6篇
  1979年   5篇
  1978年   7篇
  1977年   11篇
  1976年   6篇
  1974年   5篇
  1973年   4篇
  1972年   6篇
  1970年   6篇
排序方式: 共有1763条查询结果,搜索用时 15 毫秒
41.
Primates - Habitat fragmentation is one of the major types of anthropogenic change, though fragmented landscapes predate human intervention. At present, the Central Highlands of Madagascar are...  相似文献   
42.
Abstract

Karyology of some species of Vicia. The species studied are: Vicia articulata, Vicia monantha, Vicia narbonensis and Vicia ervilia.

The chromosome number 2n = 14 counted for the first three species agrees with that reported in literature; that of Vicia monantha (2n=14) agrees with that quoted by Heitz, while Senn finds 2n=12.

Karyotypes of V. narbonensis and V. ervilia show appreciable differences from those reported in literature.  相似文献   
43.
Olive breeding programmes are focused on selecting for traits as short juvenile period, plant architecture suited for mechanical harvest, or oil characteristics, including fatty acid composition, phenolic, and volatile compounds to suit new markets. Understanding the molecular basis of these characteristics and improving the efficiency of such breeding programmes require the development of genomic information and tools. However, despite its economic relevance, genomic information on olive or closely related species is still scarce. We have applied Sanger and 454 pyrosequencing technologies to generate close to 2 million reads from 12 cDNA libraries obtained from the Picual, Arbequina, and Lechin de Sevilla cultivars and seedlings from a segregating progeny of a Picual × Arbequina cross. The libraries include fruit mesocarp and seeds at three relevant developmental stages, young stems and leaves, active juvenile and adult buds as well as dormant buds, and juvenile and adult roots. The reads were assembled by library or tissue and then assembled together into 81 020 unigenes with an average size of 496 bases. Here, we report their assembly and their functional annotation.  相似文献   
44.
Phytoplankton size structure is key for the ecology and biogeochemistry of pelagic ecosystems, but the relationship between cell size and maximum growth rate (μmax) is not yet well understood. We used cultures of 22 species of marine phytoplankton from five phyla, ranging from 0.1 to 106 μm3 in cell volume (Vcell), to determine experimentally the size dependence of growth, metabolic rate, elemental stoichiometry and nutrient uptake. We show that both μmax and carbon‐specific photosynthesis peak at intermediate cell sizes. Maximum nitrogen uptake rate (VmaxN) scales isometrically with Vcell, whereas nitrogen minimum quota scales as Vcell0.84. Large cells thus possess high ability to take up nitrogen, relative to their requirements, and large storage capacity, but their growth is limited by the conversion of nutrients into biomass. Small species show similar volume‐specific VmaxN compared to their larger counterparts, but have higher nitrogen requirements. We suggest that the unimodal size scaling of phytoplankton growth arises from taxon‐independent, size‐related constraints in nutrient uptake, requirement and assimilation.  相似文献   
45.
Despite extensive research on face perception, few studies have investigated individuals’ knowledge about the physical features of their own face. In this study, 50 participants indicated the location of key features of their own face, relative to an anchor point corresponding to the tip of the nose, and the results were compared to the true location of the same individual’s features from a standardised photograph. Horizontal and vertical errors were analysed separately. An overall bias to underestimate vertical distances revealed a distorted face representation, with reduced face height. Factor analyses were used to identify separable subconfigurations of facial features with correlated localisation errors. Independent representations of upper and lower facial features emerged from the data pattern. The major source of variation across individuals was in representation of face shape, with a spectrum from tall/thin to short/wide representation. Visual identification of one’s own face is excellent, and facial features are routinely used for establishing personal identity. However, our results show that spatial knowledge of one’s own face is remarkably poor, suggesting that face representation may not contribute strongly to self-awareness.  相似文献   
46.
Human family X polymerases contribute both to genomic stability and variability through their specialized functions in DNA repair. Polμ participates in the repair of spontaneous double strand breaks (DSB) by non homologous end-joining (NHEJ), and also in the V(D)J recombination process after programmed DSBs. Polμ plays this dual role due to its template-dependent and terminal transferase (template-independent) polymerization activities. In this study we evaluated if Polμ could be regulated by Cdk phosphorylation along the cell cycle. In vitro kinase assays showed that the S phase-associated Cdk2/cyclin A complex was able to phosphorylate Polμ. We identified Ser12, Thr21 (located in the BRCT domain) and Ser372 (located in loop1) as the target residues. Mutation of these residues to alanine indicated that Ser372 is the main phosphorylation site. Mobilization of loop1, which mediates DNA end micro-synapsis, is crucial both for terminal transferase and NHEJ. Interestingly, the phospho-mimicking S372E mutation specifically impaired these activities. Our evidences suggest that Polμ could be regulated in vivo by phosphorylation of the BRCT domain (Ser12/Thr21) and of Ser372, affecting the function of loop1. Consequently, Polμ’s most distinctive activities would be turned off at specific cell-cycle phases (S and G2), when these promiscuous functions might be harmful to the cell.  相似文献   
47.
Amyloidosis is an uncommon syndrome consisting of a number of disorders having in common an extracellular deposit of fibrillary proteins. This results in functional and structural changes in the affected organs, depending on deposit location and severity.Amyloid infiltration of the thyroid gland may occur in 50% and up to 80% of patients with primary and secondary amyloidosis respectively. Amyloid goiter (AG) is a true rarity, usually found associated to secondary amyloidosis. AG may require surgical excision, usually because of compressive symptoms.We report the case of a patient with a big AG occurring in the course of a secondary amyloidosis associated to polyarticular onset juvenile idiopathic arthritis who underwent total thyroidectomy. Current literature is reviewed, an attempt is made to provide action guidelines, and some surgical considerations on this rare condition are given.  相似文献   
48.
49.
50.
Hyperthermia is a predictor of poor outcome in ischemic (IS) and intracerebral hemorrhagic (ICH) stroke. Our aim was to study the plausible mechanisms involved in the poor outcome associated to hyperthermia in stroke. We conducted a case-control study including patients with IS (n = 100) and ICH (n = 100) within the first 12 hours from symptom onset. Specifically, IS and ICH patients were consecutively included into 2 subgroups, according to the highest body temperature within the first 24 hours: Tmax <37.5°C and Tmax ≥37.5°C, up to reach 50 patients per subgroup of temperature for both IS and ICH patients. Body temperature was determined at admission and every 4 hours during the first 48 hours. Main outcome variable was poor functional outcome (modified Rankin scale score >2) at 3 months. Serum levels of glutamate and active MMP-9 were measured at admission. Our results showed that Tmax ≥37.5°C within the first 24 hours was independently associated with poor outcome in both IS (OR, 12.43; 95% CI, 3.73–41.48; p<0.0001) and ICH (OR, 4.29; 95% CI, 1.32–13.91; p = 0.015) after adjusting for variables with a proven biological relevance for outcome. However, when molecular markers levels were included in the logistic regression model, we observed that glutamate (OR, 1.01; 95% CI, 1.00–1.02; p = 0.001) and infarct volume (OR, 1.06; 95% CI, 1.01–1.10; p = 0.015) were the only variables independently associated to poor outcome in IS, and active MMP-9 (OR, 1.04; 95% CI, 1.00–1.08; p = 0.002) and National Institute of Health Stroke Scale (NIHSS) at admission (OR, 1.29; 95% CI, 1.13–1.49; p<0.0001) in ICH. In conclusion, these results suggest that although the outcome associated to hyperthermia is similar in human IS and ICH, the underlying mechanisms may be different.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号