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   3799篇
  免费   294篇
  4093篇
  2024年   7篇
  2023年   42篇
  2022年   99篇
  2021年   127篇
  2020年   133篇
  2019年   127篇
  2018年   161篇
  2017年   133篇
  2016年   202篇
  2015年   241篇
  2014年   254篇
  2013年   310篇
  2012年   337篇
  2011年   284篇
  2010年   161篇
  2009年   165篇
  2008年   178篇
  2007年   172篇
  2006年   143篇
  2005年   123篇
  2004年   93篇
  2003年   88篇
  2002年   84篇
  2001年   56篇
  2000年   62篇
  1999年   60篇
  1998年   9篇
  1997年   12篇
  1996年   11篇
  1995年   16篇
  1994年   5篇
  1993年   7篇
  1992年   16篇
  1991年   17篇
  1990年   15篇
  1989年   16篇
  1988年   10篇
  1987年   10篇
  1986年   8篇
  1985年   8篇
  1984年   12篇
  1983年   7篇
  1982年   8篇
  1981年   5篇
  1980年   9篇
  1979年   8篇
  1978年   7篇
  1971年   6篇
  1968年   3篇
  1967年   4篇
排序方式: 共有4093条查询结果,搜索用时 14 毫秒
991.
The legitimacy of the name Cryptococcus mollis for the type of the genus Cryptococcus is shown. The type material, available in the Rijksherbarium at Leiden University was investigated by transmission electron microscopy. The cell wall of most of the cells showed a structure, similar to that of basidiomycetous yeasts. Bud scars were also found.  相似文献   
992.
Comparative genomic studies are revealing that, in sharp contrast with the strong stability found in birds and mammals, sex determination mechanisms are surprisingly labile in cold‐blooded vertebrates, with frequent transitions between different pairs of sex chromosomes. It was recently suggested that, in context of this high turnover, some chromosome pairs might be more likely than others to be co‐opted as sex chromosomes. Empirical support, however, is still very limited. Here we show that sex‐linked markers from three highly divergent groups of anurans map to Xenopus tropicalis scaffold 1, a large part of which is homologous to the avian sex chromosome. Accordingly, the bird sex determination gene DMRT1, known to play a key role in sex differentiation across many animal lineages, is sex linked in all three groups. Our data provide strong support for the idea that some chromosome pairs are more likely than others to be co‐opted as sex chromosomes because they harbor key genes from the sex determination pathway.  相似文献   
993.
While human immunodeficiency virus (HIV)-1 chemokine co-receptors 5 tropism and the GWGR motif in the envelope third variable region (V3 loop) have been associated with a slower disease progression, their influence on antiretroviral response remains unclear. The impact of baseline V3 characteristics on treatment response was evaluated in a randomised, double blind, prospective cohort study with patients initiating highly active antiretroviral therapy with lopinavir or efavirenz plus azithothymidine/3TC (1:1) over 48 weeks. Similar virological and immunological responses were observed for both treatment regimens. The 43 individuals had a mean baseline CD4 T cell count of 119 cells/mm(3) [standard deviation (SD) = 99] and a mean viral load of 5.09 log(10) copies/mL (SD = 0.49). The GWGR motif was not associated with a CD4 T cell response, but predicted R5 tropism by the geno2pheno([clinical20%]) algorithm correlated with higher CD4 T cell levels at all monitoring points (p < 0.05). Moreover, higher false-positive rates (FPR) values from this analysis revealed a strong correlation with CD4 T cell recovery (p < 0.0001). Transmitted drug resistance mutations, documented in 3/41 (7.3%) cases, were unrelated to the assigned antiretroviral regimen and had no impact on patient outcomes. In conclusion, na?ve HIV-1 R5 infected patients exhibited higher CD4 T cell counts at baseline; this difference was sustained throughout therapy. The geno2pheno([clinical]) option FPR positively correlated with CD4 T cell gain and may be useful in predicting CD4 T cell recovery.  相似文献   
994.
The goal of this study was to analyse possible structural and ultrastructural differences between the secretory disk of male and functionally female flowers of Tapirira guianensis (Anacardiaceae) at different developmental stages. Studies were carried out using light, scanning and transmission electron microscopy. Biochemical tests were employed to determine the proportion of sugars in the nectar of the floral morphotypes: they were found to be similar, both predominantly composed of sucrose. In addition to sugars, lipids and phenolic substances were identified in anthetic flowers; thus, the secretory disk is a mixed secretion gland, also called a sensu lato nectary. During anthesis, granulocrine and eccrine secretory mechanisms occur in both floral morphotypes. After anthesis and fertilization of the functionally female flower, only the lipophilic and phenolic secretion continues until the early stages of fruit development. An intrastaminal secretory disk that produces both nectar and lipids is reported for the first time in Anacardiaceae. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179 , 533–544.  相似文献   
995.
996.
Objective To test whether strict implementation of a standardised protocol for the management of malaria and provision of a financial incentive for health workers reduced mortality.Design Randomised controlled intervention trial.Setting Paediatric ward at the national hospital in Guinea-Bissau. All children admitted to hospital with severe malaria received free drug kits.Participants 951 children aged 3 months to 5 years admitted to hospital with a diagnosis of malaria randomised to normal or intervention wards.Interventions Before the start of the study, all personnel were trained in the use of the standardised guidelines for the management of malaria, including strict follow-up procedures. Nurses and doctors were randomised to work on intervention or control wards. Personnel in the intervention ward received a small financial incentive ($50 (£25; €35)/month for nurses and $160 for doctors) and their compliance with standard case management was closely monitored.Main outcome measures In-hospital mortality and cumulative mortality within 4 weeks of hospital admission.Results In-hospital mortality was 5% for the intervention group and 10% in the control group (risk ratio 0.48, 95% confidence interval 0.29 to 0.79). The effect may have been stronger in patients with positive malaria slides (0.36, 0.16 to 0.80). Cumulative mortality 4 weeks after discharge was also lower in the intervention group (0.61, 0.40 to 0.95).Conclusions Supervising healthcare workers to adhere to a standardised treatment protocol was associated with greatly reduced in-hospital mortality. Financial incentives may be important for the dedication and compliance of staff members.Trial registration Clinical Trials NCT00465777.  相似文献   
997.
A monoclonal antibody (MAb) specific for rainbow trout thrombocytes was produced and its reactivity was demonstrated by flow cytometry and immuno-electron microscopy. Flow cytometry analysis showed that this MAb (TTL-7D11) reacted positively with about 30% of the peripheral blood leucocytes (PBL) and about 1%, 2%, and 11% of the pronephros, mesonephros, and spleen cells, respectively. Electron microscopy using immunogold labeling demonstrated that this MAb reacted strongly with thrombocytes, where gold beads could be seen attached only to the membrane and canalicular system of these cells. Positive and negative leucocytes for this MAb were obtained by magnetic cell separation. In the positive fraction, 96% of the cells were thrombocytes, while in the negative fraction no more than 3% were, which clearly showed a high purity of the positive fraction. Aggregation studies showed that about 75% of the positive fraction cells aggregated after being mixed with U-46619 thromboxane-mimetic, whereas in the negative fraction only 10% of the cells did so. Thus, utilizing the TTL-7D11 we have succeeded in isolating a pure thrombocyte population, and this would facilitate further studies, particularly on their characteristics and function(s).  相似文献   
998.
The International Journal of Life Cycle Assessment - Packaging can be a critical aspect in the environmental performance of pharmaceutical products; however, few life cycle assessment studies were...  相似文献   
999.
Many bacterial populations harbour substantial numbers of hypermutable bacteria, in spite of hypermutation being associated with deleterious mutations. One reason for the persistence of hypermutators is the provision of novel mutations, enabling rapid adaptation to continually changing environments, for example coevolving virulent parasites. However, hypermutation also increases the rate at which intraspecific parasites (social cheats) are generated. Interspecific and intraspecific parasitism are therefore likely to impose conflicting selection pressure on mutation rate. Here, we combine theory and experiments to investigate how simultaneous selection from inter- and intraspecific parasitism affects the evolution of bacterial mutation rates in the plant-colonizing bacterium Pseudomonas fluorescens. Both our theoretical and experimental results suggest that phage presence increases and selection for public goods cooperation (the production of iron-scavenging siderophores) decreases selection for mutator bacteria. Moreover, phages imposed a much greater growth cost than social cheating, and when both selection pressures were imposed simultaneously, selection for cooperation did not affect mutation rate evolution. Given the ubiquity of infectious phages in the natural environment and clinical infections, our results suggest that phages are likely to be more important than social interactions in determining mutation rate evolution.  相似文献   
1000.
Ultraviolet-B radiation (UVBR) affects plants in many important ways, including reduction of growth rate and primary productivity, and changes in ultrastructures. Rice (Oryza sativa) is one of the most cultivated cereals in the world, along with corn and wheat, representing over 50 % of agricultural production. In this study, we examined O. sativa plants exposed to ambient outdoor radiation and laboratory-controlled photosynthetically active radiation (PAR) and PAR + UVBR conditions for 2 h/day during 30 days of cultivation. The samples were studied for morphological and ultrastructural characteristics, and physiological parameters. PAR + UVBR caused changes in the ultrastructure of leaf of O. sativa and leaf morphology (leaf index, leaf area and specific leaf area, trichomes, and papillae), plant biomass (dry and fresh weight), photosynthetic pigments, phenolic compounds, and protein content. As a photoprotective acclimation strategy against PAR + UVBR damage, an increase of 66.24 % in phenolic compounds was observed. Furthermore, PAR + UVBR treatment altering the levels of chlorophylls a and b, and total chlorophyll. In addition, total carotenoid contents decreased after PAR + UVBR treatment. The results strongly suggested that PAR + UVBR negatively affects the ultrastructure, morphology, photosynthetic pigments, and growth rates of leaf of O. sativa and, in the long term, it could affect the viability of this economically important plant.  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