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   22134篇
  免费   2054篇
  国内免费   2584篇
  2024年   33篇
  2023年   193篇
  2022年   258篇
  2021年   846篇
  2020年   697篇
  2019年   906篇
  2018年   904篇
  2017年   739篇
  2016年   1009篇
  2015年   1400篇
  2014年   1670篇
  2013年   1794篇
  2012年   2058篇
  2011年   1960篇
  2010年   1390篇
  2009年   1244篇
  2008年   1388篇
  2007年   1279篇
  2006年   1106篇
  2005年   1048篇
  2004年   906篇
  2003年   865篇
  2002年   772篇
  2001年   438篇
  2000年   318篇
  1999年   268篇
  1998年   234篇
  1997年   185篇
  1996年   133篇
  1995年   103篇
  1994年   95篇
  1993年   68篇
  1992年   84篇
  1991年   62篇
  1990年   43篇
  1989年   50篇
  1988年   42篇
  1987年   25篇
  1986年   21篇
  1985年   24篇
  1984年   14篇
  1983年   15篇
  1982年   16篇
  1981年   8篇
  1980年   6篇
  1979年   8篇
  1977年   4篇
  1972年   7篇
  1970年   5篇
  1965年   4篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
报道了产自滇东南的中国兰科植物一新记录种--聚叶钗子股(Luisia appressifolia Averyanov)。研究发现,已往文献报道该种时,对其花部特征(如花颜色、萼片及花瓣形状、大小等)的描述仅依据腊叶标本,因标本严重失水导致描述信息不准或缺失。本文根据新鲜植株和花部材料对该种相关形态特征给予了补充。  相似文献   
942.
943.
Modulation of Immune check point regulators, especially the PD-1/PD-L1 axis, plays a critical role in successful management of a small proportion of lung cancer patients, but not so effective in the rest of lung cancer patients. A better understanding of immunotherapy non-responsive or resistant patients therefore warranted for future development of novel therapeutics. The newly identified regulator CMTM6 (CKLF-like MARVEL transmembrane domain containing 6) has been reported to serves as the stabilizer of PD-L1 and enhances the inhibitory effect of PD-L1 on immune system in both cell line and animal models, but its clinical relevance associated with PD-L1 is unknown and the current study is designed to address this question. The study using immunohistochemistry demonstrated that CMTM6 positivity from 15 out of 19 types of cancers with our in-house tissue microarray, and PD-L1 expression is always found only in CMTM6 positive cancers. CMTM6 and PD-L1 expression were analyzed in 81 lung cancer patient sample, and we observed that CMTM6 expression correlated with cancer histotypes and inversely correlated with cancer metastases, but not with patients’ age and gender. No PD-L1 expression was observed in negative CMTM6 samples. Higher expression PD-L1 is also associated with higher CMTM6 expression. In summary, CMTM6 expression is associated with PD-L1 expression, as well as lung cancer histotypes and metastasis. The results thus for the first time confirmed earlier reports on CMTM6/PD-L1 connection, from a clinical aspect of analysis.  相似文献   
944.
Microalgal biomass was hydrolyzed using a solid acid catalyst with the aid of liquid acid. The use of solid acid as the main catalyst instead of liquid acid was to omit subsequent neutralization and/or desalination steps, which are commonly required in using the resulting hydrolysates for microbial fermentation. The hydrolysis of 10 g/L of lipid-extracted Chlorella vulgaris containing 12.2% carbohydrates using 7.6 g/L Amberlyst 36 and 0.0075 N nitric acid at 150°C resulted in 1.08 g/L of mono-sugars with a yield of 88.5%. For hydrolysis of higher concentrations of the biomass over 10 g/L, the amount of Amberlyst 36 needed to be increased in proportion to the biomass concentration to maintain similar levels of hydrolysis performance. Increasing the solid acid concentration protected the surface of the solid acid from being severely covered by cell debris during the reaction. A hydrolysate of lipid-extracted C. vulgaris 50 g/L was used, with no post-treatment of desalination, for the cultivation of Klebsiella oxytoca producing 2,3-butanediol. Cell growth in the hydrolysate was found to be almost the same as in the conventional medium with the same monosaccharide composition, confirming its fermentation compatibility. It was noticeable that the yield of 2,3-butanediol with the hydrolysate was observed to be 2.6 times higher than that with the conventional medium. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2729, 2019  相似文献   
945.
The genus Elaeocarpus contains approximately 360 species and occurs in mesic forest communities from India, through to China, Southeast Asia, New Guinea, Australia, and New Caledonia. Elaeocarpus fossils are best known from the Eocene to the Miocene of Australia and the late Pliocene–early Pleistocene of India, but have not been documented from East Asia before. Here we describe six new species of Elaeocarpus, E. nanningensis sp. nov. from the late Oligocene Yongning Formation of the Nanning Basin, E. presikkimensis sp. nov. from the Miocene Erzitang Formation of the Guiping Basin, E. prerugosus sp. nov., E. prelacunosus sp. nov., E. preserratus sp. nov., and E. preprunifolioides sp. nov. from the late Miocene Foluo Formation of the Nankang Basin in Guangxi, South China. This is the first reliable report for the genus occurring in East Asia, and the fossils indicate that Elaeocarpus had colonized this region by the late Oligocene and represented by a morphologically diverse group of species by the late Miocene. This sheds new insights into the timing and migration patterns of the genus in East Asia. Elaeocarpus is typically a rainforest genus occurring in mesic forests. Based on the habitat of their morphologically similar modern relatives we propose that these three sedimentary basins were warm and wet adjacent to mountainous regions with the evergreen or rain forests during the late Oligocene to Miocene.  相似文献   
946.
A ring species consists of two reproductively isolated forms connected by a chain of intergrading populations encircling a geographic barrier. The mountains encircling the Sichuan Basin in southwestern China harbor great species diversity and endemism, and they are candidate regions for ring species. Here, we examined a potential ring species complex with a ring distribution surrounding the Sichuan Basinand the reproductive barrier between sibling species Rhodiola yunnanensis and R. henryi of the R. yunnanensis complex. In this study, we test the hypothesis that R. yunnanensis and R. henryi diverged by the ring-species model using an amplicon sequencing strategy targeting the introns of 27 single-copy nuclear genes and 14 chloroplast DNA sequences. Our studies indicated that the R. yunnanensis complex is monophyletic, originates at the late Miocene, and forms its current ring distribution pattern after the LIG. In addition, clear genetic intergradation was not found among R. henryi populations within the distribution ring. All these findings suggest that the divergence of two sibling species was not driven by the geographic isolation, and they were not originated from the ring-species model; however, the basin-surrounding distribution pattern and reproductive barrier between them meet some criteria for being a ring species.  相似文献   
947.
Zhang  Qi  Li  Danyang  Dong  Xue  Zhang  Xiaowen  Liu  Junwu  Peng  Lili  Meng  Bo  Hua  Qi  Pei  Xinyu  Zhao  Lu  Hu  Xiaoxi  Zhang  Yang  Pan  Zhenwei  Lu  Yanjie  Yang  Baofeng 《中国科学:生命科学英文版》2022,65(6):1198-1212
Science China Life Sciences - Diabetic cardiomyopathy (DCM) is a common complication in diabetic patients. The molecular mechanisms of DCM remain to be fully elucidated. The intronic long noncoding...  相似文献   
948.
Intestinal flora provides an important contribution to the development of pulmonary tuberculosis (PTB). We performed a cross-sectional study in 52 healthy controls (HCs) and 83 patients with untreated active PTB to assess the differences in their microbiomic and metabolic profiles in faeces via V3-V4 16S rRNA gene sequencing and gas chromatography–mass spectrometry. Patients with PTB had considerable reductions in phylogenetic alpha diversity and the production of short-chain fatty acids, dysbiosis of the intestinal flora and alterations in the faecal metabolomics composition compared with HCs. Significant alterations in faecal metabolites were associated with changes in the relative abundance of specific genera. Our study describes the imbalance of the gut microbiota and altered faecal metabolomics profiles in patients with PTB; the results indicate that the gut microbiota and faecal metabolomic profiles can be used as potential preventive and therapeutic targets for PTB.  相似文献   
949.
Acinetobacter baumannii poses a serious threat to human health, mainly because of its widespread distribution and severe drug resistance. However, no licensed vaccines exist for this pathogen. In this study, we created a conjugate vaccine against A. baumannii by introducing an O-linked glycosylation system into the host strain. After demonstrating the ability of the vaccine to elicit Th1 and Th2 immune responses and observing its good safety in mouse a model, the strong in vitro bactericidal activity and prophylactic effects of the conjugate vaccine against infection were further demonstrated by evaluating post-infection tissue bacterial loads, observing suppressed serum pro-inflammatory cytokine levels. Additionally, the broad protection from the vaccine was further proved via lethal challenge with A. baumannii. Overall, these results indicated that the conjugate vaccine could elicit an efficient immune response and provide good protection against A. baumannii infection in murine sepsis models. Thus, the conjugate vaccine can be considered as a promising candidate vaccine for preventing A. baumannii infection.  相似文献   
950.
每日盐度波动对真盐生植物盐地碱蓬种内相互作用沿盐度梯度的影响 土壤盐度的异质性是河口潮间带的一个突出的环境特征,影响植物的生长和盐沼中生物相互作用的转变。本研究旨在探究盐度梯度和盐度波动对一种真盐生植物的种内相互作用的交互影响。  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