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   4452篇
  免费   237篇
  国内免费   265篇
  2023年   62篇
  2022年   64篇
  2021年   104篇
  2020年   123篇
  2019年   133篇
  2018年   87篇
  2017年   103篇
  2016年   113篇
  2015年   127篇
  2014年   216篇
  2013年   298篇
  2012年   177篇
  2011年   224篇
  2010年   153篇
  2009年   168篇
  2008年   222篇
  2007年   222篇
  2006年   200篇
  2005年   178篇
  2004年   176篇
  2003年   154篇
  2002年   128篇
  2001年   124篇
  2000年   114篇
  1999年   121篇
  1998年   76篇
  1997年   70篇
  1996年   73篇
  1995年   80篇
  1994年   74篇
  1993年   63篇
  1992年   54篇
  1991年   51篇
  1990年   39篇
  1989年   44篇
  1988年   42篇
  1987年   43篇
  1986年   50篇
  1985年   51篇
  1984年   31篇
  1983年   33篇
  1982年   42篇
  1981年   25篇
  1980年   42篇
  1979年   31篇
  1978年   34篇
  1977年   20篇
  1976年   18篇
  1975年   24篇
  1972年   16篇
排序方式: 共有4954条查询结果,搜索用时 125 毫秒
991.
992.
Kunduru is an important Anatolian landrace having peculiar traits that are appreciated by farmers and breeders. 33 accessions known as Kunduru collected by ICARDA from six geographical provinces of Turkey, were used to study the phenotypic and genotypic intra-diversity. Kunduru landraces exhibited high intra-diversity for most of the studied morphological traits. GPC (12.10–14.90%), vitreousness (75–100%), TKW (31.80–56.70 g), YP (4.70–8.00 ppm), b*-value (14.30–19.50), ash content (1.60–2.0%) and gluten strength (14–60 ml) showed marked variations. Gliadin and glutenin banding patterns showed high polymorphism. 65 alleles were detected with 14 SSR markers, giving a mean of 6.77 alleles per locus. The average PIC value was 0.44 and ranged from 0.11 to 0.70. The average genetic distance between pairs of landraces was 0.47 and ranged between 0.11 and 0.72. This study showed that Kunduru landraces maintains high allelic variation. PCoA indicated that eco-geographical variables have a significant effect on SSR diversity as well as morphological traits. Many of the landraces studied are in danger of disappearing from the local farmers' fields; this study demonstrates the importance of maintaining and conserving this precious genetic resources.  相似文献   
993.
The benzomorphan scaffold has great potential as lead structure and the nature of the N-substituent is able to influence affinity, potency, and efficacy at all three opioid receptors. Building upon these considerations, we synthesized a new series of LP1 analogues by introducing naphthyl or heteroaromatic rings in propanamide side chain of its N-substituent (915). In vitro competition-binding assays in HEK293 cells stably expressing MOR, DOR or KOR showed that in compound 9 the 1-naphthyl ring led to the retention of MOR affinity (KiMOR = 38 ± 4 nM) displaying good selectivity versus DOR and KOR. In the electrically stimulated GPI, compound 9 was inactive as agonist but produced an antagonist potency value (pA2) of 8.6 in presence of MOR agonist DAMGO. Moreover, subcutaneously administered it antagonized the antinociceptive effects of morphine with an AD50 = 2.0 mg/kg in mouse-tail flick test. Modeling studies on MOR revealed that compound 9 fit very well in the binding pocket but in a different way in respect to the agonist LP1. Probably the replacement of its N-substituent on the III, IV and V TM domains reflects an antagonist behavior. Therefore, compound 9 could represent a potential lead to further develop antagonists as valid therapeutic agents and useful pharmacological tools to study opioid receptor function.  相似文献   
994.
明确物种生境空间分布格局及其与环境因素的关系,对了解该物种的生境需求和适宜生境空间分布至关重要。生境评价和预测是对物种进行有效保护的基础。以鹅喉羚(Gazella subgutturosa)为研究对象,以其重要栖息地新疆博州艾比湖国家级湿地自然保护区为研究区域,选取115个鹅喉羚分布点数据和23个环境变量因子,应用MAXENT模型分析其生境空间分布及主要影响因子,划分了鹅喉羚在研究区域的适宜生境,并对它的栖息地特征进行了分析。探讨了鹅猴羚生境选择与环境因子的关系。结果表明:气温日较差是影响鹅喉羚生境分布的主要环境因子。植被类型,坡度和最干月降水量对艾比湖鹅喉羚的生境选择影响不大。除了温度和降水在内的19项生物气候变量是鹅猴羚选择生境的重要因素之外,海拔和坡向等地形特征也影响鹅猴羚的生境选择性。鹅喉羚的高度适宜生境区主要分布在研究区域的北部和东部,中度及低度适宜生境区则分布于高度适宜生境区的边缘,而非适宜生境区主要集中在西部地区。研究不仅提供了鹅喉羚在艾比湖的实际分布状况及其栖息地特征,也为鹅喉羚在栖息地方面的研究,即鹅猴羚的栖息地选择和环境因子的关系方面提供了一个重要的依据。  相似文献   
995.
Heart failure with preserved ejection fraction (HFpEF) is a growing healthcare burden worldwide and its prevalence is increasing. Diagnosing HFpEF is challenging and relies upon the presence of symptoms and/or signs of heart failure, preserved left ventricular systolic function, and evidence of diastolic dysfunction. Current diagnostic algorithms mainly rely on echocardiography (E/e’) and biomarkers (NT-proBNP). However, only a minority of patients with HFpEF are identified, and especially HFpEF patients at an early stage of the disease are easily missed. We propose to incorporate invasive stress testing, by means of right heart catheterisation at rest and during exercise, and accurate assessment of right ventricular function, by means of cardiac magnetic resonance imaging. These additions to the current diagnostic work-up will improve diagnostic sensitivity and accurate staging of HFpEF patients.  相似文献   
996.
WONG  M. Y. 《Biometrika》1989,76(1):141-148
  相似文献   
997.
.Using fixed sporozoites in a 3-layer immunofluorescence assay (TLIFA), class-specific, parasite-specific antibody responses in chicks to single-pulse infection with Eimeria tenella have been studied in gut contents and bile as well as plasma and feces. After infection with 103 oocysts, IgA antibody was first detected in the duodenal lumen, then in bile, plasma, cecum, and the distal small intestine. The kinetics of the bile IgA response correlated with that in plasma and peaked 9 days post-infection (d.p.i.); IgM was detected in gut contents and bile as well as plasma, and IgG was occasionally detected in gut contents, especially in the duodenum. In some experiments, IgA was detected in gut contents and bile to at least 21 d.p.i. Infection with 103 oocysts resulted in an earlier and increased response and relatively high IgG titers in cecal contents. Coproantibody was detected inconsistently and at low titer. When sporozoites that excysted in vitro were incubated in specific, antibody-positive (9 d.p.i.) cecal contents, some complement-mediated IgG-associated anti-sporozoite effects were observed; however, the major effect of cecal contents and the only effect of bile was a non-lethal agglutination of living sporozoites. By fractionation of cecal contents and imtnunoblotting this was confirmed to be IgA mediated; IgA antibodies in cecal contents and bile after infection were shown to bind to sporozoite membrane antigens by surface fluorescence as well as agglutination. Agglutination detected anti-sporozoite antibody in gut contents and bile up to 21 d.p.i., peaking between 7 and 13 d.p.i., corresponding with TLIFA results. The immunofluorescence studies reveal the extremely labile nature of the sporozoite surface antigens and support the hypothesis that naturally elaborated IgA antibodies are involved in the modulation of complexed sporozoite surface antigens.  相似文献   
998.
Spatial pattern and ecological analysis   总被引:65,自引:0,他引:65  
  相似文献   
999.
杭州湾、钱塘江流域沿岸外环境的肉毒梭菌分布很广。调查22个点检出阳性21个点。外环境各种样品的总检出率为75.95%,上海地区的检出率最低(29.41%),其它地区间的检出率无显著差异。绍兴市区生产的酱豆面制品肉毒梭菌的检出率为20%。检出的所有菌株几乎均为无毒株,其中2株弱毒株其毒力弱到无法定型。生化试验结果显示,被试大多数菌株的生化特性与A、B、F型的PL株相似,但有13株菌株的个别生化反应与试验菌株的总体不同。  相似文献   
1000.
More extensive genetic tests have been performed on a series of 832 X-ray-induced specific-locus mutations in the ad-3 region of a 2-component heterokaryon (H-12) of Neurospora crassa, reported earlier (Webber and de Serres 1965). Using a new tester strains and techniques for performing large-scale genetic tests (heterokaryon, dikaryon and trikaryon) to characterize ad-3 mutants induced in 2-component heterokaryons, new data have been obtained on this sample of X-ray-induced ad-3 mutants. These new data show that unexpectedly high frequencies of both single-locus (gene/point) mutations and multilocus deletions in the ad-3 region have additional, but separate, sites of resessive lethal (RLCL) damage in the immediately adjacent genetic regions. The frequencies of these X-ray-induced multiple-locus mutants in the ad-3 region are orders of magnitude higher than expected on the basis of target theory and classical models of chromosome structure during interphase. Current models of interphase chromosome structure in higher eukaryotes as revealed by chromosome “painting” offer a possible explanation of the Neurospora data.  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