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   949篇
  免费   53篇
  1002篇
  2022年   13篇
  2021年   14篇
  2020年   9篇
  2019年   9篇
  2018年   13篇
  2017年   10篇
  2016年   21篇
  2015年   26篇
  2014年   45篇
  2013年   53篇
  2012年   63篇
  2011年   48篇
  2010年   30篇
  2009年   30篇
  2008年   26篇
  2007年   37篇
  2006年   38篇
  2005年   30篇
  2004年   29篇
  2003年   23篇
  2002年   30篇
  2001年   23篇
  2000年   25篇
  1999年   15篇
  1997年   12篇
  1996年   15篇
  1995年   8篇
  1993年   7篇
  1992年   22篇
  1991年   17篇
  1990年   24篇
  1989年   11篇
  1988年   11篇
  1987年   11篇
  1986年   12篇
  1985年   17篇
  1984年   9篇
  1983年   8篇
  1982年   7篇
  1981年   7篇
  1979年   14篇
  1978年   8篇
  1975年   9篇
  1974年   14篇
  1973年   11篇
  1971年   8篇
  1970年   7篇
  1969年   7篇
  1968年   8篇
  1966年   7篇
排序方式: 共有1002条查询结果,搜索用时 0 毫秒
21.
Recent investigations on the nutritive role of different carbohydrates for the survival and growth of Oryzaephilus mercator show that fructose, glucose, dextrin and glycogen are of high nutritive value, fructose being even better than natural control diet (wheat + 5% yeast). Maltose, soluble starch, mannitol and sorbitol, although support some growth, are not of as good nutritive value as control diet. All pentoses, sorbose, galactose, lactose, raffinose, potato starch, inulin and dulcitol are of no nutritive value.  相似文献   
22.
Carpel closure and stigmatic localization in the Alismatales have accompanied decreases in carpel size and numbers of ovules per carpel and increases in carpel numbers per flower. The most specialized carpels are uniovulate and indehiscent and occur acyclically in great numbers in each flower, with strong trends toward monoecism and even dioecism. The least specialized carpels in the order are open, multiovulate, and conduplicate, with poor differentiation of style and stigma. The Alismatales show a broad range from primitive to specialized gynoecial features.  相似文献   
23.
24.
25.
BackgroundEnterotoxigenic Escherichia coli (ETEC) is one of the top aetiologic agents of diarrhea in children under the age of 5 in low-middle income countries (LMICs). The lack of point of care diagnostic tools for routine ETEC diagnosis results in limited data regarding the actual burden and epidemiology in the endemic areas. We evaluated performance of the novel Rapid LAMP based Diagnostic Test (RLDT) for detection of ETEC in stool as a point of care diagnostic assay in a resource-limited setting.MethodsWe conducted a cross-sectional study of 324 randomly selected stool samples from children under 5 presenting with moderate to severe diarrhea (MSD). The samples were collected between November 2012 to September 2013 at selected health facilities in Zambia. The RLDT was evaluated by targeting three ETEC toxin genes [heat labile toxin (LT) and heat stable toxins (STh and STp)]. Quantitative PCR was used as the “gold standard” to evaluate the diagnostic sensitivity and specificity of RLDT for detection of ETEC. We additionally described the prevalence and seasonality of ETEC.ResultsThe study included 324 participants, 50.6% of which were female. The overall prevalence of ETEC was 19.8% by qPCR and 19.4% by RLDT. The children between 12 to 59 months had the highest prevalence of 22%. The study determined ETEC toxin distribution was LT 28/321(9%), ST 18/321(6%) and LT/ST 16/321(5%). The sensitivity and specificity of the RLDT compared to qPCR using a Ct 35 as the cut-off, were 90.7% and 97.5% for LT, 85.2% and 99.3% for STh and 100% and 99.7% for STp, respectively.ConclusionThe results of this study suggest that RLDT is sufficiently sensitive and specific and easy to implement in the endemic countries. Being rapid and simple, the RLDT also presents as an attractive tool for point-of-care testing at the health facilities and laboratories in the resource-limited settings.  相似文献   
26.
27.
Warming, eutrophication (nutrient fertilization) and brownification (increased loading of allochthonous organic matter) are three global trends impacting lake ecosystems. However, the independent and synergistic effects of resource addition and warming on autotrophic and heterotrophic microorganisms are largely unknown. In this study, we investigate the independent and interactive effects of temperature, dissolved organic carbon (DOC, both allochthonous and autochthonous) and nitrogen (N) supply, in addition to the effect of spatial variables, on the composition, richness, and evenness of prokaryotic and eukaryotic microbial communities in lakes across elevation and N deposition gradients in the Sierra Nevada mountains of California, USA. We found that both prokaryotic and eukaryotic communities are structured by temperature, terrestrial (allochthonous) DOC and latitude. Prokaryotic communities are also influenced by total and aquatic (autochthonous) DOC, while eukaryotic communities are also structured by nitrate. Additionally, increasing N availability was associated with reduced richness of prokaryotic communities, and both lower richness and evenness of eukaryotes. We did not detect any synergistic or antagonistic effects as there were no interactions among temperature and resource variables. Together, our results suggest that (a) organic and inorganic resources, temperature, and geographic location (based on latitude and longitude) independently influence lake microbial communities; and (b) increasing N supply due to atmospheric N deposition may reduce richness of both prokaryotic and eukaryotic microbes, probably by reducing niche dimensionality. Our study provides insight into abiotic processes structuring microbial communities across environmental gradients and their potential roles in material and energy fluxes within and between ecosystems.  相似文献   
28.
International Journal of Peptide Research and Therapeutics - Elevation high plasma cholesterol plays a significant role in promoting the incidence of atherosclerosis and coronary heart disease. In...  相似文献   
29.
30.
Abstract

The present paper deals with the embryological studies of ten species of Taraxacum, out of which nine polyploid species are apomictic and one diploid species T. wallichii shows normal sexual behaviour. Occasional formation of normal megaspore tetrads and triads are observed in tetraploid and triploid species.

Autonomous development of endosperm from the secondary nucleus seems to be the rule. The growth of endosperm is relatively slow.  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