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   1411244篇
  免费   126294篇
  国内免费   1959篇
  2021年   18079篇
  2019年   16281篇
  2018年   18188篇
  2017年   16909篇
  2016年   28462篇
  2015年   42962篇
  2014年   51055篇
  2013年   77452篇
  2012年   41213篇
  2011年   30927篇
  2010年   45087篇
  2009年   46005篇
  2008年   28656篇
  2007年   26904篇
  2006年   31669篇
  2005年   32517篇
  2004年   31706篇
  2003年   28935篇
  2002年   26864篇
  2001年   35075篇
  2000年   32416篇
  1999年   32270篇
  1998年   25717篇
  1997年   25586篇
  1996年   25030篇
  1995年   23186篇
  1994年   22940篇
  1993年   22052篇
  1992年   28160篇
  1991年   26726篇
  1990年   25416篇
  1989年   26057篇
  1988年   23945篇
  1987年   22605篇
  1986年   21700篇
  1985年   23401篇
  1984年   23047篇
  1983年   20391篇
  1982年   20638篇
  1981年   19816篇
  1980年   18555篇
  1979年   19052篇
  1978年   18083篇
  1977年   17210篇
  1976年   16369篇
  1975年   16157篇
  1974年   16680篇
  1973年   16995篇
  1972年   14445篇
  1971年   13321篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
A new method for quantification of airborne birch and grass pollen allergens collected on porous polytetrafluoroethylene filters has been developed. In this method, the allergens firmly adsorbed to the sampling filter of 25 mm in diameter are reacted with specific antibodies conjugated to alkaline phosphatase, generating a matrix-bound allergen-antibody-phosphatase complex. The filter is then floated on a chemiluminescent enzyme substrate solution. The light intensity of the product is linearly related to the amount of allergen over a large mass range, 0-1000 SQ (1 SQ is about 250 pg of protein). This direct on sampling filter in solution (DOSIS) technique demonstrated intra-assay precisions between 6-16% and 11-15% for the levels of 1-100 SQ units of grass allergen Phl p 5 and 4-400 SQ units of birch allergen Bet v 1, respectively. The limits of quantification for the corresponding allergens were estimated to 0.5 and 2 SQ units. Application of DOSIS to analysis of the grass pollen allergen concentrations of outdoor air for 12 days in July 1998 revealed a correlation coefficient of 0.69 between pollen grain and allergen concentrations for the dry weather period. After rainy days large amounts of grass allergens were present even in the absence of pollen grains.  相似文献   
993.
994.
1. The ability to achieve optimal camouflage varies between microhabitats in heterogeneous environments, potentially restricting individuals to a single habitat or imposing a compromise on crypsis to match several habitats. However, animals may exhibit morphological and behavioural attributes that enhance crypsis in different habitats. 2. We used an undescribed fish species, Galaxias‘nebula’, to investigate two objectives. First, we examined two potential methods of enhancing crypsis: change in colour pattern and selection of a suitable background. Second, we characterised the colour pattern of this unstudied fish and assessed its capacity for crypsis. 3. No background selection was apparent but the area of dark pigment expressed varied between backgrounds, which may negate the requirement to be choosy about habitats. The capacity to change colour and selection of a background that maximises crypsis are most likely separate, non‐mutually exclusive strategies. 4. Galaxias‘nebula’ exhibits polymorphic, non‐interchangeable colour patterns that have elements of both background pattern matching and disruptive colouration. This, coupled with habitat characteristics, suggests a combination of generalist and specialist strategies of habitat use. The fish’s camouflage strategy and air‐breathing ability may be key to survival under increasing pressure from habitat degradation and invasive predators.  相似文献   
995.
996.
997.
998.
To effectively integrate DNA sequence analysis and classical nematode taxonomy, we must be able to obtain DNA sequences from formalin-fixed specimens. Microdissected sections of nematodes were removed from specimens fixed in formalin, using standard protocols and without destroying morphological features. The fixed sections provided sufficient template for multiple polymerase chain reaction-based DNA sequence analyses.  相似文献   
999.
Flow cytometer measurements were made of the basal variations in peripheral blood functional monocytes and granulocytes over the course of a training season (January to November) of a cycling team. Parallel determinations were made of plasma concentration of catecholamines (chromatography) and cortisol (RIA) in a search for neuroendocrine markers. The results showed the greatest phagocytic capacity to occur in the central months (March, May, and July), coinciding with the greatest number and highest level of competitive events with good correlation with a peak in epinephrine during these months (r(2) = 0.998 for monocytes and r(2) = 0.674 for granulocytes). No good correlations were found between phagocytosis and norepinephrine or cortisol. The highest values for phagocytosis and epinephrine concentration were found in May. These results suggest that blood epinephrine concentration could be a good neuroendocrine marker of sportspeople's phagocytic response.  相似文献   
1000.
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