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   20674篇
  免费   1745篇
  国内免费   61篇
  22480篇
  2022年   152篇
  2021年   239篇
  2020年   141篇
  2019年   199篇
  2018年   242篇
  2017年   200篇
  2016年   339篇
  2015年   625篇
  2014年   675篇
  2013年   1035篇
  2012年   1134篇
  2011年   1170篇
  2010年   718篇
  2009年   683篇
  2008年   1068篇
  2007年   1105篇
  2006年   1014篇
  2005年   1016篇
  2004年   950篇
  2003年   935篇
  2002年   909篇
  2001年   224篇
  2000年   183篇
  1999年   262篇
  1998年   266篇
  1997年   194篇
  1996年   208篇
  1995年   182篇
  1994年   199篇
  1993年   166篇
  1992年   179篇
  1991年   168篇
  1990年   146篇
  1989年   157篇
  1988年   172篇
  1987年   158篇
  1986年   147篇
  1985年   183篇
  1984年   200篇
  1983年   189篇
  1982年   205篇
  1981年   239篇
  1980年   207篇
  1979年   163篇
  1978年   191篇
  1977年   169篇
  1976年   141篇
  1975年   146篇
  1974年   164篇
  1973年   161篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
63.
A Gram-negative, psychrophilic bacterium, designated GB-2, was isolated from fetal calf serum and analyzed for its morphological, physiological, and biochemical characteristics. Gliding motility, sensitivity to actinomycin D, the presence of the pigment flexirubin and cytochrome c, growth on a variety of carbohydrates, the production of acid fermentation products, and a 34.9 mol% guanine+cytosine (G+C) content of bacterial DNA indicate GB-2 to be a member either of the generaCytophaga orFlexibacter. Growth of GB-2 was optimized in a simple defined medium to facilitate isolation and characterization of bacterial products. Liquid growth of GB-2 resulted in the release of significant quantities of a macromolecule free of both endotoxin and protein into the growth supernatant, which activated the proliferation of murine lymphocytes. The relationship between this bacterium and its end-products to other species of theCytophaga/Flexibacter group is discussed.  相似文献   
64.
65.
66.
67.
Thirty-nine isolates of Cryptococcus neoformans, nineteen serotype AD and twenty serotype BC, were assayed for susceptibility to eight antifungal agents using an in vitro agar dilution assay. Media employed were Kimmig agar and yeast nitrogen base supplemented with 10% glucose. The antifungal agents used were ketoconazole, amphotericin B, 5-fluorocytosine, nystatin, miconazole, BAY N 7133, ICI 153,066, and itraconazole. No clinically significant differences in in vitro minimum inhibitory concentrations were detected between serotypes AD and BC against any of the compounds tested. An adverse medium effect was observed in two of the assays, but the outcome of the AD/BC comparison was not affected. This is the first report in which the in vitro antifungal susceptibilities of Cryptococcus neoformans serotypes are analyzed.  相似文献   
68.
Increased longevity and recently intensified emphasis on gerodontology mandate that dental students'exposure to clinical care of the elderly be enhanced. The extent to which individual students can be exposed to geriatric dental care depends on the availability of elderly patients to dental school clinics and the treatment needs of these patients. The purposes of this study were (1) to determine the dental treatment needs of geriatric patients who seek care at a dental school, and (2) to ascertain if differences exist between the needs of older versus relatively young geriatric patients. Data collected on 128 elderly patients during a three month period indicate that 57% of the aged were either edentulous at examination or treatment planned to become edentulous and receive two full dentures. The remaining 43% were treatment planned to remain dentulous and receive therapies other than full upper and lower dentures. More older geriatric patients required full dentures than their younger cohorts. More young elderly required prophylaxes, scalings, root planing therapy, dental restorations, and partial dentures. Additional to their denture requirements, aged patients appear to have sufficient non-prosthetic needs to allow for meaningful gerodontic experience by students.  相似文献   
69.
Properties of Mutants in Galactose Taxis and Transport   总被引:29,自引:17,他引:12  
beta-Methylgalactoside (mgl) permease mutants of Escherichia coli, which are defective in three genes, mglA, mglB, and mglC, were assayed for galactose taxis and galactose transport. The mglB product is the galactose-binding protein. Previous evidence, supported by our new findings, shows that the galactose-binding protein is the recognition component for galactose taxis as well as for galactose transport. Most mutants defective in mglB showed strong effects on both chemotaxis and transport; however, a couple showed effects chiefly on one process or the other, thus allowing a separation of chemotaxis and transport. The mglA and mglC products have not yet been identified, but they must be components of the galactose transport machinery since mutants defective in mglA or mglC, or both, showed strongly reduced transport. Although some of these mutants showed little chemotaxis, most gave close to wild-type chemotactic responses. Thus, transport is not required for galactose taxis. The bacteria detect changes in the fraction of binding protein associated with galactose, not changes in the rate of transport.  相似文献   
70.
Does New Guinea cannibalism have nutritional value?   总被引:1,自引:0,他引:1  
This paper examines the question of the nutritional value of cannibalism. Although other authors have concluded that the practice does not have such value, we argue that this cannot properly be determined except in the context of the total subsistence economy and local human ecology. The paper also presents a format for the empirical investigation of foodgetting and new ethnographic information about New Guinea cannibalism. Our major conclusion is that this practice does have nutritional value for certain human groups, specifically tropical peoples living at lowmedium population densities and exploiting a diverse range of animal foods.  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