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   41441篇
  免费   77篇
  国内免费   104篇
系统科学   207篇
丛书文集   800篇
教育与普及   101篇
理论与方法论   245篇
现状及发展   17466篇
研究方法   1651篇
综合类   20530篇
自然研究   622篇
  2013年   316篇
  2012年   555篇
  2011年   1208篇
  2010年   244篇
  2008年   695篇
  2007年   757篇
  2006年   791篇
  2005年   766篇
  2004年   707篇
  2003年   735篇
  2002年   746篇
  2001年   1347篇
  2000年   1252篇
  1999年   792篇
  1992年   742篇
  1991年   613篇
  1990年   624篇
  1989年   659篇
  1988年   638篇
  1987年   647篇
  1986年   660篇
  1985年   765篇
  1984年   648篇
  1983年   544篇
  1982年   474篇
  1981年   455篇
  1980年   581篇
  1979年   1280篇
  1978年   1106篇
  1977年   1061篇
  1976年   777篇
  1975年   856篇
  1974年   1246篇
  1973年   1044篇
  1972年   1073篇
  1971年   1341篇
  1970年   1732篇
  1969年   1273篇
  1968年   1253篇
  1967年   1239篇
  1966年   1106篇
  1965年   787篇
  1964年   180篇
  1959年   450篇
  1958年   693篇
  1957年   554篇
  1956年   449篇
  1955年   386篇
  1954年   445篇
  1948年   260篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
Summary A new bromotyrosine-derived alkaloid with antileukemic activity, purealidin A (5), has been isolated from the Okinawan marine spongePsammaplysilla purea and its chemical structure elucidated on the basis of the spectroscopic data.  相似文献   
22.
H Namdari  E J Bottone 《Experientia》1991,47(5):434-436
Aeromonas caviae isolated from stools of diarrheic formula-fed infants and environmental sources produce acetic acid when grown in glucose broth, which is bactericidal (suicide phenomenon). A. caviae grows anaerobically in a minimal medium or under permissive conditions such as the intestinal tract of formula-fed infants. These isolates adhered to HEp-2 cells and produced a cytotoxic and a cytotonic enterotoxin which underscore their enteropathogenicity.  相似文献   
23.
This article presents a case example involving the renaming of an organizational change process from BPR to outsourcing. The paper discusses the important ramifications of such a name change in that the recognition of outsourcing allowed an organizational commitment to counseling and termination advice. The paper discusses two ways of looking at the name change, one from a critical theory perspective and one from a realist perspective. It demonstrates how theory can provide useful yet markedly different interpretations of such organizational events. Critical theory operates from within what can be termed the transitive epistemological dimension, whereas critical realism tends to emphasize the importance of ontological issues. Each has important things to say about the situation and improves our understanding overall.  相似文献   
24.
P Villa  J Courtin 《Nature》1991,351(6328):613-614
  相似文献   
25.
P Meyrand  J Simmers  M Moulins 《Nature》1991,351(6321):60-63
  相似文献   
26.
Analysis of 78 Huntington's disease (HD) chromosomes with multi-allele markers revealed 26 different haplotypes, suggesting a variety of independent HD mutations. The most frequent haplotype, accounting for about one third of disease chromosomes, suggests that the disease gene is between D4S182 and D4S180. However, the paucity of an expected class of chromosomes that can be related to this major haplotype by assuming single crossovers may reflect the operation of other mechanisms in creating haplotype diversity. Some of these mechanisms sustain alternative scenarios that do not require a multiple mutational origin for HD and/or its positioning between D4S182 and D4S180.  相似文献   
27.
D C Gautam  L Kapoor 《Experientia》1991,47(3):280-282
Genotoxic effects of dithane M-45 were studied on the bone marrow cells of male albino mice (Lacca strain) in vivo. Different doses (30 mg, 40 mg and 300 mg/kg b.wt) of dithane M-45 were injected intraperitoneally and their effects were investigated after time intervals of 1, 2, 5 and 10 days. The chromosomal aberrations observed in the bone marrow cells of male mice after treatment with dithane M-45 were fragments, rings, dicentric chromosomes, terminal chromatid deletions, chromatid gaps and breaks. In addition to these chromosomal aberrations, physiological effects such as uneven stretching of chromatin material, end-to-end chromosomal associations, exchange configurations, clumping, stickiness and centromeric associations were also observed.  相似文献   
28.
BARNOTHY J 《Nature》1948,161(4096):681
  相似文献   
29.
30.
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