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   17689篇
  免费   58篇
  国内免费   87篇
系统科学   74篇
丛书文集   121篇
教育与普及   45篇
理论与方法论   41篇
现状及发展   7452篇
研究方法   862篇
综合类   9040篇
自然研究   199篇
  2013年   146篇
  2012年   276篇
  2011年   450篇
  2009年   115篇
  2008年   345篇
  2007年   391篇
  2006年   392篇
  2005年   383篇
  2004年   337篇
  2003年   356篇
  2002年   311篇
  2001年   688篇
  2000年   673篇
  1999年   412篇
  1994年   312篇
  1992年   385篇
  1991年   273篇
  1990年   332篇
  1989年   291篇
  1988年   254篇
  1987年   313篇
  1986年   313篇
  1985年   386篇
  1984年   279篇
  1983年   264篇
  1982年   246篇
  1981年   226篇
  1980年   217篇
  1979年   597篇
  1978年   434篇
  1977年   402篇
  1976年   350篇
  1975年   357篇
  1974年   433篇
  1973年   393篇
  1972年   352篇
  1971年   415篇
  1970年   569篇
  1969年   433篇
  1968年   446篇
  1967年   412篇
  1966年   383篇
  1965年   287篇
  1959年   145篇
  1958年   242篇
  1957年   155篇
  1956年   154篇
  1955年   138篇
  1954年   140篇
  1948年   124篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
    
Summary The seedling growth ofBrassica campestris var.varuna, has been studied, as affected by water-stress and gibberellin treatments. A boost in the net GA response due to water-stress, has been observed. Thus presence of GA can overcome the water-stress effects.Publication No. 26 from Dept of Biosciences, Himachal Pradesh University, Simla. Authors are grateful to CSIR, New Delhi, for JRF to SB.  相似文献   
82.
Summary A 2-dimensional thin-layer method has been developed for the separation on cellulose of adenine and guanine derivatives. Using incubated rat cerebral cortex slices it was shown that noradrenaline and acetylcholine stirnulated cAMP and cGMP production respectively but glutamate and -aminobutyric acid stimulated production of both cyclic nucleotides.  相似文献   
83.
Summary Eradication of replicating bone marrow cells of rat by means of combined administration of single doses of hydroxyurea and vinblastin is followed within 9–10 h by an inflow of lymphoid cells of extramedullary origin in the range of 13,200,000/femur. The rat bone marrow with a high content of lymphoid cells was previously shown to be concentrated in stem cells. The factor(s) which convey the information of decrease of replicating marrow cells to extramedullary sites is at present unknown.Acknowledgment. This study was supported by a grant from the Chief Scientist Office, Ministry of Health, Israel.Hydroxyurea for this investigation was given as a gift by the Squibb Institute of Medical Research, Princeton, N.J. USA, to which the authors are indebted.  相似文献   
84.
Summary The effects of time and various doses of testosterone on the responsiveness of lactate dehydrogenase of pituitary of 7-, 38- and 78-week-old rats were studied. The activity of the enzyme increases in 78-week-old rats. Castration decreases the enzyme activity at all ages. Maximum increase in the enzyme activity is seen with 50 and 100 g of testosterone 4 h after administration of hormone to castrated rats. No further time and dose-dependent effect is observed. The magnitude of increase for the enzyme is higher at the age of 38 weeks and decreases in 78-week-old rats.To whom all correspondence should be addressed. The authors are grateful to Professor M.S. Kanungo, Biochemistry Laboratory, Department of Zoology, Banaras Hindu University for the suggestions and encouragement. K.V.G. thanks the PL-480 for a research fellowship.  相似文献   
85.
Under well defined conditions ribosomal RNAs purified from Escherichia coli can be degraded by ribonuclease U2 giving rise to RNA fragments of 60--70 nucleotides. In vitro, these fragments are efficiently transcribed into a complementary DNA by DNA polymerase RNA dependent, partially purified from extracts of E. coli. In vivo, "RNA-fragments-U2" inhibit the development of plant tumors.  相似文献   
86.
Summary Prostaglandins (PGs) E2, F2, and I2 were examined for their effects on the electrical and mechanical activities of isolated rat, rabbit and guinea-pig hearts. All PGs produced dysrhythmias in rat hearts at low concentrations only, while higher concentrations were antiarrhythmic. Guinea-pig hearts were less responsive while rabbit hearts were completely resistant.We thank the Manitoba Heart Foundation for financial support and Dr J. Pike (Upjohn, Kalamazoo, MI, USA) for gifts of prostaglandins. Dr M. Karmazyn is a Fellow of the Canadian Heart Foundation.  相似文献   
87.
Summary After a dose of 3.0 Gy in the peak position of the pion depth-dose curve, the ratios between observed chromosome aberration yields under conditions of oxygenation and of anoxia were obtained for lymphocytes exposed at 3 depths in a plastic phantom. These ratios were 3.7, 1.9 and 1.3 in the plateau, peak and post-peak positions, respectively, suggesting a corresponding decrease in the oxygen enhancement ratio.Acknowledgments. We wish to thank Dr D.H. Reading, Mr M.A. Hynes and Mr W. Spinks for their assistance at the Rutherford Laboratory's -II beam line. This work was partly supported by Euratom Contract 171-76-1 BIO-UK.  相似文献   
88.
Summary 2-Halogeno-ethanols change the active site structure of -chymotrypsin more rapidly and effectively than ethanol, 1-propanol and urea, probably before producing an extensive conformation change.  相似文献   
89.
Summary Protein from resting or phytohemagglutinin-stimulated human peripheral blood T-lymphocytes, pulse-labeled in vitro for 1 h with3H-leucine, had a half-life of 30 h.  相似文献   
90.
Summary DNA-containing areas in various phages (T4, , T7 and 29) could be observed at the light microscopic level using ethidium bromide epi-fluorescent microscopy. The fluorescent intensity per phage was in linear proportion to the DNA content in each phage.Acknowledgements. This work was supported in part by the grant No. 521708 and No. 511212 from the Japan Ministry of Education, Science and Culture.  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