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   16360篇
  免费   42篇
  国内免费   42篇
系统科学   70篇
丛书文集   108篇
教育与普及   31篇
理论与方法论   57篇
现状及发展   5847篇
研究方法   770篇
综合类   9233篇
自然研究   328篇
  2013年   187篇
  2012年   246篇
  2011年   580篇
  2010年   102篇
  2008年   263篇
  2007年   339篇
  2006年   338篇
  2005年   310篇
  2004年   308篇
  2003年   286篇
  2002年   293篇
  2001年   694篇
  2000年   682篇
  1999年   373篇
  1998年   48篇
  1992年   344篇
  1991年   272篇
  1990年   299篇
  1989年   318篇
  1988年   286篇
  1987年   268篇
  1986年   294篇
  1985年   317篇
  1984年   270篇
  1983年   249篇
  1982年   204篇
  1981年   189篇
  1980年   212篇
  1979年   506篇
  1978年   395篇
  1977年   367篇
  1976年   309篇
  1975年   350篇
  1974年   497篇
  1973年   379篇
  1972年   386篇
  1971年   497篇
  1970年   614篇
  1969年   422篇
  1968年   450篇
  1967年   391篇
  1966年   387篇
  1965年   260篇
  1959年   135篇
  1958年   225篇
  1957年   162篇
  1956年   115篇
  1955年   115篇
  1954年   110篇
  1948年   90篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Summary The male antifertility agent 1-amino-3-chloropropan-2-ol (ACP,I) has been shown to be metabolized to-chlorohydrin (III) and metabolites of-chlorohydrin. This accounts for the similar antifertility and renal toxicity effects of both compounds.  相似文献   
102.
Summary Abnormal-8-tetrahydrocannabidiol (ABN-8-THC) failed to elicit central nervous system and cardiovascular effects in laboratory animals. Abnormal-cannabidiol (ABN-CBD) was also devoid of overt behavioral effects but produced marked hypotension with only slight bradycardia in anesthesized dogs.Acknowledgment. This work was supported by NIDA (grant No. DA-00574-01 and DA-00490) and Virginia Heart Society (grant No. RR-05697)  相似文献   
103.
104.
Summary Naja mossambica pallida venom administered i.v. (300 /kg) produces an initial brief fall in blood pressure, due to a direct myocardial depressant effect, and a sustained fall due to central depressant effect.This study was supported by University of Nairobi research grants (670–376). We also thank Mr E. Njogu for photographic assistance.  相似文献   
105.
Molecular analysis of spontaneous somatic mutants.   总被引:17,自引:0,他引:17  
K Adetugbo  C Milstein  D S Secher 《Nature》1977,265(5592):299-304
Eukaryotic structural gene mutations occurring spontaneously in a mouse myeloma cell line offer the opportunity to study somatic mutation in animal cells at the molecular level. Studies on the myeloma protein and on mRNA have enabled us to characterise four such mutants representing four different mutation mechanisms. The results may have some bearing on the origin of antibody diversity.  相似文献   
106.
Summary Using the Fell technique of organ culture of cartilage in a chemically defined medium, it has been shown that prostaglandin A1 at a concentration of 25 g/ml caused chondrocyte death in chick embryonic limb rudiments. An equimolar concentration of PGE2 was not toxic to the cells.Acknowledgments. We are grateful for the support of C. J. K. by the Medical Research Council and of D. L. G. by the Arthritis and Rheumatism Council for Research. Our thanks are due to Dame Honor Fell, F. R. S. for invaluable advice and guidance, to Dr J. Pike (Upjohn Co.) for supplies of prostaglandin A1, and to Dr Sylvia Fitton-Jackson for the gift of culture medium.  相似文献   
107.
Seed germination in response to diurnal fluctuations of temperature   总被引:20,自引:0,他引:20  
Thompson K  Grime JP  Mason G 《Nature》1977,267(5607):147-149
Diurnal fluctuations in temperature may initiate or accelerate germination in certain flowering plants, and the effectiveness of the stimulus varies according to the amplitude of fluctuation and the presence or absence of light. Attempts to assess the adaptive significance of the phenomenon, however, have been limited by the scarcity of data for species of contrasted ecology. We report here an investigation of germination responses to fluctuating temperatures, conducted on seeds of herbaceous species collected from native populations near Sheffield. The results suggest that requirements for diurnal fluctuations in temperature are characteristic of the germination of species from particular types of habitat and provide mechanisms which cause seeds to germinate at times and in places favourable for seedling establishment.  相似文献   
108.
Chlordiazepoxide selectively augments GABA action in spinal cord cell cultures   总被引:24,自引:0,他引:24  
D W Choh  D H Farb  G D Fischbach 《Nature》1977,269(5626):342-344
  相似文献   
109.
Summary A dissociation between inhibition of RNA synthesis and cell lysis was observed when thymocytes of adrenalectomized rats were incubated with high concentrations of pregn-4-ene-11-ol-3, 20-dione and pregna-1,4-diene-11-ol-3,20-dione. In contrast, no dissociation of these effects was found with the typical glucocorticoids cortisol and corticosterone, nor with their 1,4-diene analogs under the same conditions.Acknowledgments. This work was supported by grants from the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina to the Instituto de Biologia y Medicina Experimental and the Programa de Regulación Hormonal y Metabólica as well as by financial help from the Secretaría de Ciencia y Tecnología.  相似文献   
110.
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