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   228810篇
  免费   15341篇
  国内免费   52篇
  244203篇
  2021年   1895篇
  2018年   3226篇
  2017年   3102篇
  2016年   3805篇
  2015年   3423篇
  2014年   4687篇
  2013年   6643篇
  2012年   7947篇
  2011年   8538篇
  2010年   5982篇
  2009年   5100篇
  2008年   7500篇
  2007年   7835篇
  2006年   7463篇
  2005年   6955篇
  2004年   7068篇
  2003年   6803篇
  2002年   6761篇
  2001年   9314篇
  2000年   8976篇
  1999年   7021篇
  1998年   2162篇
  1997年   1991篇
  1996年   1845篇
  1992年   5478篇
  1991年   5688篇
  1990年   5655篇
  1989年   5631篇
  1988年   5219篇
  1987年   4869篇
  1986年   4448篇
  1985年   4684篇
  1984年   3706篇
  1983年   3030篇
  1982年   2024篇
  1981年   1732篇
  1980年   1737篇
  1979年   3342篇
  1978年   2581篇
  1977年   2393篇
  1976年   2346篇
  1975年   2838篇
  1974年   3121篇
  1973年   3093篇
  1972年   2758篇
  1971年   2608篇
  1970年   2215篇
  1969年   2166篇
  1968年   2041篇
  1967年   1803篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
The study of the specific epidemiological and clinical features of Q fever revealed the existence of an active focus of infection among humans due to their contacts with agricultural animals in one of the districts of the region. The focus was manifested by group morbidity among the cattle-tending personnel of a dairy farm. The source of this infection was cattle. The infection was transferred mainly through the air. The disease took a moderately severe course. The study of the rickettsial contamination of humans, animals and ticks suggested the presence of the active epidemic process and made it possible to work out concrete antiepidemic measures.  相似文献   
12.
13.
Mutants of Bacillus amyloliquefaciens resistant to at least 10 micrograms/ml of tunicamycin were isolated and shown to be pleiotropic. The mutants were more resistant to streptomycin, chloramphenicol, kanamycin and neomycin than was the parent strain but less resistant to penicillin G and tetracycline. They were more autolytic, presumably due to an altered cell wall. The mutants produced reduced levels of amylase, penicillinase and both metal and serine protease besides having an enhanced sporulation frequency and being more motile.  相似文献   
14.
The content of neurospecific proteins S-100, GFA and D2 was measured in malignant cerebral tumors by electrophoresis with the use of monospecific antisera. Concomitant measurement of proteins S-100 and GFA is a more reliable diagnostic criterion as to the tumor histogenesis than study of each protein alone. D2 protein appeared to be the most stable specific marker.  相似文献   
15.
16.
17.
The paper presents data on a change in lysozyme content in tissues of spleen, liver and heart in fry of the Lena River sturgeon exposed to the presence of sublethal concentrations of Hg2+, Cd2+, and Cu2+ under conditions of chronic experiment. It has been shown that the lysozyme content in fish tissues varies and has a phasic character. The amplitude of fluctuations of this parameter depends on the moment of sampling, nature of the toxicant, and structural-functional organization of the studied organs.  相似文献   
18.
19.
An agar-degrading marine bacterium identified as a Microscilla species was isolated from coastal California marine sediment. This organism harbored a single 101-kb circular DNA plasmid designated pSD15. The complete nucleotide sequence of pSD15 was obtained, and sequence analysis indicated a number of genes putatively encoding a variety of enzymes involved in polysaccharide utilization. The most striking feature was the occurrence of five putative agarase genes. Loss of the plasmid, which occurred at a surprisingly high frequency, was associated with loss of agarase activity, supporting the sequence analysis results.  相似文献   
20.
Self-consistent steady-state axisymmetric configurations of a plasma envelope with a uniform anisotropic conductivity around a rotating magnetized spherical body are considered. A set of electrodynamic and magnetohydrodynamic equations is analyzed under the assumption that the mass velocity of a moving weakly ionized plasma has only the azimuthal component. The equations describing the profile of the angular frequency of the rotating plasma envelope, the magnetic field, the conduction currents, and the plasma density distribution are solved in the limit of a strong anisotropy of the conductivity of a weakly ionized gas. The applicability of the results obtained to a qualitative interpretation of the phenomena occurring in the plasmaspheres of magnetized planets is discussed.  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