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   225682篇
  免费   15113篇
  国内免费   131篇
  240926篇
  2021年   1846篇
  2019年   1726篇
  2018年   3200篇
  2017年   3083篇
  2016年   3730篇
  2015年   3283篇
  2014年   4538篇
  2013年   6424篇
  2012年   7696篇
  2011年   8184篇
  2010年   5815篇
  2009年   4941篇
  2008年   7273篇
  2007年   7545篇
  2006年   7249篇
  2005年   6758篇
  2004年   6892篇
  2003年   6618篇
  2002年   6575篇
  2001年   9310篇
  2000年   8990篇
  1999年   7021篇
  1998年   2136篇
  1997年   1976篇
  1996年   1832篇
  1992年   5485篇
  1991年   5702篇
  1990年   5661篇
  1989年   5632篇
  1988年   5216篇
  1987年   4874篇
  1986年   4448篇
  1985年   4682篇
  1984年   3696篇
  1983年   3029篇
  1982年   2017篇
  1980年   1729篇
  1979年   3341篇
  1978年   2579篇
  1977年   2377篇
  1976年   2331篇
  1975年   2829篇
  1974年   3120篇
  1973年   3088篇
  1972年   2747篇
  1971年   2608篇
  1970年   2213篇
  1969年   2159篇
  1968年   2032篇
  1967年   1799篇
排序方式: 共有10000条查询结果,搜索用时 0 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