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   394902篇
  免费   45488篇
  国内免费   360篇
  2016年   3578篇
  2015年   5367篇
  2014年   6251篇
  2013年   8978篇
  2012年   9967篇
  2011年   10032篇
  2010年   6673篇
  2009年   6305篇
  2008年   8934篇
  2007年   9406篇
  2006年   8952篇
  2005年   8701篇
  2004年   8626篇
  2003年   8225篇
  2002年   8216篇
  2001年   18221篇
  2000年   18765篇
  1999年   14901篇
  1998年   4906篇
  1997年   5164篇
  1996年   4817篇
  1995年   4623篇
  1994年   4554篇
  1993年   4558篇
  1992年   12075篇
  1991年   11664篇
  1990年   11318篇
  1989年   10975篇
  1988年   10362篇
  1987年   9880篇
  1986年   9386篇
  1985年   9472篇
  1984年   7828篇
  1983年   6736篇
  1982年   5353篇
  1981年   4986篇
  1980年   4522篇
  1979年   7635篇
  1978年   6164篇
  1977年   5664篇
  1976年   5348篇
  1975年   6005篇
  1974年   6637篇
  1973年   6577篇
  1972年   6097篇
  1971年   5517篇
  1970年   4774篇
  1969年   4723篇
  1968年   4305篇
  1967年   3622篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
A series of measurements of blood pressure in normotensive and hypertensive subjects showed that measurements made with a sphygmomanometer with the arm dependent by the side were consistently higher than those made with the arm horizontal at heart level. The mean difference in a group of 90 hypertensive outpatients was 11/12 mm Hg. Failure to appreciate the importance of arm position may lead to erroneous measurements of blood pressure. This has important implications for clinical practice and research.  相似文献   
72.
73.
The design and characteristics of inexpensive and simply constructed equal-energy response photosynthetic irradiance sensors is described for use particularly where several cells are required in comparative ecological studies either above or below water. The dimensions of the sensors can be changed proportionally to suit different applications or components. The response of the sensor to irradiance at varying angles corresponds very closely to that required by the cosine law. The sensor is comparatively insensitive to other environmental variables in field use and gave a stable output; the long term drift was proportional to electrical output but in continuous use, drift is regular and could reach -0.08 year-1 of the total. The spectral range and cosine response is discussed in comparison to other more expensive (x 5–10) commercially available, sensors and to local standards.  相似文献   
74.
75.
Spinosyns A and D are the active ingredients in an insect control agent produced by fermentation of Saccharopolyspora spinosa. Spinosyns are macrolides with a 21-carbon, tetracyclic lactone backbone to which the deoxysugars forosamine and tri-O-methylrhamnose are attached. The spinosyn biosynthesis genes, except for the rhamnose genes, are located in a cluster that spans 74 kb of the S. spinosa genome. DNA sequence analysis, targeted gene disruptions and bioconversion studies identified five large genes encoding type I polyketide synthase subunits, and 14 genes involved in sugar biosynthesis, sugar attachment to the polyketide or cross-bridging of the polyketide. Four rhamnose biosynthetic genes, two of which are also necessary for forosamine biosynthesis, are located outside the spinosyn gene cluster. Duplication of the spinosyn genes linked to the polyketide synthase genes stimulated the final step in the biosynthesis — the conversion of the forosamine-less pseudoaglycones to endproducts. Duplication of genes involved in the early steps of deoxysugar biosynthesis increased spinosyn yield significantly. Journal of Industrial Microbiology & Biotechnology (2001) 27, 399–402. Received 31 May 2001/ Accepted in revised form 09 July 2001  相似文献   
76.
77.
The effect of glutathione (GSH) depletion by L-buthionine-[S,R]-sulphoximine (BSO) on tumor necrosis factor-alpha (TNF-alpha)-induced adhesion molecule expression and mononuclear leukocyte adhesion to human umbilical vein endothelial cells (HUVECs) was investigated. Cells with marked depletion of cytoplasmic GSH, but with an intact pool of mitochondrial GSH, only slightly enhanced TNF-alpha-induced E-selectin and vascular cell adhesion molecule-1 (VCAM-1) expression, compared with the control. However, TNF-a-induced expression of both molecules was markedly enhanced when the mitochondrial GSH pool was diminished to <15% of the control. In contrast, TNF-alpha-induced intercellular adhesion molecule-1 (ICAM-1) expression was not affected by the depletion of either cytoplasmic or mitochondrial GSH. Marked enhancement of TNF-alpha-induced adhesion molecule expression by the depletion of mitochondrial GSH resulted in increased in mononuclear leukocyte adhesion to treated HUVECs, compared with the control. These effects parallel reactive oxygen species (ROS) formation by the depletion of mitochondrial but not cytoplasmic GSH. Our findings demonstrate that depletion of mitochondrial GSH renders more ROS generation in HUVECs, and mitochondrial GSH modulates TNF-alpha-induced adhesion molecule expression and mononuclear leukocyte adhesion in HUVECs.  相似文献   
78.
In 1990 and 1994, two males of Golden Eagles were found with small lead particles in their stomachs. Both individuals died soon after they had been taken sitting on the soil in bad condition. There are some findings of dead eagles in the same area which showed no sign of having been injured. The two cases of presumed lead poisoning coincide with intense efforts to reduce the number of ChamoixRupicapra rupicapra by hunting. The bullets used contain a nucleus of lead and eagles were seen taking the entrails of the shot chamoix.  相似文献   
79.
80.
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