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   414485篇
  免费   47559篇
  国内免费   368篇
  2016年   3926篇
  2015年   5887篇
  2014年   6805篇
  2013年   9770篇
  2012年   10902篇
  2011年   10970篇
  2010年   7300篇
  2009年   6885篇
  2008年   9748篇
  2007年   10273篇
  2006年   9791篇
  2005年   9528篇
  2004年   9478篇
  2003年   9020篇
  2002年   9009篇
  2001年   18603篇
  2000年   19137篇
  1999年   15300篇
  1998年   5188篇
  1997年   5413篇
  1996年   5087篇
  1995年   4822篇
  1994年   4808篇
  1993年   4776篇
  1992年   12378篇
  1991年   11938篇
  1990年   11599篇
  1989年   11255篇
  1988年   10603篇
  1987年   10124篇
  1986年   9600篇
  1985年   9765篇
  1984年   8091篇
  1983年   6953篇
  1982年   5620篇
  1981年   5258篇
  1980年   4775篇
  1979年   7864篇
  1978年   6387篇
  1977年   5869篇
  1976年   5523篇
  1975年   6163篇
  1974年   6831篇
  1973年   6788篇
  1972年   6237篇
  1971年   5658篇
  1970年   4909篇
  1969年   4873篇
  1968年   4414篇
  1967年   3747篇
排序方式: 共有10000条查询结果,搜索用时 375 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.
Nine fatty acid–peptide hybrid molecules were constructed using the general formula CH3(CH2) n CO-Phe Asp Cys-amide and tested for their ability to inhibit cell lysis induced by the membrane-active peptide melittin. All of these molecules, where n = 4–14, inhibited the action of melittin to some extent, but the longer carbon chains were most effective. Several potential inhibitors were also constructed with conservative substitutions in the peptide portion of the molecule. All were effective to varying degrees. We concluded that in the hexapeptide inhibitor published by Blondelle et al. (1993), the role of the first three residues is only to provide hydrophobic interaction with the melittin and has no particular amino acid sequence specificity. Some of these inhibitors were found to inhibit the lytic activity of a melittin analogue which had only superficial sequence similarity to melittin and also a truncated form of melittin, indicating the generality of the action of the inhibitors.Deceased 5/4/98  相似文献   
77.
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   
78.
79.
80.
Synaptonemal complexes and meiosis in myxomycetes   总被引:4,自引:0,他引:4  
Synaptonemal complexes (SC) have been observed in spores 18–24 hr past cleavage in natural fruitings of Physarum cinereum, P. bogoriense, Hemitrichia stipitata, Tubifera ferruginosa, and Arcyria incarnata. Laboratory fruitings of Arcyria cinerea, Stemonitis herbatica, and a homothallic isolate of Physarum pusillum also have SC's present in spores during the same postcleavage period. The presence of these paired chromosomes of meiotic prophase in spores of species collected in nature and in a diversity of taxa suggests that the usual position of meiosis in Myxomycetes is inside the postcleavage spore. Criteria are proposed for evaluating the validity of the SC as an indicator of meiosis.  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