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   87757篇
  免费   7776篇
  国内免费   43篇
  2023年   240篇
  2022年   595篇
  2021年   1579篇
  2020年   894篇
  2019年   1122篇
  2018年   1462篇
  2017年   1253篇
  2016年   2173篇
  2015年   3690篇
  2014年   4107篇
  2013年   4889篇
  2012年   6633篇
  2011年   6587篇
  2010年   4236篇
  2009年   3866篇
  2008年   5531篇
  2007年   5537篇
  2006年   5334篇
  2005年   5105篇
  2004年   4972篇
  2003年   4742篇
  2002年   4453篇
  2001年   921篇
  2000年   673篇
  1999年   1018篇
  1998年   1260篇
  1997年   834篇
  1996年   773篇
  1995年   682篇
  1994年   644篇
  1993年   696篇
  1992年   589篇
  1991年   551篇
  1990年   497篇
  1989年   422篇
  1988年   450篇
  1987年   372篇
  1986年   337篇
  1985年   423篇
  1984年   529篇
  1983年   415篇
  1982年   509篇
  1981年   497篇
  1980年   427篇
  1979年   307篇
  1978年   334篇
  1977年   288篇
  1976年   268篇
  1975年   216篇
  1974年   250篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Uteroferrin, a purple-colored, iron-containing acid phosphatase, with many of the properties of a lysosomal hydrolase, transports iron from the mother to the conceptus in pregnant pigs. Uteroferrin, however, is but one member of what may be a broad class of iron-containing phosphatases with unusual spectral properties which result from a novel type of di-iron active site. The biological function of uteroferrin is unknown. We argue here that the in vivo function of uteroferrin, despite its undoubted ability to act as a potent acid phosphatase, is that of a transplacental iron transporter.  相似文献   
42.

Background  

Phylogenies capture the evolutionary ancestry linking extant species. Correlations and similarities among a set of species are mediated by and need to be understood in terms of the phylogenic tree. In a similar way it has been argued that biological networks also induce correlations among sets of interacting genes or their protein products.  相似文献   
43.
Three acetotrophicMethanosarcina species, which included marine, nonmarine, and thermophilic strains, were grown on acetate in a 10-liter pH auxostat. Specific growth rates and molar growth yields were constant throughout growth. Cell yields were up to 18-fold greater than previously reported. These properties of the pH auxostat indicate that it is a preferred culture method for the biochemical study of methanogenesis from acetate.  相似文献   
44.
The ocelotLeopardus pardalis Linnaeus, 1758 is an endangered felid in the United States currently restricted to southern Texas. The objectives of our study were to obtain data on ocelot parturition dates, fecundity, sex ratios, den characteristics, and first year survival, all of which are critical in development of population viability models. Sixteen parturition events were recorded ranging from mid-April to late December for 12 wild ocelots. Cumulatively, litters consisted of 1 or 2 kittens (ˉ = 1.2 ± 0.44 SD). Cumulative sex ratio was 1∶2.5 (male:female); however, there was no significant difference between the observed sex ratio and a 1∶1 sex ratio. Ten den sites were in close proximity (≤ 10 m) to dense thornshrub. Adult female ocelots used 2 to 4 den sites for each litter with distance between consecutively occupied dens ranging from 110 to 280 m (ˉ = 158 m ± 93 SD). An estimated annual survival for ocelots 0 to 1 year of age was 0.68. Evidence suggests that ocelots in the wild may breed more frequently than had been previously hypothesized.  相似文献   
45.
46.
47.
Hupfer  Michael  Dollan  Anja 《Hydrobiologia》2003,506(1-3):635-640
Hydrobiologia - To observe effects on the phosphorus retention mechanisms of a lake after re-colonisation by macrophytes, Potamogeton crispus L. and Elodea canadensis Michx. were planted in lab...  相似文献   
48.
49.
50.
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