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   1724538篇
  免费   173053篇
  国内免费   1614篇
  2021年   18347篇
  2019年   16310篇
  2018年   18946篇
  2017年   17700篇
  2016年   29205篇
  2015年   43602篇
  2014年   51799篇
  2013年   78068篇
  2012年   49690篇
  2011年   40598篇
  2010年   47632篇
  2009年   47768篇
  2008年   37917篇
  2007年   36920篇
  2006年   39886篇
  2005年   40360篇
  2004年   39826篇
  2003年   37158篇
  2002年   35221篇
  2001年   54889篇
  2000年   52972篇
  1999年   47120篇
  1998年   28257篇
  1997年   28196篇
  1996年   27339篇
  1995年   25561篇
  1994年   25209篇
  1993年   24658篇
  1992年   40588篇
  1991年   38777篇
  1990年   37457篇
  1989年   37540篇
  1988年   34580篇
  1987年   33417篇
  1986年   31380篇
  1985年   32864篇
  1984年   30353篇
  1983年   26742篇
  1982年   24852篇
  1981年   23424篇
  1980年   22016篇
  1979年   27344篇
  1978年   23845篇
  1977年   22433篇
  1976年   21216篇
  1975年   21583篇
  1974年   22769篇
  1973年   23001篇
  1972年   20056篇
  1971年   18550篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
The RAINFOR database: monitoring forest biomass and dynamics   总被引:1,自引:0,他引:1  
Problem: Data from over 100 permanent sample plots which have been studied for 10–20 years need a suitable system for storage which allows simple data manipulation and retrieval for analysis. Methods: A relational database linking tree records, taxonomic nomenclature and corresponding environmental data has been built in MS Access as part of the RAINFOR project. Conclusion: The database allows flexible and long‐term use of a large amount of data: more than 100 tree plots across Amazonia, incorporating over 80 000 records of individual trees and over 300 000 total records of tree diameter measurements from successive censuses. The database is designed to enable linkages to existing soil, floristic or plant‐trait databases. This database will be a useful tool for exploring the impact of environmental factors on forest structure and dynamics at local to continental scales, and long term changes in forest ecology. As an early example of its potential, we explore the impact of different methodological assumptions on estimates of tropical forest biomass and carbon storage.  相似文献   
902.
Circumferential and radial components of the yolk cell surface movements were measured in the loach embryos at the late blastula stage within 40–50 min after puncture or indentation by an obliquely directed glass rod. The yolk cell surface was preliminarily marked by coal particles. It was shown that even closely located regions of the surface differed markedly in the rate and direction of their movements. In the vicinity of puncture, the yolk cell surface at first contracted in both circumferential and radial directions and then widened, but did not reach the initial values. In more remote areas, this surface continued to contract in the circumferential direction, but was extended in the radial direction. The degree of its contraction along different radii was unequal. The reaction to oblique indentation was anisotropic: the closest area of the yolk cell surface, located along the direction of indentation, contracted in both circumferential and radial directions and formed a fold “leaking” onto the rod, while the opposite area contracted in the circumferential direction, but extended in the radial direction. A conclusion was drawn that the yolk cell surface is a multivariant mechanosensitive system. Its active responses to mechanical influences obey the same patterns as multicellular embryonic tissues.  相似文献   
903.
904.
905.
906.
907.
908.
909.
910.
Solubilized sarcoplasmic reticulum (SSR) was prepared by solubilizing fragmented sarcoplasmic reticulum (FSR) with a nonionic detergent (C12E8) then displacing the detergent with Tween 80, using a DEAE-cellulose column. The UV absorption of SSR decreased reversibly at about 286 and 292 nm on removal of free Ca2+ ions, while no change in the fluorescence spectrum was detectable. On the other hand, the fluorescence intensity of FSR decreased 3-4% on removal of free Ca2+ ions, as previously reported by Dupont [(1976) Biochem. Biophys. Res. Commun. 71, 544-550]. The UV absorption of FSR increased reversibly at about 270-280 nm on removal of free Ca2+ ions, but the rate of the change was very slow (k = about 0.1 min-1).  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