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   24473篇
  免费   1854篇
  国内免费   11篇
  26338篇
  2024年   26篇
  2023年   92篇
  2022年   280篇
  2021年   482篇
  2020年   277篇
  2019年   374篇
  2018年   525篇
  2017年   425篇
  2016年   740篇
  2015年   1187篇
  2014年   1312篇
  2013年   1523篇
  2012年   2056篇
  2011年   2076篇
  2010年   1280篇
  2009年   1036篇
  2008年   1640篇
  2007年   1534篇
  2006年   1380篇
  2005年   1274篇
  2004年   1332篇
  2003年   1082篇
  2002年   1002篇
  2001年   464篇
  2000年   395篇
  1999年   333篇
  1998年   253篇
  1997年   192篇
  1996年   159篇
  1995年   145篇
  1994年   122篇
  1993年   105篇
  1992年   140篇
  1991年   119篇
  1990年   86篇
  1989年   89篇
  1988年   70篇
  1987年   67篇
  1986年   73篇
  1985年   65篇
  1984年   57篇
  1983年   60篇
  1982年   52篇
  1981年   49篇
  1980年   46篇
  1979年   34篇
  1978年   38篇
  1976年   27篇
  1975年   24篇
  1974年   27篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
ABSTRACT   The phrase persistent vulnerability reflects the enduring relationship of the rural population in Ceará with a highly variable climate. Persistence underscores the historical and unyielding nature of this vulnerability. Yet contrary to once-catastrophic rates of mortality etched in a public consciousness, no one dies from severe droughts and few people flee them as in the past. Government relief and social transfers have become the institutionalized form of adaptation, giving way to the counterintuitive reality that drought stabilizes the food and income supply for poor people. We analyze how maladaptive risk reduction, which is embedded in clientilistic social relations, undermines resilience, and we examine pathways toward a more sustainable adaptive relationship.  相似文献   
3.
4.
Habitat fragmentation and flow regulation are significant factors related to the decline and extinction of freshwater biota. Pelagic-broadcast spawning cyprinids require moving water and some length of unfragmented stream to complete their life cycle. However, it is unknown how discharge and habitat features interact at multiple spatial scales to alter the transport of semi-buoyant fish eggs. Our objective was to assess the relationship between downstream drift of semi-buoyant egg surrogates (gellan beads) and discharge and habitat complexity. We quantified transport time of a known quantity of beads using 2–3 sampling devices at each of seven locations on the North Canadian and Canadian rivers. Transport time was assessed based on median capture time (time at which 50% of beads were captured) and sampling period (time period when 2.5% and 97.5% of beads were captured). Habitat complexity was assessed by calculating width∶depth ratios at each site, and several habitat metrics determined using analyses of aerial photographs. Median time of egg capture was negatively correlated to site discharge. The temporal extent of the sampling period at each site was negatively correlated to both site discharge and habitat-patch dispersion. Our results highlight the role of discharge in driving transport times, but also indicate that higher dispersion of habitat patches relates to increased retention of beads within the river. These results could be used to target restoration activities or prioritize water use to create and maintain habitat complexity within large, fragmented river systems.  相似文献   
5.
Induction of inflammation by turpentine injection caused 1.5–2-fold increase of both sialy- and galactosyltransferase activity in liver homogenates. The effect was apparent after 12 h turpentine treatment. Serum sialytransferase activity started to increase in the inflamed rats after 18 h, reaching a maximum of 4-fold at 48 h. In contrast, galactosyltransferase activity in serum showed no significant increase. The coordinated and temporal increase of sialytransferase activity in liver and serum suggest involvement of a specific mechanism for the preferential release of this enzyme into serum.  相似文献   
6.
7.
8.
SP-40,40 is a serum glycoprotein consisting of two different subunits (alpha and beta) assembled into a dimer by disulfide bonds. Northern blot hybridization, using total RNA from several cell lines, showed that SP-40,40 is expressed in glioblastoma and testicular tumor cells, as well as hepatoma cells. Spot blot hybridization of flow-sorted human chromosomes, using a SP-40,40 cDNA fragment as a probe, localized the gene for SP-40,40 to human chromosome 8. This gene has been given the designation CLI, for complement lysis inhibitor, by the Human Gene Nomenclature Committee.  相似文献   
9.
10.
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