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   1446367篇
  免费   138386篇
  国内免费   1451篇
  1586204篇
  2021年   18011篇
  2019年   16180篇
  2018年   17870篇
  2017年   16575篇
  2016年   28255篇
  2015年   42670篇
  2014年   50694篇
  2013年   76961篇
  2012年   39557篇
  2011年   28390篇
  2010年   44096篇
  2009年   45379篇
  2008年   26845篇
  2007年   25199篇
  2006年   30012篇
  2005年   31066篇
  2004年   30282篇
  2003年   27760篇
  2002年   25780篇
  2001年   38489篇
  2000年   36252篇
  1999年   35107篇
  1998年   26043篇
  1997年   26016篇
  1996年   25483篇
  1995年   23660篇
  1994年   23461篇
  1993年   22684篇
  1992年   30877篇
  1991年   29527篇
  1990年   28179篇
  1989年   28620篇
  1988年   26500篇
  1987年   25248篇
  1986年   24087篇
  1985年   25940篇
  1984年   25067篇
  1983年   22189篇
  1982年   21950篇
  1981年   21238篇
  1980年   19774篇
  1979年   21746篇
  1978年   19827篇
  1977年   18889篇
  1976年   17993篇
  1975年   17844篇
  1974年   18287篇
  1973年   18560篇
  1972年   16140篇
  1971年   14518篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
995.
Zingiber anamalayanum (Zingiberaceae), a new species from the southern western Ghats, India is described and illustrated. A phenetic analyses to assess the similarity indices and the uniqueness of the new species have been performed. A diagnostic key to Zingiber spp. in south India is provided.  相似文献   
996.
997.
998.
999.
Bacopa monniera Wettst. (BM, syn. Herpestis monniera L; Scrophulariaceae), is an Ayurvedic drug used as a rasayana. Its fresh juice was earlier reported to have significant antiulcerogenic activity. In continuation, methanolic extract of BM (BME) standardized to bacoside-A content (percentage-38.0 ± 0.9), when given in the dose of 10–50 mg/kg, twice daily for 5 days, showed dose-dependent anti-ulcerogenic on various gastric ulcer models induced by ethanol, aspirin, 2 h cold restraint stress and 4 h pylorus ligation. BME in the dose of 20 mg/kg, given for 10 days, twice daily showed healing effects against 50% acetic acid-induced gastric ulcers. Further work was done to investigate the possible mechanisms of its action by studying its effect on various mucosal offensive acid-pepsin secretion and defensive factors like mucin secretion, mucosal cell shedding, cell proliferation and antioxidant activity in rats. BME 20 mg/kg showed no effect on acid-pepsin secretion, increased mucin secretion, while it decreased cell shedding with no effect on cell proliferation. BME showed significant antioxidant effect per se and in stressed animals. Thus, the gastric prophylactic and curative effects of BME may be due to its predominant effect on mucosal defensive factors.  相似文献   
1000.
The diet of the viperine snake was compared with food availability in the Ebro Delta, a wetland largely occupied by rice fields, in 1990 and 1991. Snake selection of prey type and size was studied seasonally and by snake group: males, females and immature snakes. Overall, feeding activity (percentage of individuals with prey and number of prey per stomach) increased with food availability. Diet analysis showed that viperine snakes mainly foraged on the green frog Rana perezi (adults and tadpoles) and the carp Cyprinus earpio. Conversely, viperine snakes rejected the mosquito fish Gambusia holbroki which is the most abundant species in autumn, when Natrix maura has a low feeding activity. Statistical comparisons between viperine snake diet and prey availability showed that males selected small carp, immature snakes selected tadpoles and, in spring, females selected frogs. The selection of small carp by males may reflect a sexual divergence of trophic niche related to sexual size dimorphism, as females are larger than males. As tadpoles are presumably easier to catch than fish, tadpole selection by immature individuals may reflect variance in capture abilities. In spring, the selection of frogs by females overlapped with vitellogenesis, suggesting that females compensate for the cost of reproduction by selecting green frogs, which have a greater biomass and higher energy content than fish. Carps eaten in spring were smaller than in summer. Moreover, in summer viperine snakes selected smaller carp than the available mean size. This divergent tendency between carp size selection and carp size availability reveals how seasonal diet shifts in prey size selection may be a response to an increase in prey size.  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