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   1392179篇
  免费   124579篇
  国内免费   1952篇
  1518710篇
  2021年   17958篇
  2019年   16218篇
  2018年   18004篇
  2017年   16728篇
  2016年   28292篇
  2015年   42757篇
  2014年   50824篇
  2013年   77124篇
  2012年   39399篇
  2011年   28679篇
  2010年   44474篇
  2009年   45578篇
  2008年   26868篇
  2007年   24921篇
  2006年   29827篇
  2005年   30788篇
  2004年   30029篇
  2003年   27346篇
  2002年   25366篇
  2001年   34997篇
  2000年   32343篇
  1999年   32191篇
  1998年   25625篇
  1997年   25539篇
  1996年   24985篇
  1995年   23166篇
  1994年   22911篇
  1993年   22019篇
  1992年   28133篇
  1991年   26687篇
  1990年   25374篇
  1989年   26019篇
  1988年   23908篇
  1987年   22555篇
  1986年   21643篇
  1985年   23361篇
  1984年   23016篇
  1983年   20364篇
  1982年   20595篇
  1981年   19789篇
  1980年   18529篇
  1979年   19036篇
  1978年   18058篇
  1977年   17173篇
  1976年   16333篇
  1975年   16146篇
  1974年   16672篇
  1973年   16974篇
  1972年   14419篇
  1971年   13306篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
141.
142.
143.
144.
When cotyledonary tissue of G., barbadense cotton are treated with the mutagen ethyl methanesulfonate and then germinated, an enhanced, unscheduled DNA synthesis response is observed, along with a concomitant increase in the thymidine triphosphate precursor pool size. The implications of these results are discussed in this paper.  相似文献   
145.
146.
147.
148.
Nitrogen fertigation of greenhouse-grown cucumber   总被引:2,自引:0,他引:2  
Summary This greenhouse study investigated the response of trickle-irrigated cucumber (Cucumis sativa cv. ‘Petita’) to three N levels applied with every irrigation via the irrigation stream. The plants were grown in pots filled with 12 kg of soil. Water containing 5.8, 11.8, or 17.8 mmol N/l, and uniformly supplied with 2.0 and 3.9 mmol/l of P and K, respectively, was applied two to three times daily. In all treatments of 0.3 leaching fraction was allowed. The resulting total N applications were 15.7, 31., and 47.2 g N/plant. The total amount of water applied was 1851/plant. Total N and NO3-N, in lajinae and petioles, increased with increasing N level whereas P and K in generated decreased. Although different NO3/NH4 ratios in the treatments may have influeced the response to N, it could be concluded that the highest yield was obtained with 11.8 mmol N/1 due to increased number of fruit. In the root volume of this treatment the NO3-N concentration in the soil solution was aroun 7 mmol/1 for most of the growing season. The dry matter concentration of fruits was not affected by the N levels. It was concluded that 11.8 mmol N/1 applied with every irrigation via the irrigation stream is adequate to cover the needs of greenhous-grown cucumber for higher yield (9.42 kg/plant over a harvesting period of 93 days).  相似文献   
149.
OBJECTIVE--To test the hypothesis that farmers are at high risk of hip osteoarthritis and to investigate possible causes for such a hazard. DESIGN--Cross sectional survey. SETTING--Five rural general practices. SUBJECTS--167 male farmers aged 60-76 and 83 controls from mainly sedentary jobs. All those without previous hip replacement underwent radiography of the hip. MAIN OUTCOME MEASURES--Hip replacement for osteoarthritis or radiological evidence of hip osteoarthritis. RESULTS--Prevalence of hip osteoarthritis was higher in farmers than controls and especially in those who had farmed for over 10 years (odds ratio 9.3, 95% confidence interval 1.9 to 44.5). The excess could not be attributed to any one type of farming, and heavy lifting seems the likely explanation. CONCLUSIONS--Manual handling in agriculture should be limited where possible. Consideration should be given to making hip osteoarthritis a prescribed industrial disease in farmers. There may be wider implications for the prevention of hip osteoarthritis in the general population.  相似文献   
150.
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