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   1308950篇
  免费   115706篇
  国内免费   1418篇
  1426074篇
  2021年   17804篇
  2020年   12602篇
  2019年   16172篇
  2018年   17502篇
  2017年   16322篇
  2016年   27964篇
  2015年   42389篇
  2014年   50387篇
  2013年   76684篇
  2012年   35394篇
  2011年   23351篇
  2010年   43159篇
  2009年   44623篇
  2008年   22392篇
  2007年   20385篇
  2006年   25900篇
  2005年   26956篇
  2004年   26426篇
  2003年   23988篇
  2002年   22265篇
  2001年   31327篇
  2000年   28550篇
  1999年   29409篇
  1998年   25191篇
  1997年   24945篇
  1996年   24598篇
  1995年   22753篇
  1994年   22588篇
  1993年   21638篇
  1992年   25953篇
  1991年   24197篇
  1990年   22864篇
  1989年   23769篇
  1988年   21787篇
  1987年   20699篇
  1986年   19596篇
  1985年   21546篇
  1984年   21741篇
  1983年   19295篇
  1982年   20190篇
  1981年   19401篇
  1980年   18150篇
  1979年   18113篇
  1978年   17058篇
  1977年   16548篇
  1976年   15667篇
  1975年   15012篇
  1974年   15426篇
  1973年   15903篇
  1972年   13486篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
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   
143.
144.
145.
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   
146.
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   
147.
Synthesis of prenylquinones in chloroplasts   总被引:3,自引:0,他引:3  
  相似文献   
148.
149.
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