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   1194403篇
  免费   99787篇
  国内免费   2242篇
  2021年   17936篇
  2020年   12797篇
  2019年   16400篇
  2018年   17200篇
  2017年   15857篇
  2016年   27806篇
  2015年   42632篇
  2014年   50727篇
  2013年   77033篇
  2012年   31773篇
  2011年   18605篇
  2010年   41939篇
  2009年   43864篇
  2008年   18145篇
  2007年   15321篇
  2006年   21896篇
  2005年   22853篇
  2004年   22149篇
  2003年   20009篇
  2002年   18132篇
  2001年   28396篇
  2000年   25145篇
  1999年   26817篇
  1998年   24620篇
  1997年   24417篇
  1996年   24020篇
  1995年   22102篇
  1994年   22013篇
  1993年   20933篇
  1992年   22640篇
  1991年   20888篇
  1990年   19182篇
  1989年   20416篇
  1988年   18563篇
  1987年   17200篇
  1986年   16429篇
  1985年   18412篇
  1984年   19174篇
  1983年   17063篇
  1982年   18763篇
  1981年   18158篇
  1980年   16903篇
  1979年   14946篇
  1978年   15049篇
  1977年   14608篇
  1976年   13849篇
  1975年   12904篇
  1974年   12952篇
  1973年   13413篇
  1972年   11054篇
排序方式: 共有10000条查询结果,搜索用时 31 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