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   1390275篇
  免费   129551篇
  国内免费   1456篇
  2021年   17971篇
  2019年   16197篇
  2018年   17740篇
  2017年   16334篇
  2016年   28161篇
  2015年   42693篇
  2014年   50786篇
  2013年   77025篇
  2012年   39731篇
  2011年   28523篇
  2010年   44001篇
  2009年   45286篇
  2008年   26718篇
  2007年   24758篇
  2006年   29469篇
  2005年   30431篇
  2004年   29678篇
  2003年   27065篇
  2002年   24946篇
  2001年   37994篇
  2000年   35335篇
  1999年   34133篇
  1998年   25953篇
  1997年   25806篇
  1996年   25270篇
  1995年   23375篇
  1994年   23204篇
  1993年   22352篇
  1992年   28880篇
  1991年   26971篇
  1990年   25598篇
  1989年   26373篇
  1988年   24163篇
  1987年   22879篇
  1986年   21587篇
  1985年   23230篇
  1984年   23068篇
  1983年   20392篇
  1982年   20810篇
  1981年   20027篇
  1980年   18685篇
  1979年   19186篇
  1978年   17905篇
  1977年   17240篇
  1976年   16442篇
  1975年   15754篇
  1974年   16224篇
  1973年   16556篇
  1972年   13901篇
  1971年   12859篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
172.
173.
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   
174.
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   
175.
176.
Synthesis of prenylquinones in chloroplasts   总被引:3,自引:0,他引:3  
  相似文献   
177.
178.
179.
180.
An earlier report (1) of an adverse effect of high doses of oxytocin on human memory included results of studies on women receiving oxytocin as part of the treatment to induce 2nd trimester therapeutic abortion. These women served as their own controls. We have now been able to study a group of women who have been treated in all ways like the original group, with the exception that they did not receive oxytocin. The results from this external control corroborate the finding that oxytocin affected memory.  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