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   1771233篇
  免费   176985篇
  国内免费   1640篇
  2021年   18386篇
  2018年   19367篇
  2017年   18132篇
  2016年   29401篇
  2015年   43474篇
  2014年   51673篇
  2013年   77846篇
  2012年   51052篇
  2011年   42907篇
  2010年   48346篇
  2009年   48257篇
  2008年   39953篇
  2007年   39092篇
  2006年   41549篇
  2005年   42091篇
  2004年   41371篇
  2003年   38545篇
  2002年   36564篇
  2001年   56134篇
  2000年   54272篇
  1999年   48067篇
  1998年   28369篇
  1997年   28392篇
  1996年   27424篇
  1995年   25623篇
  1994年   25247篇
  1993年   24742篇
  1992年   41385篇
  1991年   39846篇
  1990年   38609篇
  1989年   38600篇
  1988年   35799篇
  1987年   34610篇
  1986年   32436篇
  1985年   33927篇
  1984年   31038篇
  1983年   27336篇
  1982年   25081篇
  1981年   23641篇
  1980年   22260篇
  1979年   27975篇
  1978年   24402篇
  1977年   22907篇
  1976年   21798篇
  1975年   22319篇
  1974年   23759篇
  1973年   23778篇
  1972年   20894篇
  1971年   19216篇
  1970年   16807篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
221.
222.
223.
224.
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   
225.
Electron microscopy of the adrenal medulla and cortex during the intermediate period of endotoxin shock has revealed severe destructive changes in parenchymal cells. These changes are the reason for synthetic, secretory and trophic disturbances of glandular functions. Ultrastructural lesions indicate that stress exposure exceeds the adaptive capacity of cells, the majority of which are exhausted and killed.  相似文献   
226.
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   
227.
228.
Synthesis of prenylquinones in chloroplasts   总被引:3,自引:0,他引:3  
  相似文献   
229.
230.
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