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   1367646篇
  免费   122302篇
  国内免费   1406篇
  2021年   17858篇
  2019年   16130篇
  2018年   18003篇
  2017年   16782篇
  2016年   28178篇
  2015年   42265篇
  2014年   50262篇
  2013年   76401篇
  2012年   37307篇
  2011年   26307篇
  2010年   43940篇
  2009年   45182篇
  2008年   25247篇
  2007年   23327篇
  2006年   28286篇
  2005年   29362篇
  2004年   28594篇
  2003年   25964篇
  2002年   24110篇
  2001年   33436篇
  2000年   30867篇
  1999年   31047篇
  1998年   25405篇
  1997年   25306篇
  1996年   24768篇
  1995年   22932篇
  1994年   22715篇
  1993年   21815篇
  1992年   27261篇
  1991年   25809篇
  1990年   24543篇
  1989年   25250篇
  1988年   23474篇
  1987年   22368篇
  1986年   21207篇
  1985年   23084篇
  1984年   22848篇
  1983年   20226篇
  1982年   20596篇
  1981年   19779篇
  1980年   18526篇
  1979年   19004篇
  1978年   17842篇
  1977年   17147篇
  1976年   16413篇
  1975年   15982篇
  1974年   16625篇
  1973年   16948篇
  1972年   14534篇
  1971年   13324篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
172.
Protein metal-binding sites.   总被引:2,自引:0,他引:2  
Metal ions have a role in a variety of important functions in proteins including protein folding, assembly, stability, conformational change, and catalysis. The presence or absence of a given metal ion is crucial to the conformation or activity of over one third of all proteins. Recent developments have been made in the understanding and design of metal-binding sites in proteins, an important and rapidly advancing area of protein engineering.  相似文献   
173.
Experiments were made on rabbit fed an atherogenic diet (0.5 g/kg cholesterol) singly for 15 and 24 h and repeatedly for 3, 9 and 30 days. At early stages of lipid metabolism distress the interrelationship was established between blood rheological disorders and microcirculatory abnormalities. The dependence of the initial reaction of some rheological characteristics on their initial level was marked.  相似文献   
174.
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   
175.
176.
177.
178.
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   
179.
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   
180.
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   
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