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   1606049篇
  免费   152728篇
  国内免费   1556篇
  2021年   18154篇
  2019年   16293篇
  2018年   18917篇
  2017年   17500篇
  2016年   29005篇
  2015年   43350篇
  2014年   51486篇
  2013年   77899篇
  2012年   45238篇
  2011年   35727篇
  2010年   46547篇
  2009年   47124篇
  2008年   32927篇
  2007年   31780篇
  2006年   35241篇
  2005年   36196篇
  2004年   35370篇
  2003年   32737篇
  2002年   30713篇
  2001年   48049篇
  2000年   45810篇
  1999年   42076篇
  1998年   27257篇
  1997年   27140篇
  1996年   26430篇
  1995年   24499篇
  1994年   24288篇
  1993年   23509篇
  1992年   35931篇
  1991年   34255篇
  1990年   33099篇
  1989年   33606篇
  1988年   31005篇
  1987年   29427篇
  1986年   27705篇
  1985年   29437篇
  1984年   27801篇
  1983年   24529篇
  1982年   23303篇
  1981年   22146篇
  1980年   20661篇
  1979年   24185篇
  1978年   21492篇
  1977年   20501篇
  1976年   19647篇
  1975年   19518篇
  1974年   20455篇
  1973年   20689篇
  1972年   17947篇
  1971年   16486篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
881.
Summary A field experiment was conducted using15N-labelled urea on a Reddish Brown Lateritic (Peleustult) soil. Growing two crops on flat land and on soil ridges of 15 cm height produced similar comparative effects from fertilizer on maize. However, fertilizer applied by broadcasting on maize with a 50 cm effective band followed by incorporating was more useful to mungbean than that applied by banding below the cereal seed rows when crops were grown on flat land. The reverse was observed when crops were grown on ridges. It was deduced that the maize cultivar was not likely to affect comparative efficiencies of fertilizer. For fertilizer application at sowing, broadcasting in 50 cm maize effective band followed by incorporating was slightly superior to banding below maize seed rows. Side-dressing of fertilizer to maize at 4 weeks after sowing was superior to application at sowing. Evenly-split application, at sowing and at 4 weeks after sowing, was either only slightly superior or comparable to non-split application by banding below maize seed rows at sowing, depending on placement method of the first application. Soil moisture status as a possible factor rendering discrepancy in the comparative efficiencies obtained by different authors is discussed.  相似文献   
882.
Study of the posterior abdominal epidermis in hypophysectomized/thyroidectomized male and female tokays following surgery, and subsequent androgen therapy, indicates that, contrary to a previous model, all aspects of β-gland differentiation are under direct androgenic control. On the other hand, another epidermal specialization, the digital foot-pad, shows a pattern of histogenesis directly comparable to that of β-glands, but is unaffected by androgens. These data are discussed with respect to the evolution of glandular epidermal specializations in gekkonid lizards and the possible role of androgens in modifying the control of cell differentiation in lizard epidermis.  相似文献   
883.
884.
885.
A high-performance liquid chromatographic procedure is reported for reproducibly and sensitively quantitating caffeine and its N-demethylated metabolite paraxanthine in micro-samples. A 5-μm reversed-phase radial compression column and 214-nm fixed wavelength ultraviolet detector were used to attain a sensitivity sufficient to quantitate these compounds at concentratios as low as 80 ng/ml using only 25 μl of sample. The assay is applicable to microliter samples of whole blood, serum, plasma, saliva, amniotic, cerebro-spinal and gastric fluids such as might be obtained in studies involving small animals or neonates. The utility of the assay is illustrated with caffeine and paraxanthine levels measured in several maternal and fetal fluids following constant-rate intravenous infusion of caffeine into a rabbit throughout pregnancy.  相似文献   
886.
887.
888.
889.
A double antibody radioimmunoassay technique is described for the measurement of anti-zona antibodies in the peripheral plasma of marmosets actively immunized against intact cow zonae pellucidae. The method has been shown to be a reliable, repeatable indicator of antibody titer, enabling direct comparison of the response obtained between marmosets. This is the first report of a procedure describing the active immunization of a primate against zona antigens, and the first time a radioimmunoassay technique has been used to monitor profiles of anti-cow zona antibody production following active immunization. The method should prove to be a useful tool in the evaluation of zona antigens as agents for immunocontraception.  相似文献   
890.
Physicians with an interest in pain have long suggested that the poor complain more and have a higher prevalence of neuroticism than do higher social groups. This assumption was tested by analysing the pain patterns in 500 consecutive patients attending a pain relief clinic. Results implied that scores for presenting pain, anxiety, and depression were similar to all social groups. After treatment scores for residual pain were significantly lower in all social classes, with greatest reduction in classes III, IV, and V. Almost identical results were obtained in a subgroup of patients with cancer but not in a subgroup with sciatica. That patients from the lower social classes have a higher perception of pain and are more neurotic than other group is a myth, probably resulting from poor communication between clinicians and patients of dissimilar socioeconomic class.  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