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   770521篇
  免费   79304篇
  国内免费   1664篇
  851489篇
  2018年   16895篇
  2017年   15526篇
  2016年   15007篇
  2015年   11944篇
  2014年   13698篇
  2013年   18585篇
  2012年   25205篇
  2011年   33675篇
  2010年   25562篇
  2009年   21051篇
  2008年   28406篇
  2007年   30355篇
  2006年   19300篇
  2005年   18745篇
  2004年   19131篇
  2003年   18196篇
  2002年   17611篇
  2001年   32872篇
  2000年   32947篇
  1999年   25845篇
  1998年   8776篇
  1997年   9000篇
  1996年   8468篇
  1995年   7968篇
  1994年   7697篇
  1993年   7775篇
  1992年   21051篇
  1991年   20842篇
  1990年   20140篇
  1989年   19497篇
  1988年   18225篇
  1987年   17151篇
  1986年   16064篇
  1985年   15646篇
  1984年   12775篇
  1983年   11090篇
  1982年   8203篇
  1981年   7302篇
  1980年   6868篇
  1979年   11858篇
  1978年   9442篇
  1977年   8487篇
  1976年   7786篇
  1975年   8956篇
  1974年   9649篇
  1973年   9452篇
  1972年   8595篇
  1971年   8049篇
  1970年   6730篇
  1969年   6592篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
92.
With DNA sequencing now getting cheaper more quickly than data storage or computation, the time may have come for genome informatics to migrate to the cloud.  相似文献   
93.
94.
95.
Culturing the dorsal iris epithelium of a newt with a pituitary gland in organ culture greatly enhances the ability of the iris epithelium to produce advanced lens regenerates in vitro. In an attempt to elucidate the mechanism by which the pituitary enhances lens regeneration irido-corneal complexes from adult newts were cultured in medium to which various substances had been added either singly or in numerous combinations. Prolactin, insulin, hydrocortisone, and thyroxine failed to enhance the production of advanced lens regenerates in any of the doses or combinations tested. Similarly, addition of 50 microgram/ml of sodium or calcium ascorbate had no effect on the progress of lens regeneration in vitro. Addition of dibutyryl cyclic-AMP caused an inhibition of depigmentation and regeneration at high doses. The results of these experiments show that the effects of the pituitary cannot be duplicated by hormones which other authors have asserted to be beneficial to limb or tail regenerates in vitro. The results with cyclic AMP suggest that prolonged exposure to high doses of cyclic AMP inhibit regeneration and indicate that further studies on the fluctations in cyclic AMP levels throughout the process of lens regeneration must be done.  相似文献   
96.
Puerto Rican populations of two species of sea anemones (Bunodosoma cavernata and B. granulifera) which had previously been considered one were assayed electrophoretically for enzymes encoded by 12 loci. The two species shared no common allozymes at 6 of the 12 loci. Genetic distance and identity values based on these allozymes were computed for the Puerto Rican populations and for B. cavernata from Florida and B. granulifera from Panama. The Puerto Rican populations of both species had much higher genetic identities for their geographically distant conspecifics than for each other. These results indicate that the two species are reproductively isolated and should be considered as separate valid species. Average heterozygosities are presented which are the first published for coelenterate species.  相似文献   
97.
98.
Rice fields are an alternative habitat for waterbirds and provide food and shelter for many avian species, but there is a lack of information about how the use of rice fields translates into population level effects. The aim of this study was to test the relationship between the use of rice fields by European waterbirds and trends in their populations. We tested this relationship during the autumn migration season and during the breeding season. Based on counts conducted over the last 23 years in natural marshes and areas of rice fields in Doñana (SW Spain), an index of rice field use was constructed for 76 bird species, which was then compared to these species’ European population trends obtained from the literature. A positive relationship was found between waterbird population trends and the use of rice fields during autumn migration season. Our study suggests that changes in the Common Agriculture Policy in Europe leading to reductions in areas of rice cultivation may have important effects on waterbirds. The restoration of former marsh areas and the maintenance of rice cultivation would seem to be more environmentally friendly approaches than the use of these areas to grow alternative crops or solar farms.  相似文献   
99.
Previous studies have shown that the concentration of red blood cell (RBC) magnesium is significantly lower in subjects carrying an HLA-BW 35 antigen (p less than 0.001) than in non-carriers. As this finding might be related to modifications of the RBC membrane sialoglycoconjugates, RBC sialic acid was comparatively determined in BW 35+ and BW 35- subjects. Pyruvate-kinase activity mean RBC volume, and reticulocyte count have also been determined in order to estimate whether some significant variations in the level of these age markers could be detected between the HLA BW 35+ and BW 35- subjects. A significant negative correlation between sialic acid and RBC magnesium concentrations was observed for the whole population tested (n 57, p less than 0.005), 61% of the BW 35+ and only 25% of the BW 35- individuals having sialic acid values above, and magnesium values below the overall mean (p less than 0.01). The variance of mean RBC volume was also larger for the BW 35+ group. Other determinations did not show any significant variations, suggesting that the results are not related to RBC age.  相似文献   
100.
Fumaria capreolata was taken into cell suspension culture. The culture yielded a biomass of about 12 g dry weight per liter of medium; the dried cells contained ca. 0.1% of alkaloids. Besides choline, the following ten known isoquinoline alkaloids were isolated from the cell extract in crystalline form: coptisine, dehydrocheilanthifoline; (+)-isoboldine; magnoflorine; N-methylcoclaurine; (+)-reticuline; (–)-pallidine; protopine; sanguinarine; (–)-scoulerine. This is the most diverse isoquinoline alkaloid spectrum thus far published for a cell suspension culture.  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