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   879683篇
  免费   100739篇
  国内免费   567篇
  980989篇
  2018年   8116篇
  2017年   7674篇
  2016年   11100篇
  2015年   15079篇
  2014年   17571篇
  2013年   25280篇
  2012年   28330篇
  2011年   28822篇
  2010年   19385篇
  2009年   17901篇
  2008年   25302篇
  2007年   26177篇
  2006年   24126篇
  2005年   23437篇
  2004年   23166篇
  2003年   22055篇
  2002年   21219篇
  2001年   42252篇
  2000年   42381篇
  1999年   33516篇
  1998年   11439篇
  1997年   12060篇
  1996年   11399篇
  1995年   10454篇
  1994年   10359篇
  1993年   10208篇
  1992年   26926篇
  1991年   25799篇
  1990年   24978篇
  1989年   24545篇
  1988年   22296篇
  1987年   21170篇
  1986年   19651篇
  1985年   19281篇
  1984年   16098篇
  1983年   13620篇
  1982年   10428篇
  1981年   9458篇
  1980年   8910篇
  1979年   15169篇
  1978年   11592篇
  1977年   10726篇
  1976年   9825篇
  1975年   10686篇
  1974年   11572篇
  1973年   11299篇
  1972年   10266篇
  1971年   9429篇
  1970年   8011篇
  1969年   7682篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Nucleotide sequence of the ethidium efflux gene from Escherichia coli   总被引:4,自引:0,他引:4  
The nucleotide sequence of the gene specifying the ethidium efflux system of Escherichia coli has been determined. The translated open reading frame has identified a membrane-bound polypeptide of 110 amino acids (11,960 Da) which shares 42% identity with a staphylococcal protein specifying resistance to ethidium.  相似文献   
992.
A crown-of-thorns starfish control program was conducted at Grub Reef (central Great Barrier Reef) in an area (0.64 km2) which encompassed 53 individual patch reefs. During a two week period, 15 divers injected 3175 starfish with copper sulphate. The program was considered unsuccessful. Although starfish abundance had declined significantly after the control efforts, biological surveys indicated that a relatively large number of starfish remained. The surveys also indicated a general decline in the number of starfish along the reef perimeter, outside the control area. The total cost of the control program was $35 per starfish. These results have important implications for the implementation of future control programs and highlight the need to undertake before and after biological surveys to assess the effectiveness of the control efforts.  相似文献   
993.
994.
995.
Regulation of tobacco acetolactate synthase gene expression.   总被引:4,自引:0,他引:4       下载免费PDF全文
S J Keeler  P Sanders  J K Smith    B J Mazur 《Plant physiology》1993,102(3):1009-1018
  相似文献   
996.
The genetic diversity at the ELA DQβ locus was investigated using polymerase chain reaction and DNA sequencing. Based upon serological methods 16 class II homozygous animals were selected and their genomic DNA was used. A DQβ gene from an equine cDNA library was also sequenced. Our methology and the similarity between the genomic and the cDNA sequences suggest that the studied locus is expressed on equine lymphocytes. In the predicted amino acid sequence the most extensive variation is located at residues 56–60. The pattern of these five amino acids is strongly correlated to the serological ELA class II specificities (W13, W22, W23, Be200). The alleles corresponding to the W23 specificity are the most divergent among the equine DQβ alleles and also from other mammalian DQβ sequences.  相似文献   
997.
Common white facial and leg markings have a multifactorial mode of inheritance in Equus caballus. Evidence for the complexity of the genetic component is the observation that chestnut (e/e) horses have more extensive white markings than do bay (E/-) horses. Computerized records obtained from the Arabian Horse Registry of America, Inc., were used to determine if heterozygous (E/e) bay horses have more extensive white facial markings than do homozygous (E/E) bay horses. Thirty-five sire families were analyzed. Each sire family consists of a sire, his foals, and the dams of those foals. The facial region was divided into five areas, and each horse was given a score from 0 to 5 according to the number of areas with whiteness. Since dams and foals with E/E genotypes cannot be identified in these sire families, mean facial scores were compared in dams and foals that were E/e and E/-. It was assumed that if a difference exists between E/e and E/E horses, the presence of E/E horses in the E/- group would reduce the mean of the E/- group. The results show that Arabian horses with the genotype E/e have more white markings than do horses with the genotype E/-, leading to the conclusion that horses with the genotypes e/e, E/e, and E/E vary as to the quantitative expression of white facial markings, with heterozygotes having an intermediate expression.  相似文献   
998.
999.
The effects of natural blooms of Heterosigma akashiwo on freshwater-and saltwater-acclimated juvenile chinook salmon were assessed. Rates of fish mortality in the blooms were independent of acclimation of fish to seawater and the ambient oxygen levels, but were dependent on concentration of algae and ambient water temperatures. No pathological abnormality to gills or other internal organs in the fish were evident. Aeration or oxygenation of fish cages did not enhance or inhibit fish survival in a H. akashiwo bloom. Cause of death was considered to be due to a labile ichthyotoxic agent.  相似文献   
1000.
A psychological test battery comprising reasoning ability, name and number checking, letter cancellation and associative recall was administered to a group of 70 student nurses at 2-hr intervals from 0800 to 2000 to determine if their performance varied at different times of day. Simultaneously, their oral temperature was recorded. A 7 x 7 latin square design was utilized, and latin square repeated measure analysis of variance was employed. The results indicated variations in oral temperature as the day advanced. While accuracy of performance in cognitive tasks was found to be superior during the early morning, noon or early evening hours, the speed of performance was highest during the period from late-morning to late-evening.  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