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   863048篇
  免费   95723篇
  国内免费   500篇
  959271篇
  2018年   8063篇
  2016年   10848篇
  2015年   14096篇
  2014年   16830篇
  2013年   23998篇
  2012年   26765篇
  2011年   27459篇
  2010年   18699篇
  2009年   17309篇
  2008年   24547篇
  2007年   25515篇
  2006年   23824篇
  2005年   22996篇
  2004年   22722篇
  2003年   21540篇
  2002年   21045篇
  2001年   39698篇
  2000年   39665篇
  1999年   31304篇
  1998年   10650篇
  1997年   11215篇
  1996年   10442篇
  1995年   9709篇
  1994年   9362篇
  1993年   9425篇
  1992年   25465篇
  1991年   24693篇
  1990年   24066篇
  1989年   23392篇
  1988年   21741篇
  1987年   20546篇
  1986年   19239篇
  1985年   18943篇
  1984年   15665篇
  1983年   13435篇
  1982年   10167篇
  1981年   9165篇
  1980年   8755篇
  1979年   14968篇
  1978年   11637篇
  1977年   10674篇
  1976年   9973篇
  1975年   10915篇
  1974年   12040篇
  1973年   11702篇
  1972年   10819篇
  1971年   9879篇
  1970年   8590篇
  1969年   8294篇
  1968年   7919篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
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   
992.
993.
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   
994.
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   
995.
996.
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   
997.
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   
998.
The generation and characterization of new sheep-hamster cell hybrids is reported from the fusion of sheep white blood cells with six different hamster auxotrophs. Selection from these and previously generated cell hybrids has led to the production of a panel of 30 hybrids covering the complete sheep genome of 28 chromosomes. Over half of the cell hybrids in this panel contain single sheep chromosomes. By complementation, the following new assignments have been made using the panel: phosphoribosyl N-formylglycinamide amidotransferase (PRFGA) to sheep chromosome (chr) 11; adenylosuccinate synthetase (ADSS) to sheep chr 12; adenylosuccinate lyase (ADSL) to sheep chr 3q; 3-hydroxy-3-methylglutaryl-coenzyme A synthase (HMGCS) to sheep chr 16; dihydrofolate reductase (DHFR) to sheep chr 5; and adenine phosphoribosyltransferase (APRT) to sheep chr 14. The gene phosphoribosylaminoinidazole-carboxamide formyltransferase/Inosinicase (PRACFT) has now been regionally assigned to chr 2q. By isozyme analysis, phosphogluconate dehydrogenase (PGD) was assigned to sheep chr 12, anchoring the sheep syntenic group U1 to this chromosome, and mannose phosphate isomerase (MPI) was assigned to sheep chr 18. Furthermore, the chromosomal assignment of 110 microsatellites was confirmed using this cell panel.  相似文献   
999.
In order to develop direct methods for determining the extent of metabolic incorporation of isotopically labeled amino acids into a protein, the determination of deuterated tryptophan in [2H5]tryptophan-bacteriorhodopsin was investigated. The isotopically modified protein was subjected to alkaline hydrolysis. After phenyl isothiocyanate derivatization of the hydrolysate, the mixture was separated by reversed-phase liquid chromatography. Field desorption mass spectrometry and thermospray mass spectrometry were investigated for their ability to determine the ratio between [2H5]tryptophan and total tryptophan in the collected fractions. In order to check the procedure a set of known tryptophan/[2H5]tryptophan mixtures were passed through the same derivatization, HPLC separation, and lyophilization procedure as used for the biological samples.  相似文献   
1000.
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