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   11476篇
  免费   3642篇
  国内免费   4篇
林业   438篇
农学   654篇
基础科学   11篇
  2270篇
综合类   649篇
农作物   725篇
水产渔业   2841篇
畜牧兽医   5915篇
园艺   81篇
植物保护   1538篇
  2023年   29篇
  2022年   70篇
  2021年   248篇
  2020年   547篇
  2019年   1091篇
  2018年   1005篇
  2017年   1048篇
  2016年   1077篇
  2015年   942篇
  2014年   975篇
  2013年   1222篇
  2012年   799篇
  2011年   890篇
  2010年   808篇
  2009年   469篇
  2008年   529篇
  2007年   383篇
  2006年   374篇
  2005年   341篇
  2004年   324篇
  2003年   282篇
  2002年   308篇
  2001年   217篇
  2000年   221篇
  1999年   88篇
  1998年   32篇
  1997年   30篇
  1996年   25篇
  1995年   26篇
  1994年   20篇
  1993年   29篇
  1992年   29篇
  1991年   28篇
  1990年   37篇
  1989年   40篇
  1988年   31篇
  1987年   40篇
  1986年   21篇
  1985年   34篇
  1984年   24篇
  1983年   29篇
  1981年   21篇
  1978年   27篇
  1977年   24篇
  1976年   29篇
  1975年   18篇
  1974年   22篇
  1973年   23篇
  1972年   22篇
  1970年   17篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
CARL LUDWIG     
  相似文献   
992.
993.
994.
Searches for extrasolar planets have uncovered an astonishing diversity of planetary systems, yet the frequency of solar system analogs remains unknown. The gravitational microlensing planet search method is potentially sensitive to multiple-planet systems containing analogs of all the solar system planets except Mercury. We report the detection of a multiple-planet system with microlensing. We identify two planets with masses of approximately 0.71 and approximately 0.27 times the mass of Jupiter and orbital separations of approximately 2.3 and approximately 4.6 astronomical units orbiting a primary star of mass approximately 0.50 solar mass at a distance of approximately 1.5 kiloparsecs. This system resembles a scaled version of our solar system in that the mass ratio, separation ratio, and equilibrium temperatures of the planets are similar to those of Jupiter and Saturn. These planets could not have been detected with other techniques; their discovery from only six confirmed microlensing planet detections suggests that solar system analogs may be common.  相似文献   
995.
The primary structure of human apolipoprotein (apo) B-48 has been deduced and shown by a combination of DNA excess hybridization, sequencing of tryptic peptides, cloned complementary DNAs, and intestinal messenger RNAs (mRNAs) to be the product of an intestinal mRNA with an in-frame UAA stop codon resulting from a C to U change in the codon CAA encoding Gln2153 in apoB-100 mRNA. The carboxyl-terminal Ile2152 of apoB-48 purified from chylous ascites fluid has apparently been cleaved from the initial translation product, leaving Met2151 as the new carboxyl-terminus. These data indicate that approximately 85% of the intestinal mRNAs terminate within approximately 0.1 to 1.0 kilobase downstream from the stop codon. The other approximately 15% have lengths similar to hepatic apoB-100 mRNA even though they have the same in-frame stop codon. The organ-specific introduction of a stop codon to a mRNA appears unprecedented and might have implications for cryptic polyadenylation signal recognition and RNA processing.  相似文献   
996.
Through a discursive and organizational analysis we seek to understand the Biosafety Protocol and the place of socioeconomic regulation of agricultural biotechnology in it. The literature on the Protocol has been fairly extensive, but little of it has explored debates over socioeconomic regulation during the negotiation process or the regulatory requirements specified in the final document. This case is especially important at a time when the spread of neoliberalism is increasingly associated with deregulation, because it sheds light on the conditions under which circumvention of the market is deemed legitimate and socio-economic regulation of agricultural technology is possible. Daniel Lee Kleinman is a professor in the Department of Rural Sociology at the University of Wisconsin, Madison, where he is also affiliated with the Holtz Center for Science and Technology Studies and the Integrated Liberal Studies Program. He is the author and editor of a number of books, including Impure Cultures: University Biology and the World of Commerce (2003). Abby J. Kinchy is a PhD candidate in the Departments of Sociology and Rural Sociology at the University of Wisconsin, Madison. Her current research examines the controversies surrounding the genetic “contamination” of Mexican maize and Canadian canola.  相似文献   
997.
Genetics aims to understand the relation between genotype and phenotype. However, because complete deletion of most yeast genes ( approximately 80%) has no obvious phenotypic consequence in rich medium, it is difficult to study their functions. To uncover phenotypes for this nonessential fraction of the genome, we performed 1144 chemical genomic assays on the yeast whole-genome heterozygous and homozygous deletion collections and quantified the growth fitness of each deletion strain in the presence of chemical or environmental stress conditions. We found that 97% of gene deletions exhibited a measurable growth phenotype, suggesting that nearly all genes are essential for optimal growth in at least one condition.  相似文献   
998.
The neuron-specific protein PGP 9.5 is a ubiquitin carboxyl-terminal hydrolase   总被引:42,自引:0,他引:42  
A complementary DNA (cDNA) for ubiquitin carboxyl-terminal hydrolase isozyme L3 was cloned from human B cells. The cDNA encodes a protein of 230 amino acids with a molecular mass of 26.182 daltons. The human protein is very similar to the bovine homolog, with only three amino acids differing in over 100 residues compared. The amino acid sequence deduced from the cDNA was 54% identical to that of the neuron-specific protein PGP 9.5. Purification of bovine PGP 9.5 confirmed that it is also a ubiquitin carboxyl-terminal hydrolase. These results suggest that a family of such related proteins exists and that their expression is tissue-specific.  相似文献   
999.
Lee HA 《Science (New York, N.Y.)》1960,131(3413):1609-1611
Geological investigations, archeological studies, and radiocarbon dates indicate a similarity of events around Hudson Bay, commencing at the time Hudson Bay Basin was freed of glacier ice. The sea that then spread around Hudson Bay 7000 to 8000 years ago is here named "Tyrrell Sea." The subsequent rate of land emergence decreased exponentially.  相似文献   
1000.
Ma DD  Lee CS  Au FC  Tong SY  Lee ST 《Science (New York, N.Y.)》2003,299(5614):1874-1877
Small-diameter (1 to 7 nanometers) silicon nanowires (SiNWs) were prepared, and their surfaces were removed of oxide and terminated with hydrogen by a hydrofluoric acid dip. Scanning tunneling microscopy (STM) of these SiNWs, performed both in air and in ultrahigh vacuum, revealed atomically resolved images that can be interpreted as hydrogen-terminated Si (111)-(1 x 1) and Si (001)-(1 x 1) surfaces corresponding to SiH3 on Si (111) and SiH2 on Si (001), respectively. These hydrogen-terminated SiNW surfaces seem to be more oxidation-resistant than regular silicon wafer surfaces, because atomically resolved STM images of SiNWs were obtained in air after several days' exposure to the ambient environment. Scanning tunneling spectroscopy measurements were performed on the oxide-removed SiNWs and were used to evaluate the electronic energy gaps. The energy gaps were found to increase with decreasing SiNW diameter from 1.1 electron volts for 7 nanometers to 3.5 electron volts for 1.3 nanometers, in agreement with previous theoretical predictions.  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