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   48972篇
  免费   2688篇
  国内免费   33篇
林业   1701篇
农学   1661篇
基础科学   251篇
  5835篇
综合类   8196篇
农作物   1545篇
水产渔业   2717篇
畜牧兽医   26328篇
园艺   574篇
植物保护   2885篇
  2019年   416篇
  2018年   777篇
  2017年   823篇
  2016年   743篇
  2015年   607篇
  2014年   750篇
  2013年   1736篇
  2012年   1347篇
  2011年   1690篇
  2010年   1154篇
  2009年   1096篇
  2008年   1682篇
  2007年   1593篇
  2006年   1497篇
  2005年   1380篇
  2004年   1394篇
  2003年   1371篇
  2002年   1184篇
  2001年   1913篇
  2000年   1984篇
  1999年   1498篇
  1998年   569篇
  1997年   499篇
  1996年   449篇
  1995年   612篇
  1994年   524篇
  1993年   459篇
  1992年   1151篇
  1991年   1211篇
  1990年   1178篇
  1989年   1097篇
  1988年   1056篇
  1987年   1081篇
  1986年   1034篇
  1985年   966篇
  1984年   803篇
  1983年   683篇
  1982年   421篇
  1981年   382篇
  1979年   635篇
  1978年   500篇
  1977年   441篇
  1976年   408篇
  1975年   451篇
  1974年   523篇
  1973年   507篇
  1972年   531篇
  1971年   449篇
  1970年   424篇
  1969年   464篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
Cerumen in American Indians: genetic implications of sticky and dry types   总被引:2,自引:0,他引:2  
Occurrence of sticky and dry cerumen was determined in 483 Indians from various tribes of the United States. The elevated frequencies of the allele for dry cerumen, found in Indians of pure ancestry, support the theory of the mongoloid origin of the American Indian. Potential application of cerumen quality as a marker for genetic and anthropological studies is discussed.  相似文献   
902.
Hemoglobin Gun Hill, a new variant of adult hemoglobin, was found in a Caucasian and one of his three daughters. The abnormal hemoglobin had only half of the expected number of heme groups. Five amino acid residues appeared to be missing from the beta-globin chains. These residues occur in linear sequence in normal beta-chains in a region involved in heme-globin binding. A deletion of five amino acids in the beta-chains of hemoglobin Gun Hill is postulated. The most likely mechanism for the origin of such a hemoglobin variant would appear to be unequal crossing-over during meiosis.  相似文献   
903.
Two harvests of ocean-growing red tide, comprised mainly of Gonyaulax polyedra, were evaluated in limited trials of rat feeding. The protein of red tide (25 to 30 percent, dry basis) supported growth satisfactorily. The essential amino acid composition of the protein closely resembles that of casein, the major protein of milk. As a marine resource, plankton represents a challenge for research.  相似文献   
904.
Radio tracking of homing bats   总被引:1,自引:0,他引:1  
Neotropical bats, Phyllostomas hastatus, were released 10 kilometers from their home roost, and their homeward flights were tracked by radio. Flights of bats with unimpeded vision were strongly oriented in the homeward direction, while the flights of blindfolded bats did not show this marked orientation.  相似文献   
905.
Hemoglobin G Taegu, an electrophoretically slow hemoglobin with a structural anomaly believed to be in the beta-T-3 section of the beta chain, was the only variant found among 6700 normal Koreans. Four subjects, 0.06 percent, had the G-hemoglobin variant in addition to normal hemoglobin A. Hemoglobin E, known in numerous groups from Southeast Asia and the variant most frequently seen in Chinese subjects, was not found among the Koreans we tested.  相似文献   
906.
Virus-induced peliosis hepatitis in rats   总被引:3,自引:0,他引:3  
Inoculation of newborn Fischer, W/Fu, or Lewis rats with the 9H virus recently isolated from leukemic rat tissues resulted in the development of peliosis hepatis. The virus was recoverable from the peliotic livers. The cystic lesions were of the parenchymal peliosis type and appeared to be the result of focal hepatic necrosis followed by hemorrhage.  相似文献   
907.
Voluntary control of microsaccades during maintained monocular fixation   总被引:2,自引:0,他引:2  
A contact-lens technique was used to record eye movements made by two subjects instructed either to "fixate" stationary white-light targets or to "hold" their eyes in position in the presence of the same targets. A marked reduction in saccade rate, frequently reaching zero throughout 9.8-second trials, was observed under the "hold" instruction.  相似文献   
908.
Indium concentrations have been determined by neutron activation in four members of each of the L3, L4, L5, and L6 chondritic meteorite classes. The range of concentrations is found to be from 0.14 to 22 parts per billion, with the highest values in L3 chondrites and the lowest values in the L5 and L6 classes. Plots of indium concentration versus relative mean deviation of pyroxene iron content, total carbon concentration, and primordial argon-36 concentration show positive correlations to varying degrees. Indium concentration appears to be a valuable parameter relating to variable formation conditions of the chondritic meteorites during the early history of the solar system.  相似文献   
909.
Responding elicited in the squirrel monkey by electric shocks presented every 60 seconds was gradually altered in temporal patterning, especially when the shock was also produced by responses under a 30-second fixed-interval schedule. The initially elicited pattern of maximal responding just after each shock was altered by the recurrent shock and by the added fixed-interval schedule to a pattern of maximal responding just before each shock. Most shocks were produced by responses and the response pattern was maintained for several months, but little responding occurred when shocks were omitted.  相似文献   
910.
Continuous-wave signals transmitted from Lunar Orbiter I have been received on Earth after they have been reflected from the surface of the moon. The frequency spectrum of the reflected signals is used to locate discrete, heterogeneous, scattering centers on the lunar surface. The scattering centers are probably distinguished from the surrounding terrain by a higher surface reflectivity. Continuous-wave bistatic radar could provide an important new method for the study and mapping of planetary surfaces.  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