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   759916篇
  免费   86785篇
  国内免费   375篇
  847076篇
  2016年   8392篇
  2015年   11556篇
  2014年   13522篇
  2013年   20013篇
  2012年   21856篇
  2011年   22488篇
  2010年   15273篇
  2009年   14085篇
  2008年   19980篇
  2007年   20884篇
  2006年   19568篇
  2005年   18802篇
  2004年   18547篇
  2003年   18202篇
  2002年   17729篇
  2001年   32587篇
  2000年   32833篇
  1999年   26214篇
  1998年   9433篇
  1997年   10181篇
  1996年   9591篇
  1995年   9052篇
  1994年   8812篇
  1993年   9011篇
  1992年   22707篇
  1991年   22455篇
  1990年   21916篇
  1989年   21628篇
  1988年   20051篇
  1987年   19141篇
  1986年   17854篇
  1985年   18308篇
  1984年   15266篇
  1983年   13180篇
  1982年   10193篇
  1981年   9103篇
  1980年   8677篇
  1979年   14607篇
  1978年   11470篇
  1977年   10675篇
  1976年   10178篇
  1975年   11069篇
  1974年   11810篇
  1973年   11668篇
  1972年   10727篇
  1971年   9759篇
  1970年   8330篇
  1969年   8148篇
  1968年   7467篇
  1967年   6555篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
931.
Paracoenia fumosalis Cresson previously known only from the USA and Canada is recorded in the Palaearctic Region for the first time. The adults were collected near a thermal hydrosulfuric spring in the Geiser Valley in Kamchatka. A key to species of the genus Paracoenia occurring in Russia is given.  相似文献   
932.
Four pigeons responded under a progressive-delay procedure. In a signaled-delay condition, a chained variable interval (VI) 30-s progressive time (PT) 4-s schedule was arranged; in an unsignaled-delay condition, a tandem VI 30-s PT 4-s schedule was arranged. Two pigeons experienced a signaled-unsignaled-signaled sequence; whereas, two pigeons experienced an unsignaled-signaled-unsignaled sequence. Effects of saline and d-amphetamine were determined under each condition. At intermediate doses (1.0 and 1.78 m/kg) delay functions were shallower, area under the curve was increased, and, when possible, break points were increased compared to saline; these effects were not systematically related to signaling conditions. These effects on control by delay often were accompanied by decreased response rates at 0 s. These results suggest that stimulus conditions associated with the delay may not play a crucial role in effects of d-amphetamine and other stimulants on behavior controlled by reinforcement delay.  相似文献   
933.
934.
935.
Abstract The parasitoid wasp Trichogramma dendrolimi Matsumura sets the number of progeny allocated to its insect egg hosts according to the duration of its initial transit walk across the host surface. Although cooling the wasp reduces its walking speed, reduced temperature does not affect progeny allocation. By locally heating and cooling the wasps, the initial transit can be thermally uncoupled from the subsequent oviposition. Using this technique we show that the timing of initial transit duration is temperature-dependent. These findings suggest that short interval timing by Trichogramma differs physiologically from the temperature compensated clocks that have been described for other insects.  相似文献   
936.
937.
938.
Diploid and triploid coho salmon Oncorhynchus kisutch transgenic for growth hormone (GH) and control coho salmon were compared for differences in disease resistance and stress response. Resistance to the bacterial pathogen Vibrio anguillarum was not affected in transgenic fish relative to their non‐transgenic counterparts when they were infected at the fry stage, but was lower in transgenic fish when infected near smolting. Vaccination against vibriosis provided equal protection to both transgenic and non‐transgenic fish. Triploid fish showed a lower resistance to vibriosis than their diploid counterparts. Diploid transgenic fish and non‐transgenic fish appeared to show similar physiological and cellular stress responses to a heat shock. These studies provide information useful for both performance and ecological risk assessments of growth‐accelerated coho salmon.  相似文献   
939.
A cytophysiological study was carried out of the functional status of a halo as a response of the host plant to contact with a powdery mildew pathogen. Interactions of the powdery mildew causative agents with barley, wheat, wheat–wheat-grass hybrids, wheat-aegilops lines, and aegilops with different genotypic resistance lead to the expression of haloes during pathogens, which are induced by infection pegs of the primary growth tubes, appressoria, and hyphal lobes. Haloes are visualized using cytochemical reactions to proteins and scanning electron microscopy. The observed differences in the size of haloes and intensity of their staining (uniform or zonal) are related, to a great extent, to individual reactions of the plant cell at the penetration site and, to a lesser extent, to the level of genotypic resistance. An analysis of electron microscopy and cytochemistry studies suggests that the halo as a physiologically active zone is localized at the level of the plant cell plasmalemma. Active taxis of the cell organelles to the site of infection during the formation of a halo suggests that some kind of informational signals to changes in the cell metabolism are spread from the halo zone, which lead to compatible or incompatible interactions.  相似文献   
940.
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