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   1789528篇
  免费   173596篇
  国内免费   1868篇
  2021年   18432篇
  2018年   19688篇
  2017年   18564篇
  2016年   29713篇
  2015年   43723篇
  2014年   51997篇
  2013年   78194篇
  2012年   51356篇
  2011年   43300篇
  2010年   48526篇
  2009年   48322篇
  2008年   39404篇
  2007年   38648篇
  2006年   41076篇
  2005年   42036篇
  2004年   41164篇
  2003年   38383篇
  2002年   36317篇
  2001年   60071篇
  2000年   58457篇
  1999年   51385篇
  1998年   28635篇
  1997年   28471篇
  1996年   27416篇
  1995年   25656篇
  1994年   25223篇
  1993年   24555篇
  1992年   43359篇
  1991年   41341篇
  1990年   39926篇
  1989年   40142篇
  1988年   37216篇
  1987年   34987篇
  1986年   33189篇
  1985年   34565篇
  1984年   31506篇
  1983年   27256篇
  1982年   25013篇
  1981年   23638篇
  1980年   22036篇
  1979年   27704篇
  1978年   24209篇
  1977年   22741篇
  1976年   21345篇
  1975年   22017篇
  1974年   23378篇
  1973年   23479篇
  1972年   20533篇
  1971年   18778篇
  1970年   16420篇
排序方式: 共有10000条查询结果,搜索用时 671 毫秒
211.
The biologically important pyridoxinato(1−) ligand (anionic vitamin B6) shows the rare phenolate-hydroxymethyl chelation plus bridging mode through the pyridine-nitrogen atom towards zinc(II) to give the one-dimensional (1D) coordination polymer {(acetato-κO)-aqua-μ-[2-methyl-3-oxy-4,5-di(hydroxymethyl)pyridine-κN:O,O′]zinc(II)}·monohydrate with polar packing of adjacent chains along the polar c axis (in space group Pc) through strong inter-chain hydrogen bonding.  相似文献   
212.
Mild chymotrypsin digestion of native lipase (449 amino acids) preferentially cleaved the Phe 335-Ala 336 bond. On SDS-gel electrophoresis, 3 major bands were observed: band 1 (52 kDa) representing native lipase, bands 2 and 3 (40 and 12 kDa) representing the two lipase fragments A and B. Fragment A does not retain lipase activity but maintains its ability to adsorb to interfaces. Fragment B was identified with the lipase C-terminal region (336-449). It does not exhibit any activity towards tributyrylglycerol emulsions and any ability to adsorb to interfaces.  相似文献   
213.
1. Seasonal regulation of energy economy was studied in farm-raised blue foxes (Alopex lagopus) and raccoon dogs (Nyctereutes procyonoides; Gray, 1834). 2. Both species exhibited marked seasonal changes in body weight, body energy content, food intake and locomotor activity. Seasonal changes in food efficiency were also evident. 3. The results emphasize the important role of both energy intake and energy expenditure in the regulation process. 4. It can be concluded that regulation of seasonal energy balance is a dynamic process with various interactions between different factors.  相似文献   
214.
215.
1. As trees age, they undergo significant physiological and morphological changes. Nevertheless, tree ontogeny and its impacts on herbivores are often overlooked as determinants of plant–herbivore population dynamics and the strength of plant–herbivore interactions. 2. Juniperus (Cupressaceae) is a dominant, long‐lived conifer that serves as the sole host to a specialised assemblage of caterpillars. Over the past 150 years, several juniper species in western North America have expanded their geographic occupancy at local and regional scales, which has resulted in an increase in the number of immature trees on the landscape. Using assays in the laboratory, the effects of tree ontogeny on caterpillar performance and oviposition preference for two juniper specialist caterpillars, Callophrys gryneus (Lycaenidae) and Glena quinquelinearia (Geometridae), were examined. The study considered whether responses to tree ontogeny were consistent across caterpillar species and juniper host species. 3. Tree age was found to be a reliable predictor of caterpillar performance, with caterpillars developing more quickly and growing larger when fed foliage from young trees. Differences in the phytochemical diversity between foliage from trees of different ages might help to explain observed differences in caterpillar performance. Interestingly, the specialist butterfly, C. gryneus, displayed an oviposition preference for foliage from old‐growth Juniperus osteosperma trees, despite the fact that larvae of this species performed poorly on older trees. 4. It is concluded that young juniper trees are an important resource for the specialised Lepidopteran community and that tree ontogeny is an important component of intraspecific variation, which contributes to the structure of plant–herbivore communities.  相似文献   
216.
217.
218.
219.
220.
The marine bacteriumVibrio anguillarum causes disease in fish worldwide and is particularly devastating in aquaculture. Little is known about the ecology ofV. anguillarum in the environment and how this may relate to the pathogenicity of this organism. Combining membrane filtration and a species-specific DNA probe, culturableV. anguillarum cells were detected in water from three habitats and in chinook salmon (Onchorynchus tshawytscha) tissue samples. Results show that different marine habitats have a marked effect on cell numbers and that water temperature may play a role in the culturability and distribution ofV. anguillarum. Vibrio anguillarum was detected from the gills of salmon within 24 h of transfer of fingerlings from freshwater to seawater, with cell numbers reaching a concentration of 1.9 × 102 cells g–1 tissue 28 days post transfer.Vibrio anguillarum cell numbers were low in the colon throughout the study, andV. anguillarum was not detected in healthy kidney samples. The methodology reported in this paper allows the accurate quantification of culturableV. anguillarum cells and has allowed a preliminary study of the ecology of this species.  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