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   3612篇
  免费   290篇
  2024年   2篇
  2023年   20篇
  2022年   19篇
  2021年   101篇
  2020年   63篇
  2019年   85篇
  2018年   87篇
  2017年   81篇
  2016年   154篇
  2015年   212篇
  2014年   222篇
  2013年   311篇
  2012年   311篇
  2011年   311篇
  2010年   191篇
  2009年   149篇
  2008年   243篇
  2007年   234篇
  2006年   241篇
  2005年   177篇
  2004年   170篇
  2003年   134篇
  2002年   109篇
  2001年   35篇
  2000年   27篇
  1999年   20篇
  1998年   24篇
  1997年   22篇
  1996年   15篇
  1995年   19篇
  1994年   19篇
  1993年   16篇
  1992年   8篇
  1991年   17篇
  1990年   12篇
  1989年   9篇
  1988年   9篇
  1987年   2篇
  1986年   2篇
  1985年   4篇
  1983年   2篇
  1981年   2篇
  1980年   1篇
  1978年   2篇
  1976年   1篇
  1975年   1篇
  1974年   1篇
  1973年   1篇
  1972年   1篇
  1910年   2篇
排序方式: 共有3902条查询结果,搜索用时 31 毫秒
1.
Abstract

Several aroma hops (Humulus lupulus L.) were recently introduced in Northern Italy as a small-scale production of excellence. In this preliminary study, the American cultivar Cascade was investigated in a combined morphological and phytochemical survey. Morphological investigation on trichome structure, density and distribution was performed by scanning electron microscopy and light microscopy. Essential oil composition, α/β-acid and polyphenol profiles over 3 years were determined by GC-MS and HPLC analyses. Two types of non-glandular (simple and cystolithic) and glandular (peltate and bulbous) trichomes were observed on leaves and female inflorescences. The peltate trichomes resulted as the main sites of terpene production and accumulation. The essential oil profiles showed myrcene, β-caryophyllene, (E)-β-farnesene and humulene epoxide II as the dominant compounds over the three collection times, although with different relative abundances. The presence of two exclusive compounds, γ-muurolene and trans-γ-cadinene, characterized the investigated cv. Cascade, potentially enhancing herbal, woody and spicy aroma traits of this cultivation in Northern Italy. The bitter acid composition showed quantitative values consistent with literature data only for the second and third monitoring year. Qualitative differences in polyphenol content were also recorded, for the presence of quercetin-3-O-malonylglucoside and kaempferol-3-O-rutinoside, which may characterize this peculiar Italian cultivation.  相似文献   
2.
Helicobacter pylori adhere to Kato III and Hela S3 cells in monolayer cultures. To explore whether cell surface glycoconjugates on these two cell lines mediate binding of H. pylori, various carbohydrates, glycoproteins, and glycolipids were tested to inhibit H. pylori cell adhesion. The adhesion was measured (i) with a urease-based assay and (ii) by cells stained with fluorescein. Sodium periodate and sialidase treatment (but not α- or β-galactosidase, heparitinase, lysozyme, or trypsin) inhibited H. pylori binding to both cell lines. Sulfatides and sulfated glycoconjugates (50 μg/ml) but not heparin or a number of simple carbohydrates inhibited binding (1 mg/ml). The two H. pylori strains studied (CCUG 17874 and strain 25) showed high binding of soluble 125I-labeled heparin and other sulfated carbohydrate compounds. Received: 5 July 1996 / Accepted: 17 October 1996  相似文献   
3.
4.
5.
6.
Grooming in primates is often considered a “currency” that can be exchanged for other “services” or “commodities” such as reciprocal grooming, coalitionary support, infant handling, tolerance around food sources, active food sharing, or mating opportunities. Previous studies on primate grooming‐for‐sex exchange viewed the males as the demanding class, with the females as suppliers of mating opportunities. In this study, we examine the broader context of grooming‐for‐mating exchange in Barbary macaques in Gibraltar. Our data show that Barbary macaque males groom females with whom they are mating more frequently and for longer periods than other females, and the relationship between grooming and mating remains significant in both sexual and nonsexual contexts. In addition, females groomed males with whom they were mating more frequently and for longer periods than other males. In both sexes, grooming was observed to be far more frequent and to occur for longer durations in sexual compared to nonsexual contexts. We did not find any difference in grooming behavior between presexual and postsexual contexts. Our data suggest that there is no simple model to describe Barbary macaque grooming patterns in sexual contexts. Although our results are partly consistent with male use of grooming as payment for mating, broadly assessed grooming‐mating patterns cannot be solely explained by a male‐driven grooming‐for‐mating exchange.  相似文献   
7.
8.
9.
Diatoms, an important group of phytoplankton, bloom annually in the Southern Ocean, covering thousands of square kilometers and dominating the region''s phytoplankton communities. In their role as the major food source to marine grazers, diatoms supply carbon, nutrients and energy to the Southern Ocean food web. Prevailing environmental conditions influence diatom phenotypic traits (for example, photophysiology, macromolecular composition and morphology), which in turn affect the transfer of energy, carbon and nutrients to grazers and higher trophic levels, as well as oceanic biogeochemical cycles. The paucity of phenotypic data on Southern Ocean phytoplankton limits our understanding of the ecosystem and how it may respond to future environmental change. Here we used a novel approach to create a ‘snapshot'' of cell phenotype. Using mass spectrometry, we measured nitrogen (a proxy for protein), total carbon and carbon-13 enrichment (carbon productivity), then used this data to build spectroscopy-based predictive models. The models were used to provide phenotypic data for samples from a third sample set. Importantly, this approach enabled the first ever rate determination of carbon productivity from a single time point, circumventing the need for time-series measurements. This study showed that Chaetoceros simplex was less productive and had lower protein and carbon content during short-term periods of high salinity. Applying this new phenomics approach to natural phytoplankton samples could provide valuable insight into understanding phytoplankton productivity and function in the marine system.  相似文献   
10.
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