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   914篇
  免费   68篇
  国内免费   2篇
林业   65篇
农学   14篇
基础科学   3篇
  127篇
综合类   184篇
农作物   18篇
水产渔业   107篇
畜牧兽医   368篇
园艺   41篇
植物保护   57篇
  2023年   12篇
  2022年   9篇
  2021年   28篇
  2020年   31篇
  2019年   37篇
  2018年   34篇
  2017年   21篇
  2016年   30篇
  2015年   30篇
  2014年   24篇
  2013年   53篇
  2012年   56篇
  2011年   80篇
  2010年   40篇
  2009年   30篇
  2008年   47篇
  2007年   62篇
  2006年   60篇
  2005年   53篇
  2004年   43篇
  2003年   37篇
  2002年   36篇
  2001年   11篇
  2000年   10篇
  1999年   5篇
  1995年   3篇
  1994年   2篇
  1993年   7篇
  1992年   4篇
  1991年   4篇
  1990年   6篇
  1989年   4篇
  1988年   2篇
  1987年   2篇
  1986年   4篇
  1985年   6篇
  1984年   5篇
  1983年   5篇
  1980年   4篇
  1978年   6篇
  1977年   6篇
  1976年   7篇
  1975年   3篇
  1974年   2篇
  1973年   2篇
  1969年   2篇
  1963年   2篇
  1961年   2篇
  1954年   2篇
  1942年   2篇
排序方式: 共有984条查询结果,搜索用时 593 毫秒
101.
102.
103.
Three disaccharides were isolated and purified from a commercial total invert sugar (TIS). The structures of these compounds were determined by a combination of acid and enzymatic hydrolysis studies, chromatographic comparison to standards, and nuclear magnetic resonance spectroscopy. These carbohydrates were identified as O-alpha-D-glucopyranosyl-(1-->3)-D-fructose (IS1), O-beta-D-fructofuranosyl-(2-->6)-D-glucose (IS2), and O-alpha-D-glucopyranosyl-beta-D-glucopyranoside (IS3). On the basis of these structures a mechanism for the hydrochloric acid catalyzed hydrolysis of sucrose is proposed: protonation of the glycosidic oxygen of sucrose leading to the formation of glucopyranosyl and fructofuranosyl oxonium ions of D-glucose or D-fructose, respectively, followed by nucleophilic attack of these ions by D-glucose or D-fructose at either the alpha- or beta-face.  相似文献   
104.
Mulcahy NJ  Call J 《Science (New York, N.Y.)》2006,312(5776):1038-1040
Planning for future needs, not just current ones, is one of the most formidable human cognitive achievements. Whether this skill is a uniquely human adaptation is a controversial issue. In a study we conducted, bonobos and orangutans selected, transported, and saved appropriate tools above baseline levels to use them 1 hour later (experiment 1). Experiment 2 extended these results to a 14-hour delay between collecting and using the tools. Experiment 3 showed that seeing the apparatus during tool selection was not necessary to succeed. These findings suggest that the precursor skills for planning for the future evolved in great apes before 14 million years ago, when all extant great ape species shared a common ancestor.  相似文献   
105.
Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.  相似文献   
106.
Until now, continental shelf environments have been monitored with highly localized line-transect methods from slow-moving research vessels. These methods significantly undersample fish populations in time and space, leaving an incomplete and ambiguous record of abundance and behavior. We show that fish populations in continental shelf environments can be instantaneously imaged over thousands of square kilometers and continuously monitored by a remote sensing technique in which the ocean acts as an acoustic waveguide. The technique has revealed the instantaneous horizontal structural characteristics and volatile short-term behavior of very large fish shoals, containing tens of millions of fish and stretching for many kilometers.  相似文献   
107.
Cochlosoma anatis is a flagellated intestinal parasite that infects a variety of avian species. C. anatis infections have been associated with decreased weight gain and increased morbidity and mortality. Conditions favoring the growth of this organism in birds are current pathogenic intestinal infections and/or young age. There is little data describing the life cycle of this parasite. In this study, electron microscopy images are presented that document longitudinal binary fission of the trophozoite stage and outline the events of pseudocyst formation, which includes a rounding stage. Evidence provided here indicates that the pseudocyst stage may be a mechanism for transmission of this organism. The observations reported here provide additional evidence of homology between Cochlosoma and members of the trichomonad order.  相似文献   
108.
Detection of juice-to-juice adulteration based on chemical composition studies is a common method used by government regulatory agencies and food companies. This study investigated the use of major carbohydrate (fructose, glucose and sucrose), polyol (sorbitol), proline, and phenolic profiles as indicators of pear adulteration of apple juice (PAAJ). For this work, a total of 105 authentic apple juice samples from 13 countries and 27 authentic pear juice samples from 5 countries were analyzed. Because the major carbohydrate ranges for these juices showed significant overlap their use as markers for PAAJ detection would be very limited. It was found that sorbitol and proline means for apple and pear were significantly different; however, their broad natural ranges would afford PAAJ at levels up to 30% without detection. In addition, careful selection of the pear juice used as the adulterant would further limit the usefulness of these markers for PAAJ detection. Arbutin was conclusively identified as a marker for pear juice on the basis of its presence in all 27 authentic pear samples and its absence (<0.5 microg/mL) in all 105 apple juice samples analyzed in this study. The application of the developed HPLC-PDA method for arbutin analysis to detect PAAJ at levels as low as 2% (v/v) was demonstrated. A confirmation method for the presence of arbutin in pure pear juice and apple adulterated with pear juice was introduced on the basis of the hydrolysis of arbutin to hydroquinone employing beta-glucosidase, with reactant and product monitoring by HPLC-PDA.  相似文献   
109.
The feasibility of global polio eradication is being questioned as a result of continued transmission in a few localities that act as sources for outbreaks elsewhere. Perhaps the greatest challenge is in India, where transmission has persisted in Uttar Pradesh and Bihar despite high coverage with multiple doses of vaccine. We estimate key parameters governing the seasonal epidemics in these areas and show that high population density and poor sanitation cause persistence by not only facilitating transmission of poliovirus but also severely compromising the efficacy of the trivalent vaccine. We analyze strategies to counteract this and show that switching to monovalent vaccine may finally interrupt virus transmission.  相似文献   
110.
We report a mild method for generating the diphosphorus molecule or its synthetic equivalent in homogeneous solution; the P2 allotrope of the element phosphorus is normally obtained only under extreme conditions (for example, from P4 at 1100 kelvin). Diphosphorus is extruded from a niobium complex designed for this purpose and can be trapped efficiently by two equivalents of an organic diene to produce an organodiphosphorus compound. Diphosphorus stabilized by coordination to tungsten pentacarbonyl can be generated similarly at 25 degrees C, and in this stabilized form it still efficiently consumes two organic diene molecules for every diphosphorus unit.  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