全文获取类型
收费全文 | 1347篇 |
免费 | 108篇 |
专业分类
林业 | 56篇 |
农学 | 33篇 |
基础科学 | 2篇 |
156篇 | |
综合类 | 156篇 |
农作物 | 32篇 |
水产渔业 | 60篇 |
畜牧兽医 | 894篇 |
园艺 | 23篇 |
植物保护 | 43篇 |
出版年
2023年 | 17篇 |
2022年 | 16篇 |
2021年 | 21篇 |
2020年 | 39篇 |
2019年 | 37篇 |
2018年 | 34篇 |
2017年 | 28篇 |
2016年 | 31篇 |
2015年 | 37篇 |
2014年 | 27篇 |
2013年 | 50篇 |
2012年 | 78篇 |
2011年 | 95篇 |
2010年 | 37篇 |
2009年 | 42篇 |
2008年 | 82篇 |
2007年 | 88篇 |
2006年 | 83篇 |
2005年 | 72篇 |
2004年 | 74篇 |
2003年 | 92篇 |
2002年 | 69篇 |
2001年 | 19篇 |
2000年 | 31篇 |
1999年 | 22篇 |
1998年 | 16篇 |
1997年 | 13篇 |
1996年 | 13篇 |
1995年 | 8篇 |
1994年 | 6篇 |
1993年 | 12篇 |
1992年 | 15篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1973年 | 9篇 |
1972年 | 9篇 |
1971年 | 4篇 |
1970年 | 4篇 |
1968年 | 4篇 |
1966年 | 5篇 |
1909年 | 4篇 |
排序方式: 共有1455条查询结果,搜索用时 15 毫秒
101.
Fleischer RL Haines EL Hanneman RE Hart HR Kasper JS Lifshin E Woods RT Price PB 《Science (New York, N.Y.)》1970,167(3918):568-571
Particle tracks in Apollo 11 samples are dominantly of cosmic ray and solar origin: primary galactic and solar flare particles, likely spallation recoil tracks, and possible solar-wind heavy particles. The energy spectrum of irongroup nuclei is inferred from track density gradients in surface layers, and a limit of < 10(-7) centimeter per year is deduced for the surface erosion rate. From cosmic ray tracks in rock and core samples it is clear that the lunar soil is stirred often during each few million years. X-rays reveal augite, anorthite, olivine, ilmenite, troilite, nonmeteoritic iron, and assorted glasses, but no major structural damage. Hydrogen, helium, and other gases in the fines are compatible with expected solar wind ratios. 相似文献
102.
A large form of the copepod Pseudocalanus is found in two warm, semi-landlocked fiords in arctic Canada, together with a similar but smaller form attributable to the widespread P. minutus. The large form has the same chromosomne number as P. minutus, but has larger chromosomes and a higher nuclear DNA content. There are suggestions in the literature that other similar polytenic and cryptic species occur among copepods. 相似文献
103.
William I. Woods 《Agriculture and Human Values》2004,21(2-3):255-261
Cahokia, the largest pre-European settlement in North America, was situated on the Middle Mississippi River floodplain and flourished for approximately three hundred years from the 10th century AD onward. The site was favorably located from an environmental standpoint, being proximal to a diversity of microhabitats including expanses of open water and marshes from which the essential, renewable fish protein could be procured. More importantly, the largest local zone of soils characterized as optimal for prehistoric hoe cultivation lay immediately to the east. Here, on the floodplain and along its bordering alluvial fans, the large maize outfields were situated, while the multi-crop house gardens were placed within the habitation zone on soils that had often been culturally enriched by prior occupation. As successful as this strategy might have been for small, dispersed populations in such a plentiful environment, nucleation of large numbers of people at Cahokia provided a different adaptive context that ultimately led to ruinous consequences. The seeds for the city's destruction centered anthropogenically produced environmental degradation. Demands on wood resources for fuel and construction were enormous and agricultural field clearance was in forested rather than prairie settings. The resultant watershed deforestation produced greatly increased rates of erosion, runoff, and unseasonable downstream flooding during the summer growing season. The economic and social consequences of declining production and localized crop failures proved disastrous for this city of farmers. 相似文献
104.
105.
106.
A mule cloned from fetal cells by nuclear transfer 总被引:1,自引:0,他引:1
Woods GL White KL Vanderwall DK Li GP Aston KI Bunch TD Meerdo LN Pate BJ 《Science (New York, N.Y.)》2003,301(5636):1063
107.
108.
Giovannoni SJ Tripp HJ Givan S Podar M Vergin KL Baptista D Bibbs L Eads J Richardson TH Noordewier M Rappé MS Short JM Carrington JC Mathur EJ 《Science (New York, N.Y.)》2005,309(5738):1242-1245
The SAR11 clade consists of very small, heterotrophic marine alpha-proteobacteria that are found throughout the oceans, where they account for about 25% of all microbial cells. Pelagibacter ubique, the first cultured member of this clade, has the smallest genome and encodes the smallest number of predicted open reading frames known for a free-living microorganism. In contrast to parasitic bacteria and archaea with small genomes, P. ubique has complete biosynthetic pathways for all 20 amino acids and all but a few cofactors. P. ubique has no pseudogenes, introns, transposons, extrachromosomal elements, or inteins; few paralogs; and the shortest intergenic spacers yet observed for any cell. 相似文献
109.
Gavriil FP Gonzalez ME Gotthelf EV Kaspi VM Livingstone MA Woods PM 《Science (New York, N.Y.)》2008,319(5871):1802-1805
We report the detection of magnetar-like x-ray bursts from the young pulsar PSR J1846-0258, at the center of the supernova remnant Kes 75. This pulsar, long thought to be exclusively rotation-powered, has an inferred surface dipolar magnetic field of 4.9 x 10(13) gauss, which is higher than those of the vast majority of rotation-powered pulsars, but lower than those of the approximately 12 previously identified magnetars. The bursts were accompanied by a sudden flux increase and an unprecedented change in timing behavior. These phenomena lower the magnetic and rotational thresholds associated with magnetar-like behavior and suggest that in neutron stars there exists a continuum of magnetic activity that increases with inferred magnetic field strength. 相似文献
110.
RNA silencing genes control de novo DNA methylation 总被引:1,自引:0,他引:1
Chan SW Zilberman D Xie Z Johansen LK Carrington JC Jacobsen SE 《Science (New York, N.Y.)》2004,303(5662):1336