全文获取类型
收费全文 | 146篇 |
免费 | 2篇 |
专业分类
系统科学 | 2篇 |
教育与普及 | 1篇 |
理论与方法论 | 1篇 |
现状及发展 | 54篇 |
研究方法 | 22篇 |
综合类 | 64篇 |
自然研究 | 4篇 |
出版年
2021年 | 2篇 |
2020年 | 1篇 |
2018年 | 2篇 |
2017年 | 2篇 |
2016年 | 2篇 |
2014年 | 4篇 |
2013年 | 3篇 |
2012年 | 6篇 |
2011年 | 8篇 |
2010年 | 1篇 |
2009年 | 2篇 |
2008年 | 11篇 |
2007年 | 6篇 |
2006年 | 7篇 |
2005年 | 9篇 |
2004年 | 10篇 |
2003年 | 11篇 |
2002年 | 3篇 |
2001年 | 3篇 |
2000年 | 1篇 |
1999年 | 4篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1986年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1970年 | 4篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1967年 | 5篇 |
1957年 | 1篇 |
排序方式: 共有148条查询结果,搜索用时 0 毫秒
41.
Kempf S Srama R Horányi M Burton M Helfert S Moragas-Klostermeyer G Roy M Grün E 《Nature》2005,433(7023):289-291
High-velocity submicrometre-sized dust particles expelled from the jovian system have been identified by dust detectors on board several spacecraft. On the basis of periodicities in the dust impact rate, Jupiter's moon Io was found to be the dominant source of the streams. The grains become positively charged within the plasma environment of Jupiter's magnetosphere, and gain energy from its co-rotational electric field. Outside the magnetosphere, the dynamics of the grains are governed by the interaction with the interplanetary magnetic field that eventually forms the streams. A similar process was suggested for Saturn. Here we report the discovery by the Cassini spacecraft of bursts of high-velocity dust particles (> or = 100 km s(-1)) within approximately 70 million kilometres of Saturn. Most of the particles detected at large distances appear to originate from the outskirts of Saturn's outermost main ring. All bursts of dust impacts detected within 150 Saturn radii are characterized by impact directions markedly different from those measured between the bursts, and they clearly coincide with the spacecraft's traversals through streams of compressed solar wind. 相似文献
42.
43.
44.
45.
T. B. Gilliam R. R. Roy J. F. Taylor W. W. Heusner W. D. Van Huss 《Cellular and molecular life sciences : CMLS》1977,33(5):665-667
Summary The observed differences in the soma and nuclear diameters reflect chronic changes specific to each exercise regimen used.This study was supported by National Institute of Health Grant HD 03918, Michigan State University, East Lansing, Michigan.Acknowledgments. Gratitude is expressed to Barbara Wheaton for her technical assistance in the processing of tissues and R. E. Carrow for making his laboratory facilities available for this study. 相似文献
46.
47.
48.
A. K. Chatterjee A. D. Roy S. C. Dutta B. B. Ghosh 《Cellular and molecular life sciences : CMLS》1970,26(10):1077-1078
Résumé Le contenu de protéine, de RNA et de DNA dans les tissus de foie, de cur et de rognon a été étudié chez des rats albinos traités à l'émétine. Le traitement a réduit la concentration de la protéine et du DNA dans le foie et dans le rognon. Dans le cur, cette concentration ne fut pas altérée d'une maniére significative. On suggère que l'émétine non settlement empÊche la synthèse de la protéine, mais réduit aussi sa désagrégation. 相似文献
49.
Tom Olliff Roy Renkin Craig McClure Paul Miller Dave Price Dan Reinhart Jennifer Whipple 《西北部美国博物学家》2011,61(3)
The number of documented exotic plants in Yellowstone National Park has increased from 85 known in 1986 to over 185 today. Exotic plants are substantially impacting the parks natural and cultural resources and are a high management priority. We have adopted an integrated weed management approach with regard to exotic vegetation, emphasizing prevention, education, early detection and eradication, control, and, to a lesser degree, monitoring. The program involves over 140 staff with program expenditures averaging approximately $190,000 annually. Prevention actions include requiring approved gravel on construction projects; banning hay in the backcountry and allowing transport of only certified weed-seed-free hay through Yellowstone; requiring construction equipment to be pressurecleaned prior to entering the park; and native species revegetation after road, housing, and other construction projects have disturbed ground. Over 4500 acres, primarily along roadsides and in developed areas, are surveyed annually in early detection efforts with emphasis placed on eradicating small, new infestations of highly invasive species such as sulfur cinquefoil ( Potentilla recta L.) and leafy spurge ( Euphorbia esula L.). Control efforts focus on about 30 priority species, such as spotted knapweed ( Centaurea maculosa Lam.), oxeye daisy ( Chrysanthemum leucanthemum L.), and hoary cress ( Cardaria draba [L.] Desv.) using chemical, mechanical, and cultural techniques. A total of 2027 acres were treated during 1998, whereas control efforts for 12 species occurred on 2596 acres during the previous 3-year period, 1995-1997. Strong and expanding partnerships with other federal, state, and local agencies and private companies contribute to management efforts within the park. Future program goals emphasize increases in base funding to ensure continued weed management efforts as well as expanding survey, monitoring, and reclamation efforts. Ultimately, a more rigorous assessment of program effectiveness is desired. 相似文献
50.
After the elimination of wolves ( Canis lupis L.) in the 1920s, woody riparian plant communities on the northern range of Yellowstone National Park (YNP) declined an estimated 50%. After the reintroduction of wolves in 1995–1996, riparian willows ( Salix spp.) on YNP’s northern range showed significant growth for the first time since the 1920s. However, the pace of willow recovery has not been uniform. Some communities have exceeded 400 cm, while others are still at pre-1995 levels of <80 cm mean height. We took intensive, repeated measurements of abiotic factors, including soil and water-table characteristics, to determine whether these factors might be contributing to the varying pace of willow recovery. Willows at all of our study sites were “short” (<250 cm max. height) prior to 1995 and have recovered to varying degrees since. We contrasted “tall” (>250 cm max. height) willow sites where willows had escaped elk ( Cervus elaphus L.) browsing with “short” willow sites that could still be browsed. Unlike studies that manipulated willow height with fences and artificial dams, we examined sites that had natural growth differences in height since the reintroduction of wolves. Tall willow sites had greater water availability, more-rapid net soil nitrogen mineralization, greater snow depth, lower soil respiration rates, and cooler summer soil temperatures than nearby short willow sites. Most of these differences were measured both in herbaceous areas adjacent to the willow patches and in the willow patches themselves, suggesting that they were not effects of varying willow height recovery but were instead preexisting site differences that may have contributed to increased plant productivity. Our results agree with earlier studies in experimental plots which suggest that the varying pace of willow recovery has been influenced by abiotic limiting factors that interact with top-down reductions in willow browsing by elk. 相似文献