全文获取类型
收费全文 | 1063175篇 |
免费 | 112491篇 |
国内免费 | 377篇 |
出版年
2018年 | 10176篇 |
2017年 | 9778篇 |
2016年 | 13716篇 |
2015年 | 17824篇 |
2014年 | 21099篇 |
2013年 | 30795篇 |
2012年 | 34332篇 |
2011年 | 35174篇 |
2010年 | 23937篇 |
2009年 | 22229篇 |
2008年 | 31466篇 |
2007年 | 32537篇 |
2006年 | 30657篇 |
2005年 | 29298篇 |
2004年 | 29116篇 |
2003年 | 27937篇 |
2002年 | 27291篇 |
2001年 | 45302篇 |
2000年 | 44928篇 |
1999年 | 36292篇 |
1998年 | 13192篇 |
1997年 | 13458篇 |
1996年 | 12819篇 |
1995年 | 12239篇 |
1994年 | 12031篇 |
1993年 | 12048篇 |
1992年 | 30287篇 |
1991年 | 29550篇 |
1990年 | 28935篇 |
1989年 | 28439篇 |
1988年 | 26150篇 |
1987年 | 25070篇 |
1986年 | 23399篇 |
1985年 | 23436篇 |
1984年 | 19308篇 |
1983年 | 16914篇 |
1982年 | 12877篇 |
1981年 | 11623篇 |
1980年 | 11015篇 |
1979年 | 18347篇 |
1978年 | 14437篇 |
1977年 | 13020篇 |
1976年 | 12390篇 |
1975年 | 13737篇 |
1974年 | 14631篇 |
1973年 | 14452篇 |
1972年 | 13156篇 |
1971年 | 11841篇 |
1970年 | 10385篇 |
1969年 | 10039篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
13.
Elevationally biased avian predation as a contributor to the spatial distribution of geometrid moth outbreaks in sub‐arctic mountain birch forest 下载免费PDF全文
ADAM A. PEPI OLE PETTER L. VINDSTAD MALIN EK JANE U. JEPSEN 《Ecological Entomology》2017,42(4):430-438
1. Population dynamics and interactions that vary over a species' range are of particular importance in the context of latitudinal clines in biological diversity. Winter moth (Operophtera brumata) and autumnal moth (Epirrita autumnata) are two species of eruptive geometrids that vary widely in outbreak tendency over their range, which generally increases from south to north and with elevation. 2. The predation pressure on geometrid larvae and pupae over an elevational gradient was tested. The effects of background larval density and bird occupancy of monitoring nest boxes on predation rates were also tested. Predation on larvae was tested through exclusion treatments at 20 replicate stations over four elevations at one site, while pupae were set out to measure predation at two elevations at three sites. 3. Larval densities were reduced by bird predation at three lower elevations, but not at the highest elevation, and predation rates were 1.9 times higher at the lowest elevation than at the highest elevation. The rate of predation on larvae was not related to background larval density or nest box occupancy, although there were more eggs and chicks at the lowest elevation. There were no consistent differences in predation on pupae by elevation. 4. These results suggest that elevational variation in avian predation pressure on larvae may help to drive elevational differences in outbreak tendency, and that birds may play a more important role in geometrid population dynamics than the focus on invertebrate and soil predators of previous work would suggest. 相似文献
14.
15.
16.
17.
18.
Charles A. Kunos Jeffrey M. Fabien John P. Shanahan Christine Collen Thierry Gevaert Kenneth Poels Robbe Van den Begin Benedikt Engels Mark De Ridder 《Journal of visualized experiments : JoVE》2015,(100)
Physicians considering stereotactic ablative body radiation therapy (SBRT) for the treatment of extracranial cancer targets must be aware of the sizeable risks for normal tissue injury and the hazards of physical tumor miss. A first-of-its-kind SBRT platform achieves high-precision ablative radiation treatment through a combination of versatile real-time imaging solutions and sophisticated tumor tracking capabilities. It uses dual-diagnostic kV x-ray units for stereoscopic open-loop feedback of cancer target intrafraction movement occurring as a consequence of respiratory motions and heartbeat. Image-guided feedback drives a gimbaled radiation accelerator (maximum 15 x 15 cm field size) capable of real-time ±4 cm pan-and-tilt action. Robot-driven ±60° pivots of an integrated ±185° rotational gantry allow for coplanar and non-coplanar accelerator beam set-up angles, ultimately permitting unique treatment degrees of freedom. State-of-the-art software aids real-time six dimensional positioning, ensuring irradiation of cancer targets with sub-millimeter accuracy (0.4 mm at isocenter). Use of these features enables treating physicians to steer radiation dose to cancer tumor targets while simultaneously reducing radiation dose to normal tissues. By adding respiration correlated computed tomography (CT) and 2-[18F] fluoro-2-deoxy-ᴅ-glucose (18F-FDG) positron emission tomography (PET) images into the planning system for enhanced tumor target contouring, the likelihood of physical tumor miss becomes substantially less1. In this article, we describe new radiation plans for the treatment of moving lung tumors. 相似文献
19.
20.