全文获取类型
收费全文 | 1334篇 |
免费 | 9篇 |
国内免费 | 12篇 |
专业分类
系统科学 | 33篇 |
教育与普及 | 4篇 |
理论与方法论 | 27篇 |
现状及发展 | 178篇 |
研究方法 | 264篇 |
综合类 | 791篇 |
自然研究 | 58篇 |
出版年
2021年 | 7篇 |
2020年 | 6篇 |
2019年 | 3篇 |
2018年 | 12篇 |
2017年 | 9篇 |
2016年 | 22篇 |
2015年 | 13篇 |
2014年 | 18篇 |
2013年 | 25篇 |
2012年 | 104篇 |
2011年 | 206篇 |
2010年 | 36篇 |
2009年 | 7篇 |
2008年 | 111篇 |
2007年 | 112篇 |
2006年 | 117篇 |
2005年 | 116篇 |
2004年 | 108篇 |
2003年 | 99篇 |
2002年 | 119篇 |
2001年 | 6篇 |
2000年 | 8篇 |
1999年 | 3篇 |
1998年 | 4篇 |
1996年 | 3篇 |
1994年 | 2篇 |
1993年 | 9篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1980年 | 2篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1973年 | 6篇 |
1972年 | 1篇 |
1971年 | 4篇 |
1970年 | 1篇 |
1968年 | 2篇 |
1967年 | 1篇 |
1963年 | 2篇 |
1962年 | 2篇 |
1959年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有1355条查询结果,搜索用时 15 毫秒
131.
132.
The power function of basal metabolic rate scaling is expressed as aM(b), where a corresponds to a scaling constant (intercept), M is body mass, and b is the scaling exponent. The 3/4 power law (the best-fit b value for mammals) was developed from Kleiber's original analysis and, since then, most workers have searched for a single cause to explain the observed allometry. Here we present a multiple-causes model of allometry, where the exponent b is the sum of the influences of multiple contributors to metabolism and control. The relative strength of each contributor, with its own characteristic exponent value, is determined by the control contribution. To illustrate its use, we apply this model to maximum versus basal metabolic rates to explain the differing scaling behaviour of these two biological states in mammals. The main difference in scaling is that, for the basal metabolic rate, the O(2) delivery steps contribute almost nothing to the global b scaling exponent, whereas for the maximum metabolic rate, the O(2) delivery steps significantly increase the global b value. 相似文献
133.
Biodiversity as a barrier to ecological invasion 总被引:69,自引:0,他引:69
Biological invasions are a pervasive and costly environmental problem that has been the focus of intense management and research activities over the past half century. Yet accurate predictions of community susceptibility to invasion remain elusive. The diversity resistance hypothesis, which argues that diverse communities are highly competitive and readily resist invasion, is supported by both theory and experimental studies conducted at small spatial scales. However, there is also convincing evidence that the relationship between the diversity of native and invading species is positive when measured at regional scales. Although this latter relationship may arise from extrinsic factors, such as resource heterogeneity, that covary with diversity of native and invading species at large scales, the mechanisms conferring greater invasion resistance to diverse communities at local scales remain unknown. Using neighbourhood analyses, a technique from plant competition studies, we show here that species diversity in small experimental grassland plots enhances invasion resistance by increasing crowding and species richness in localized plant neighbourhoods. Both the establishment (number of invaders) and success (proportion of invaders that are large) of invading plants are reduced. These results suggest that local biodiversity represents an important line of defence against the spread of invaders. 相似文献
134.
135.
136.
137.
138.
139.
140.
It is known that pain perception can be altered by mood, attention and cognition, or by direct stimulation of the cerebral cortex, but we know little of the neural mechanisms underlying the cortical modulation of pain. One of the few cortical areas consistently activated by painful stimuli is the rostral agranular insular cortex (RAIC) where, as in other parts of the cortex, the neurotransmitter gamma-aminobutyric acid (GABA) robustly inhibits neuronal activity. Here we show that changes in GABA neurotransmission in the RAIC can raise or lower the pain threshold--producing analgesia or hyperalgesia, respectively--in freely moving rats. Locally increasing GABA, by using an enzyme inhibitor or gene transfer mediated by a viral vector, produces lasting analgesia by enhancing the descending inhibition of spinal nociceptive neurons. Selectively activating GABA(B)-receptor-bearing RAIC neurons produces hyperalgesia through projections to the amygdala, an area involved in pain and fear. Whereas most studies focus on the role of the cerebral cortex as the end point of nociceptive processing, we suggest that cerebral cortex activity can change the set-point of pain threshold in a top-down manner. 相似文献