全文获取类型
收费全文 | 318893篇 |
免费 | 37333篇 |
国内免费 | 137篇 |
专业分类
356363篇 |
出版年
2018年 | 2753篇 |
2016年 | 3743篇 |
2015年 | 5073篇 |
2014年 | 5996篇 |
2013年 | 8635篇 |
2012年 | 9587篇 |
2011年 | 9761篇 |
2010年 | 6603篇 |
2009年 | 5894篇 |
2008年 | 8542篇 |
2007年 | 8970篇 |
2006年 | 8381篇 |
2005年 | 8088篇 |
2004年 | 8175篇 |
2003年 | 7911篇 |
2002年 | 7807篇 |
2001年 | 14298篇 |
2000年 | 14323篇 |
1999年 | 11482篇 |
1998年 | 4022篇 |
1997年 | 4130篇 |
1996年 | 3932篇 |
1995年 | 3800篇 |
1994年 | 3803篇 |
1993年 | 3691篇 |
1992年 | 9812篇 |
1991年 | 9390篇 |
1990年 | 9217篇 |
1989年 | 8941篇 |
1988年 | 8353篇 |
1987年 | 7987篇 |
1986年 | 7215篇 |
1985年 | 7322篇 |
1984年 | 6049篇 |
1983年 | 5301篇 |
1982年 | 4175篇 |
1981年 | 3696篇 |
1980年 | 3511篇 |
1979年 | 6097篇 |
1978年 | 4556篇 |
1977年 | 4274篇 |
1976年 | 3973篇 |
1975年 | 4297篇 |
1974年 | 4655篇 |
1973年 | 4719篇 |
1972年 | 4355篇 |
1971年 | 4067篇 |
1970年 | 3438篇 |
1969年 | 3338篇 |
1968年 | 2965篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
231.
232.
233.
Mélody Porlier Marc Bélisle Dany Garant 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2009,364(1523):1543-1554
Improving our knowledge of the links between ecology and evolution is especially critical in the actual context of global rapid environmental changes. A critical step in that direction is to quantify how variation in ecological factors linked to habitat modifications might shape observed levels of genetic variability in wild populations. Still, little is known on the factors affecting levels and distribution of genetic diversity at the individual level, despite its vital underlying role in evolutionary processes. In this study, we assessed the effects of habitat quality on population structure and individual genetic diversity of tree swallows (Tachycineta bicolor) breeding along a gradient of agricultural intensification in southern Québec, Canada. Using a landscape genetics approach, we found that individual genetic diversity was greater in poorer quality habitats. This counter-intuitive result was partly explained by the settlement patterns of tree swallows across the landscape. Individuals of higher genetic diversity arrived earlier on their breeding grounds and settled in the first available habitats, which correspond to intensive cultures. Our results highlight the importance of investigating the effects of environmental variability on individual genetic diversity, and of integrating information on landscape structure when conducting such studies. 相似文献
234.
235.
236.
237.
238.
Calcium mobilization induced by phosphorylated sphingoid bases was analyzed in calf pulmonary artery endothelial cells by confocal microscopy. A sphingenine-1-phosphate (SeP) analogue, N-acetyl-sphingenine-1-phosphate (N-C2-SeP), exogenously added to these cells, caused a fast and transient intracellular rise in calcium and was as potent as SeP. A minimal concentration of 0.6 nM for N-C2-SeP versus 1 nM for SeP was determined. The N-C2-SeP-induced Ca2+-signaling, like the response to SeP, was due to a release from thapsigargin-sensitive, ryanodine-insensitive, intracellular Ca2+-stores and not to a Ca2+-influx. N-C2-SeP can be considered as a truncated ceramide-phosphate, a lipid already reported to be mitogenic (Gomez-Munoz, A., Duffy, P.A., Martin, A., O'Brien, L., Byun, H.S., Bittman, R. and Brindley, D.N. (1995) Mol. Pharmacol. 47, 833-839), an effect that might be secondary to Ca2+-mobilization. 相似文献
239.
Fall techniques that reduce fall severity may decrease the risk of hip fractures. A fundamental variable for fall severity is impact force, but impact velocity is also used. The purpose of the study was to determine whether impact velocity is valid to determine differences in fall severity between different techniques. Five young adults with martial arts (MA) experience performed sideways falls from kneeling height using three techniques: Block with arm (Block) and MA techniques with and without use of the arm to break the fall. In addition, one subject also performed MA falls from standing height. Linear regression analysis showed a moderate relation between hip impact velocity and force, which was depended on technique. In falls with comparable impact velocities, forces in MA falls were lower than forces in Block falls. Hence, differences in impact force could not be predicted by velocity. In conclusion, hip impact velocity may be useful to make an approximate prediction of impact force within fall techniques. However, to determine differences between techniques it was not always a valid predictor. When direct impact force measurements are not possible, methods combining impact velocity with energy estimates before and after impact might be more valid. 相似文献
240.
Reginald B. Cocroft 《Journal of Insect Behavior》2003,16(1):79-95
Many membracid treehoppers are attended by honeydew-harvesting ants. Ant mutualism often favors group living, which will in turn influence social interactios and communication. I investigated aspects of life history that underlie the social behavior of an aggregating, ant-attended treehopper. The number of adults, and their patterns of distribution, changes dramatically over the course of a season. Despite the relatively low vagility and high persistence in the same clump of host plants, individuals encounter a wide range of social environments. This aggregating species differs from solitary species in the clumped distribution of females, and possibly in the intensity of acoustic competition among males, but both aggregating and solitary species exhibit large temporal changes in density. A high degree of temporal and spatial variation in the social environment is probably characteristic of many insects and may be an important source of selection on insect communication. 相似文献