全文获取类型
收费全文 | 144篇 |
免费 | 0篇 |
专业分类
现状及发展 | 29篇 |
研究方法 | 15篇 |
综合类 | 99篇 |
自然研究 | 1篇 |
出版年
2012年 | 5篇 |
2011年 | 6篇 |
2010年 | 3篇 |
2009年 | 2篇 |
2008年 | 3篇 |
2007年 | 7篇 |
2006年 | 5篇 |
2005年 | 2篇 |
2004年 | 3篇 |
2003年 | 3篇 |
2002年 | 4篇 |
2001年 | 6篇 |
2000年 | 2篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 2篇 |
1970年 | 2篇 |
1969年 | 3篇 |
1968年 | 4篇 |
1967年 | 1篇 |
1965年 | 1篇 |
1962年 | 2篇 |
1960年 | 1篇 |
排序方式: 共有144条查询结果,搜索用时 390 毫秒
121.
Buchhave LA Latham DW Johansen A Bizzarro M Torres G Rowe JF Batalha NM Borucki WJ Brugamyer E Caldwell C Bryson ST Ciardi DR Cochran WD Endl M Esquerdo GA Ford EB Geary JC Gilliland RL Hansen T Isaacson H Laird JB Lucas PW Marcy GW Morse JA Robertson P Shporer A Stefanik RP Still M Quinn SN 《Nature》2012,486(7403):375-377
The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a 'fossil' record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets, supporting the model that planets form by accumulation of dust and ice particles. Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets. However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA's Kepler mission, including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation. 相似文献
122.
Thompson AM 《Nature》2012,485(7397):174
123.
Mate choice on fallow deer leks 总被引:5,自引:0,他引:5
Leks, on which males defend small clustered mating territories, may have evolved because of the unusual opportunities they provide for female choice of mating partners, and several studies of lek-breeding animals have demonstrated correlations between the mating success of males and their phenotype or behaviour. However, these could arise because (1) females select mates on the basis of male phenotypic traits; (2) males interfere with each other's mating attempts; or (3) females show preferences for particular mating territories, and larger or stronger males are more likely to win access to these territories. Here we report that when fallow bucks on a traditional lek were experimentally induced to change their territories, differences in the mating success of bucks persisted, whereas differences in the position of their territories relative to the centre of the lek did not. The observation that bucks rarely interfered with their neighbours' harems and females moved freely between bucks suggests that females choose their mates on the basis of male phenotype rather than territory type or location. In this population, the immediate factor affecting the movements of females between males was the size of a buck's harem. 相似文献
124.
Nanog safeguards pluripotency and mediates germline development 总被引:3,自引:0,他引:3
Chambers I Silva J Colby D Nichols J Nijmeijer B Robertson M Vrana J Jones K Grotewold L Smith A 《Nature》2007,450(7173):1230-1234
125.
126.
Field experiments on the pollination biology of slickspot peppergrass, Lepidium papilliferum L. (Brassicaceae), a rare species endemic to microsites in sagebrush-steppe habitat in southwestern Idaho, were conducted at 2 sites from May to July 2001. Site KB contained over 10,000 plants, whereas site WG contained less than 150 plants. Insect exclusion experiments revealed that seed production in L. papilliferum is dependent on insect-mediated pollination; median percent seed set dropped from 70% to 2% when insects were excluded from flowers. A total of 25 insect families from 5 orders visited L. papilliferum flowers: 24 families at KB and 11 families at WG. Only 9 families contained more than trace amounts of pollen on their bodies: Hymenoptera-Anthophoridae, Apidae, Colletidae, Halictidae, Sphecidae, Vespidae; Coleoptera-Cerambycidae, Dermestidae; Diptera-Syrphidae. Insects from these families are likely responsible for pollination of L. papilliferum , although some may be of only minor significance due to their infrequent visits to flowers. Two of the 4 most common and pollen-laden insects found at KB, honey bees ( Apis mellifera ) and colletid bees, were absent or rare at WG. Three other pollen-carrying families present at KB, Sphecidae, Vespidae, and Halictidae, were not found at WG. We raise several possible explanations for this disparity in pollinator communities between sites and discuss the importance of pollinator diversity to the long term viability of L. papilliferum . 相似文献
127.
128.
129.
A. Watson R. Moss R. Parr I. B. Trenholm A. Robertson 《Cellular and molecular life sciences : CMLS》1988,44(3):274-275
Summary This experiment tested whether a cyclic-type population decline can be prevented by removing territorial cock red grouse to keep breeding density below peak levels. The manipulated population did not decline, despite big decreases in food and breeding success, and more parasitic threadworms per bird than on the control. 相似文献
130.
Fusion between a myogenic cell in the satellite cell position and undamaged adult myofibre segments.
In this report we demonstrate for the first time that differentiating myogenic cells, geographically located between the plasmalemma and external lamina of myofibres in the satellite cell position, can fuse directly with the plasmalemma of undamaged segments of mature myofibres. 相似文献