全文获取类型
收费全文 | 887篇 |
免费 | 72篇 |
国内免费 | 2篇 |
出版年
2020年 | 8篇 |
2019年 | 7篇 |
2018年 | 9篇 |
2016年 | 17篇 |
2015年 | 20篇 |
2014年 | 31篇 |
2013年 | 44篇 |
2012年 | 51篇 |
2011年 | 35篇 |
2010年 | 23篇 |
2009年 | 19篇 |
2008年 | 34篇 |
2007年 | 34篇 |
2006年 | 32篇 |
2005年 | 28篇 |
2004年 | 23篇 |
2003年 | 30篇 |
2002年 | 21篇 |
2001年 | 22篇 |
2000年 | 33篇 |
1999年 | 19篇 |
1998年 | 20篇 |
1997年 | 17篇 |
1996年 | 8篇 |
1995年 | 15篇 |
1994年 | 12篇 |
1993年 | 13篇 |
1992年 | 16篇 |
1991年 | 17篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 12篇 |
1987年 | 19篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 12篇 |
1983年 | 13篇 |
1982年 | 11篇 |
1981年 | 15篇 |
1979年 | 10篇 |
1977年 | 14篇 |
1976年 | 9篇 |
1975年 | 10篇 |
1974年 | 13篇 |
1973年 | 12篇 |
1972年 | 10篇 |
1971年 | 9篇 |
1970年 | 8篇 |
1969年 | 9篇 |
1968年 | 9篇 |
排序方式: 共有961条查询结果,搜索用时 444 毫秒
871.
Phylogeny and biogeography of ratite birds inferred from DNA sequences of the mitochondrial ribosomal genes 总被引:7,自引:2,他引:5
The origin of the flightless ratite birds of the southern continents has
been debated for over a century. Whether dispersal or vicariance
(continental breakup) best explains their origin depends largely on their
phylogenetic relationships. No consensus has been reached on this issue
despite many morphological and molecular studies. To address this question
further we sequenced a 2.8-kb region of mitochondrial DNA containing the
ribosomal genes in representative ratites and a tinamou. Phylogenetic
analyses indicate that Struthio (Africa) is basal and Rhea (South America)
clusters with living Australasian ratites. This phylogeny agrees with
transferrin and DNA hybridization studies but not with sequence analyses of
some protein-coding genes. These results also require reevaluation of the
phylogenetic position of the extinct moas of New Zealand. We propose a new
hypothesis for the origin of ratites that combines elements of dispersal
and vicariance.
相似文献
872.
Kevin J. Emerson Alison M. Uyemura Keely L. McDaniel Paul S. Schmidt William E. Bradshaw Christina M. Holzapfel 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2009,195(9):825-829
Drosophila melanogaster from Australia, Europe and North America enter an adult ovarian dormancy in response to short days and low temperatures.
The independent effects of temperature and day length in the determination of dormancy have been examined only in one long-established
laboratory line (Canton-S). In all other studies of natural or laboratory populations, dormancy has been assessed at either
a single short day or a single moderately low temperature. Herein, we determine the relative roles of temperature, photoperiod,
and their interaction in the control of ovarian dormancy in D. melanogaster from two natural populations representing latitudinal extremes in eastern North America (Florida at 27°N and Maine at 44°N).
In both natural populations, temperature is the main determinant of dormancy, alone explaining 67% of the total variation
among replicate isofemale lines, whereas photoperiod has no significant effect. We conclude that ovarian dormancy in D. melanogaster is a temperature-initiated syndrome of winter-tolerant traits that represents an adaptive phenotypic plasticity in temperate
seasonal environments. 相似文献
873.
Amy D. Bradshaw 《Journal of cell communication and signaling》2009,3(3-4):239-246
SPARC is a collagen-binding matricellular protein. Expression of SPARC in adult tissues is frequently associated with excessive deposition of collagen and SPARC-null mice fail to generate a robust fibrotic response to a variety of stimuli. This review summarizes recent advancements in the characterization of the binding of SPARC to collagens and describes the results of studies that implicate a function for SPARC in the regulation of the assembly of basal lamina and fibrillar collagen in the ECM. Potential cellular mechanisms that underlie SPARC activity in ECM deposition are also explored. 相似文献
874.
875.
Corey J. A. Bradshaw Lloyd S. Davis Chris Lalas Robert G. Harcourt 《Journal of Zoology》2000,252(1):41-51
876.
877.
Hybrids have been obtained by protoplast fusion of penicillin-producing strains of Aspergillus nidulans with strains of A. rugulosus which do not produce the antibiotic at detectable levels. Induced haploidization of the hybrids allowed the recovery of stable segregants, which were screened for penicillin production. Approximately half of the segregants from each cross produced penicillin, indicating segregation of one or several linked genes which are essential for penicillin biosynthesis. These essential genes were found to be located on linkage-group VI of A. nidulans by observing segregation in multi-marked strains. The penicillin-producing segregants showed a wide range of titres. Some segregants, which had recombinant phenotypes, produced significantly more penicillin than the A. nidulans parent. 相似文献
878.
The rapid industrialisation of agriculture and forestry during the last century has contributed to a transformation of the forested landscape in southern Sweden. Palaeoecological investigation revealed how the Medieval forest-meadow system was created about 900 years ago from a deciduous forest type that had been rather stable for the previous 3000 years. The study site was a forest hollow close to where Linnaeus was born and brought up. The species-rich forest-meadow system suffered from over-grazing during the human-population peak of the nineteenth century, and was abandoned about one hundred years ago. The recent forest succession in southern Sweden has led to reduced floristic biodiversity and created conservation problems related to disappearance of open landscape, even though the remnant vegetation is moving towards the near-natural forest type. Spatially detailed palaeoecology permits documentation of baseline conditions, and places conservation debates in a valuable temporal perspective. 相似文献
879.
880.
Thomas R. Bradshaw 《BMJ (Clinical research ed.)》1902,1(2150):686-687