全文获取类型
收费全文 | 71145篇 |
免费 | 7127篇 |
国内免费 | 1162篇 |
专业分类
79434篇 |
出版年
2022年 | 584篇 |
2021年 | 1099篇 |
2020年 | 764篇 |
2019年 | 925篇 |
2018年 | 1151篇 |
2017年 | 1005篇 |
2016年 | 1395篇 |
2015年 | 1913篇 |
2014年 | 2262篇 |
2013年 | 2692篇 |
2012年 | 3058篇 |
2011年 | 3029篇 |
2010年 | 1915篇 |
2009年 | 1838篇 |
2008年 | 2461篇 |
2007年 | 2301篇 |
2006年 | 2294篇 |
2005年 | 1946篇 |
2004年 | 1797篇 |
2003年 | 1695篇 |
2002年 | 1596篇 |
2001年 | 4949篇 |
2000年 | 4449篇 |
1999年 | 3306篇 |
1998年 | 1113篇 |
1997年 | 1048篇 |
1996年 | 942篇 |
1995年 | 847篇 |
1994年 | 729篇 |
1993年 | 647篇 |
1992年 | 2065篇 |
1991年 | 1796篇 |
1990年 | 1615篇 |
1989年 | 1475篇 |
1988年 | 1238篇 |
1987年 | 1170篇 |
1986年 | 1005篇 |
1985年 | 965篇 |
1984年 | 662篇 |
1983年 | 596篇 |
1979年 | 680篇 |
1978年 | 494篇 |
1977年 | 455篇 |
1975年 | 518篇 |
1974年 | 606篇 |
1973年 | 587篇 |
1972年 | 607篇 |
1971年 | 562篇 |
1970年 | 550篇 |
1969年 | 547篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
We released seeds of two sympatric tree species, Corylus mandshurica (seed with thinner seed hull, higher nutrition) and C. heterophylla (seeds with thicker seed hull, lower nutrition) in the masting year of C. mandshurica in 2008, and C. heterophylla in 2009, respectively, to investigate how seed masting of the two sympatric Corylus species affects seed removal and dispersal fitness of the two species differently at both intra- and inter-specific levels.
At intra-specific level, the authors found mast seeding of both C. mandshurica and C. heterophylla significantly reduced seed removal, seed consumption, but increased seed dispersal distance and seed dispersal fitness of
the released seeds. Mast seeding of C. mandshurica increased seed caching of C. mandshurica. At inter-specific level, the authors found mast seeding of C. mandshurica reduced seed removal of C. heterophylla, but mast seeding of C. heterophylla did not significantly reduce seed removal of C. mandshurica. Mast seeding of C. mandshurica reduced seed consumption of C. heterophylla, while mast seeding of C. heterophylla reduced seed consumption of C. mandshurica. We found mast seeding of C. mandshurica significantly reduced seed dispersal distance of C. heterophylla, while mast seeding of C. heterophylla significantly increased seed dispersal distance of C. mandshurica. We found that mast seeding of C. mandshurica significantly increased seed dispersal fitness of C. heterophylla, while mast seeding of C. heterophylla did not significantly increase seed dispersal fitness of C. mandshurica. More studies are needed to reveal the ecological consequences of mast seeding at inter-specific or community-level. Seed
traits may attribute the differences of mast seeding at inter-specific level. Because seeds with thinner seed hull and higher
nutrition were more harvested and eaten by rodents, mast seeding of C. mandshurica might have reduced seed removal and seed consumption, but increased dispersal fitness of C. heterophylla (seeds with thicker seed hull, lower nutrition). Therefore, synchrony among species is, or is not, selectively beneficial
to the focus species depends on seed traits which determine gains from mast seeding at inter-specific level. 相似文献
32.
33.
34.
35.
36.
37.
A new purification method for cytosolic malate dehydrogenases from several sources has been developed. The procedure, employing chromatographies on 5'AMP-Sepharose, DEAE-Sephacel and Blue-Sepharose, allows for a rapid isolation of the enzyme (approximately 40 hours), in large quantities, with good yields (45-54%). The specific activity of final preparations were around 1300 I.U./mg and were judged homogeneous by polyacrylamide gradient gel and sodium dodecyl sulfate polyacrylamide gel electrophoresis, high performance size exclusion chromatography and isoelectric focusing. 相似文献
38.
39.
40.
I Lee V E Gould J A Radosevich A Thor Y X Ma J Schlom S T Rosen 《Virchows Archiv. B, Cell pathology including molecular pathology》1987,53(3):146-152
We undertook an immunohistochemical analysis of human bronchopulmonary epithelial neoplasms and pleural mesotheliomas using a monoclonal antibody which recognizes ras oncogene products (p21ras). The monoclonal antibody, RAP-5, recognizes both unaltered and certain mutated p21ras. Formalin fixed and paraffin embedded tissue samples of 187 lung epithelial tumors and 27 pleural mesotheliomas were investigated; normal and bronchiectatic lungs were similarly studied. Normal lung and pleural tissue did not immunostain except for occasional type II pneumocytes. Reactive type II pneumocytes adjacent to carcinomas and bronchiectasis immunostained consistently. Twenty four/34 (71%) squamous carcinomas immunostained. Only 8/50 (16%) adenocarcinomas immunostained focally and weakly whereas 19/24 (79%) bronchioloalveolar carcinomas immunostained. Eleven/18 (61%) large cell carcinomas immunostained with variable intensity. Eleven/13 (85%) carcinoids, 6/7 (85%) well differentiated neuroendocrine carcinomas, and 18/21 (86%) intermediate cell neuroendocrine carcinomas immunostained while none of 20 small cell neuroendocrine carcinomas immunostained. Only a few mesotheliomas were immunostained focally. Two/14 (14%) epithelial type and 1/9 (11%) biphasic type mesotheliomas immunostained weakly; none of 4 spindle cell mesotheliomas immunostained. We conclude that while at least occasional cases of most types of pulmonary epithelial neoplasms express p21ras, the frequency and intensity of the expression are distinctly greater in certain tumor types such as squamous, bronchioloalveolar, and neuroendocrine neoplasm except for the small cell type. Contrary to these lung epithelial neoplasms, most mesotheliomas did not immunostain for p21ras. Whether the enhanced p21ras expression may point to a different mechanism of transformation or may merely reflect differentiation features remains undetermined. 相似文献