全文获取类型
收费全文 | 120571篇 |
免费 | 21524篇 |
国内免费 | 6883篇 |
出版年
2024年 | 210篇 |
2023年 | 1315篇 |
2022年 | 3036篇 |
2021年 | 5329篇 |
2020年 | 5207篇 |
2019年 | 7366篇 |
2018年 | 7417篇 |
2017年 | 6700篇 |
2016年 | 7890篇 |
2015年 | 9729篇 |
2014年 | 10424篇 |
2013年 | 11449篇 |
2012年 | 10678篇 |
2011年 | 9254篇 |
2010年 | 7685篇 |
2009年 | 5933篇 |
2008年 | 5453篇 |
2007年 | 4559篇 |
2006年 | 3972篇 |
2005年 | 3447篇 |
2004年 | 2875篇 |
2003年 | 2541篇 |
2002年 | 2203篇 |
2001年 | 1855篇 |
2000年 | 1718篇 |
1999年 | 1612篇 |
1998年 | 871篇 |
1997年 | 850篇 |
1996年 | 841篇 |
1995年 | 794篇 |
1994年 | 706篇 |
1993年 | 503篇 |
1992年 | 716篇 |
1991年 | 558篇 |
1990年 | 509篇 |
1989年 | 373篇 |
1988年 | 338篇 |
1987年 | 293篇 |
1986年 | 242篇 |
1985年 | 258篇 |
1984年 | 165篇 |
1983年 | 151篇 |
1982年 | 107篇 |
1981年 | 81篇 |
1980年 | 54篇 |
1979年 | 88篇 |
1977年 | 50篇 |
1976年 | 48篇 |
1974年 | 53篇 |
1973年 | 64篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Jianjun Wang Janne Soininen Yong Zhang Beixin Wang Xiangdong Yang Ji Shen 《Journal of Biogeography》2011,38(3):595-603
Aim Data and analyses of elevational gradients in diversity have been central to the development and evaluation of a range of general theories of biodiversity. Elevational diversity patterns have, however, been severely understudied for microbes, which often represent decomposer subsystems. Consequently, generalities in the patterns of elevational diversity across different trophic levels remain poorly understood. Our aim was to examine elevational gradients in the diversity of macroinvertebrates, diatoms and bacteria along a stony stream that covered a large elevational gradient. Location Laojun Mountain, Yunnan province, China. Methods The sampling scheme included 26 sites spaced at elevational intervals of 89 m from 1820 to 4050 m elevation along a stony stream. Macroinvertebrate and diatom richness were determined based on the morphology of the specimens. Taxonomic richness for bacteria was quantified using a molecular fingerprinting method. Over 50 environmental variables were measured at each site to quantify environmental variables that could correlate with the patterns of diversity. We used eigenvector‐based spatial filters with multiple regressions to account for spatial autocorrelation. Results The bacterial richness followed an unexpected monotonic increase with elevation. Diatoms decreased monotonically, and macroinvertebrate richness showed a clear unimodal pattern with elevation. The unimodal richness pattern for macroinvertebrates was best explained by the mid‐domain effect (r2 = 0.72). The diatom richness was best explained by the variation in nutrient supply, and the increase in bacterial richness with elevation may be related to an increased carbon supply. Main conclusions We found contrasting patterns in elevational diversity among the three studied multi‐trophic groups comprising unicellular and multicellular aquatic taxa. We also found that there may be fundamental differences in the mechanisms underlying these species diversity patterns. 相似文献
102.
103.
P. Cruz C. H. Mejia‐Ruiz R. Perez‐Enriquez A. M. Ibarra 《Molecular ecology resources》2002,2(3):239-241
Five polymorphic microsatellite loci were characterized for Penaeus (Litopenaeus) vannamei. Loci were isolated using a partial Sau3A1 genomic library by the sequencing of randomly selected clones and by a biotinylated (CT)10 and (GT)10 probes screening procedure. The last strategy resulted in the most useful data. About 40% of the clones showed a previously reported satellite/microsatellite (PVS1), reducing the chance of finding new microsatellite regions. Whereas two of the microsatellite loci with more than 10 alleles will be useful for mating analysis in a breeding program, the others might prove useful for population genetic studies. 相似文献
104.
105.
106.
107.
Zhenzhen Zhang Changjiu He Yu Gao Lu Zhang Yukun Song Tianqi Zhu Kuanfeng Zhu Dongying Lv Jing Wang Xiuzhi Tian Teng Ma Pengyun Ji Wei Cui Guoshi Liu 《Aging cell》2021,20(2)
The fecundity reduction with aging is referred as the reproductive aging which comes earlier than that of chronological aging. Since humans have postponed their childbearing age, to prolong the reproductive age becomes urgent agenda for reproductive biologists. In the current study, we examined the potential associations of α‐ketoglutarate (α‐KG) and reproductive aging in mammals including mice, swine, and humans. There is a clear tendency of reduced α‐KG level with aging in the follicle fluids of human. To explore the mechanisms, mice were selected as the convenient animal model. It is observed that a long term of α‐KG administration preserves the ovarian function, the quality and quantity of oocytes as well as the telomere maintaining system in mice. α‐KG suppresses ATP synthase and alterations of the energy metabolism trigger the nutritional sensors to down‐regulate mTOR pathway. These events not only benefit the general aging process but also maintain ovarian function and delay the reproductive decline. Considering the safety of the α‐KG as a naturally occurring molecule in energy metabolism, its utility in reproduction of large mammals including humans deserves further investigation. 相似文献
108.
109.
110.