全文获取类型
收费全文 | 86879篇 |
免费 | 19475篇 |
国内免费 | 5295篇 |
出版年
2024年 | 95篇 |
2023年 | 663篇 |
2022年 | 1789篇 |
2021年 | 3123篇 |
2020年 | 3976篇 |
2019年 | 5817篇 |
2018年 | 5831篇 |
2017年 | 5609篇 |
2016年 | 6303篇 |
2015年 | 7551篇 |
2014年 | 7821篇 |
2013年 | 8651篇 |
2012年 | 7401篇 |
2011年 | 6406篇 |
2010年 | 6125篇 |
2009年 | 4623篇 |
2008年 | 4183篇 |
2007年 | 3398篇 |
2006年 | 3084篇 |
2005年 | 2827篇 |
2004年 | 2399篇 |
2003年 | 2198篇 |
2002年 | 1875篇 |
2001年 | 1505篇 |
2000年 | 1205篇 |
1999年 | 1179篇 |
1998年 | 638篇 |
1997年 | 609篇 |
1996年 | 573篇 |
1995年 | 503篇 |
1994年 | 475篇 |
1993年 | 335篇 |
1992年 | 447篇 |
1991年 | 364篇 |
1990年 | 298篇 |
1989年 | 250篇 |
1988年 | 196篇 |
1987年 | 215篇 |
1986年 | 182篇 |
1985年 | 144篇 |
1984年 | 102篇 |
1983年 | 88篇 |
1982年 | 74篇 |
1981年 | 54篇 |
1980年 | 39篇 |
1979年 | 58篇 |
1978年 | 37篇 |
1977年 | 41篇 |
1975年 | 34篇 |
1972年 | 33篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
41.
We tried to establish compatible carbon content models of individual trees for a Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantation from Fujian province in southeast China. In general, compatibility requires that the sum of components equal the whole tree, meaning that the sum of percentages calculated from component equations should equal 100%. Thus, we used multiple approaches to simulate carbon content in boles, branches, foliage leaves, roots and the whole individual trees. The approaches included (i) single optimal fitting (SOF), (ii) nonlinear adjustment in proportion (NAP) and (iii) nonlinear seemingly unrelated regression (NSUR). These approaches were used in combination with variables relating diameter at breast height (D) and tree height (H), such as D, D2H, DH and D&H (where D&H means two separate variables in bivariate model). Power, exponential and polynomial functions were tested as well as a new general function model was proposed by this study. Weighted least squares regression models were employed to eliminate heteroscedasticity. Model performances were evaluated by using mean residuals, residual variance, mean square error and the determination coefficient. The results indicated that models with two dimensional variables (DH, D2H and D&H) were always superior to those with a single variable (D). The D&H variable combination was found to be the most useful predictor. Of all the approaches, SOF could establish a single optimal model separately, but there were deviations in estimating results due to existing incompatibilities, while NAP and NSUR could ensure predictions compatibility. Simultaneously, we found that the new general model had better accuracy than others. In conclusion, we recommend that the new general model be used to estimate carbon content for Chinese fir and considered for other vegetation types as well. 相似文献
42.
43.
44.
Yang Su-Rong Sun Huan-Xin Hu Zhen-Zhen Wang Si-Heng Sun Hui Xue Yin-Jia Ye Chen-Bo 《Sleep and biological rhythms》2017,15(1):57-65
Sleep and Biological Rhythms - Chronic sleep deprivation (SD) is an overwhelming problem in young students. Firstly, we investigated whether different levels of pre-training SD had effects on... 相似文献
45.
Late Quaternary climate change,relict populations and present‐day refugia in the northern Atacama Desert: a case study from Quebrada La Higuera (18° S) 下载免费PDF全文
46.
47.
48.
49.
50.
N. Rocks C. Estrella G. Paulissen F. Quesada‐Calvo C. Gilles M. M. Guéders C. Crahay J.‐M. Foidart P. Gosset A. Noel D. D. Cataldo 《Cell proliferation》2008,41(6):988-1001
Abstract. Objectives: The ADAMs (a disintegrin and metalloproteinase) enzymes compose a family of membrane‐bound proteins characterized by their multi‐domain structure and ADAM‐12 expression is elevated in human non‐small cell lung cancers. The aim of this study was to investigate the roles played by ADAM‐12 in critical steps of bronchial cell transformation during carcinogenesis. Materials and methods: To assess the role of ADAM‐12 in tumorigenicity, BEAS‐2B cells were transfected with a plasmid encoding human full‐length ADAM‐12 cDNA, and then the effects of ADAM‐12 overexpression on cell behaviour were explored. Treatment of clones with heparin‐binding epidermal growth factor (EGF)‐like growth factor (HB‐EGF) neutralizing antibodies as well as an EGFR inhibitor allowed the dissection of mechanisms regulating cell proliferation and apoptosis. Results: Overexpression of ADAM‐12 in BEAS‐2B cells promoted cell proliferation. ADAM‐12 overexpressing clones produced higher quantities of HB‐EGF in their culture medium which may rely on membrane‐bound HB‐EGF shedding by ADAM‐12. Targeting HB‐EGF activity with a neutralizing antibody abrogated enhanced cell proliferation in the ADAM‐12 overexpressing clones. In sharp contrast, targeting of amphiregulin, EGF or transforming growth factor‐α failed to influence cell proliferation; moreover, ADAM‐12 transfectants were resistant to etoposide‐induced apoptosis and the use of a neutralizing antibody against HB‐EGF activity restored rates of apoptosis to be similar to controls.Conclusions: ADAM‐12 contributes to enhancing HB‐EGF shedding from plasma membranes leading to increased cell proliferation and reduced apoptosis in this bronchial epithelial cell line. 相似文献