全文获取类型
收费全文 | 132094篇 |
免费 | 4614篇 |
国内免费 | 895篇 |
专业分类
137603篇 |
出版年
2022年 | 301篇 |
2021年 | 525篇 |
2020年 | 347篇 |
2019年 | 425篇 |
2018年 | 12228篇 |
2017年 | 11038篇 |
2016年 | 8236篇 |
2015年 | 2187篇 |
2014年 | 1831篇 |
2013年 | 2365篇 |
2012年 | 6505篇 |
2011年 | 14752篇 |
2010年 | 13257篇 |
2009年 | 9441篇 |
2008年 | 11434篇 |
2007年 | 12979篇 |
2006年 | 1857篇 |
2005年 | 2060篇 |
2004年 | 2420篇 |
2003年 | 2267篇 |
2002年 | 2025篇 |
2001年 | 1480篇 |
2000年 | 1391篇 |
1999年 | 1129篇 |
1998年 | 618篇 |
1997年 | 583篇 |
1996年 | 563篇 |
1995年 | 537篇 |
1994年 | 496篇 |
1993年 | 541篇 |
1992年 | 1025篇 |
1991年 | 785篇 |
1990年 | 777篇 |
1989年 | 758篇 |
1988年 | 669篇 |
1987年 | 609篇 |
1986年 | 610篇 |
1985年 | 709篇 |
1984年 | 537篇 |
1983年 | 429篇 |
1982年 | 342篇 |
1981年 | 312篇 |
1980年 | 254篇 |
1979年 | 378篇 |
1978年 | 344篇 |
1977年 | 241篇 |
1974年 | 289篇 |
1973年 | 250篇 |
1972年 | 435篇 |
1971年 | 416篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
Christelle Hély Kelly K. Caylor Peter Dowty Samuel Alleaume Robert J. Swap Hank H. Shugart Christopher O. Justice 《Ecosystems》2007,10(7):1116-1132
Abstract
We present a regional fuel load model (1 km2 spatial resolution) applied in the southern African savanna region. The model is based on a patch-scale production efficiency
model (PEM) scaled up to the regional level using empirical relationships between patch-scale behavior and multi-source remote
sensing data (spatio-temporal variability of vegetation and climatic variables). The model requires the spatial distribution
of woody vegetation cover, which is used to determine separate respiration rates for tree and grass. Net primary production,
grass and tree leaf death, and herbivory are also taken into account in this mechanistic modeling approach. The fuel load
model has been calibrated and validated from independent measurements taken from savanna vegetation in Africa southward from
the equator. A sensitivity analysis on the effect of climate variables (incoming radiation, air temperature, and precipitation)
has been conducted to demonstrate the strong role that water availability has in determining productivity and subsequent fuel
load over the southern African region. The model performance has been tested in four different areas representative of a regional
increasing rainfall gradient—Etosha National Park, Namibia, Mongu and Kasama, Zambia, as well as in Kruger National Park,
South Africa. Within each area, we analyze model output from three different magnitudes of canopy coverage (<5, 30, and 50%).
We find that fuel load ranges predicted by the model are globally in agreement with field measurements for the same year.
High rainfall sustains green herbaceous production late in the dry season and delays tree leaf litter production. Effect of
water on production varies across the rainfall gradient with delayed start of green material production in more arid regions. 相似文献
63.
64.
Two cases of right lower lobe oedema are presented as a supposedly very rare sign of left heart failure after myocardial infarction in the presence of acute mitral regurgitation. Causes of unilateral oedema and its clinical relevance in the diagnostic and therapeutic perspectives are discussed, and a review of the literature on this topic is presented. 相似文献
65.
Tapas K. Nandi Hridoy R. Bairagya Bishnu P. Mukhopadhyay K. Sekar Dipankar Sukul Asim K. Bera 《Journal of biosciences》2009,34(1):27-34
The role of invariant water molecules in the activity of plant cysteine protease is ubiquitous in nature. On analysing the
11 different Protein DataBank (PDB) structures of plant thiol proteases, the two invariant water molecules W1 and W2 (W220
and W222 in the template 1PPN structure) were observed to form H-bonds with the Ob atom of Asn 175. Extensive energy minimization and molecular dynamics simulation studies up to 2 ns on all the PDB and solvated
structures clearly revealed the involvement of the H-bonding association of the two water molecules in fixing the orientation
of the asparagine residue of the catalytic triad. From this study, it is suggested that H-bonding of the water molecule at
the W1 invariant site better stabilizes the Asn residue at the active site of the catalytic triad. 相似文献
66.
67.
68.
69.
A method is described for the histochemical detection of horseradish peroxidase in Paraplast Plus embedded brain sections. The procedure uses 150-micron-thick Vibratome-cut slices of glutaraldehyde-paraformaldehyde-fixed brain tissue. Tetramethylbenzidine stabilized by diaminobenzidine/cobalt/H2O2 is used as chromogen. The Vibratome-cut slices are dehydrated through a graded series of acetone, cleared in toluol and flat-embedded in Paraplast Plus embedding medium. Serial sections can be cut as thin as 5-7 micron. The method is universal in its application and permits optimal visualization of labeled neurons with great morphological detail at the light-microscopic level. 相似文献
70.
Mingyue Wang Pranab K Mukherjee Jyotsna Chandra Ali Abdul Lattif Thomas S McCormick Mahmoud A Ghannoum 《BMC microbiology》2008,8(1):31