全文获取类型
收费全文 | 620340篇 |
免费 | 66245篇 |
国内免费 | 7330篇 |
出版年
2021年 | 9137篇 |
2020年 | 6070篇 |
2019年 | 7446篇 |
2018年 | 8858篇 |
2017年 | 7582篇 |
2016年 | 10694篇 |
2015年 | 14505篇 |
2014年 | 17225篇 |
2013年 | 21184篇 |
2012年 | 24541篇 |
2011年 | 23895篇 |
2010年 | 15781篇 |
2009年 | 14432篇 |
2008年 | 19270篇 |
2007年 | 19089篇 |
2006年 | 17798篇 |
2005年 | 16150篇 |
2004年 | 15758篇 |
2003年 | 14697篇 |
2002年 | 14211篇 |
2001年 | 26732篇 |
2000年 | 26574篇 |
1999年 | 21226篇 |
1998年 | 7272篇 |
1997年 | 7808篇 |
1996年 | 7149篇 |
1995年 | 6640篇 |
1994年 | 6446篇 |
1993年 | 6193篇 |
1992年 | 16525篇 |
1991年 | 16115篇 |
1990年 | 15292篇 |
1989年 | 14891篇 |
1988年 | 14012篇 |
1987年 | 13086篇 |
1986年 | 12132篇 |
1985年 | 11909篇 |
1984年 | 9650篇 |
1983年 | 8382篇 |
1982年 | 6329篇 |
1981年 | 5603篇 |
1979年 | 9062篇 |
1978年 | 7124篇 |
1977年 | 6498篇 |
1976年 | 6147篇 |
1975年 | 6647篇 |
1974年 | 7393篇 |
1973年 | 7265篇 |
1972年 | 6716篇 |
1971年 | 6126篇 |
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
991.
992.
993.
994.
995.
Fish finders have already been widely available in the fishing market for a number of years.However,the sizes of these fishfinders are too big and their prices are expensive to suit for the research of robotic fish or mini-submarine.The goal of thisresearch is to propose a low-cost fish detector and classifier which suits for underwater robot or submarine as a proximity sensor.With some pre-condition in hardware and algorithms,the experimental results show that the proposed design has good per-formance,with a detection rate of 100 % and a classification rate of 94 %.Both the existing type of fish and the group behaviorcan be revealed by statistical interpretations such as hovering passion and sparse swimming mode. 相似文献
996.
H. Elenga O. Peyron R. Bonnefille D. Jolly R. Cheddadi J. Guiot V. Andrieu S. Bottema G. Buchet J.-L. De Beaulieu A. C. Hamilton J. Maley R. Marchant R. Perez-Obiol M. Reille G. Riollet L. Scott H. Straka D. Taylor E. Van Campo A. Vincens F. Laarif H. Jonson 《Journal of Biogeography》2000,27(3):621-634
Pollen data from 18,000 14C yr bp were compiled in order to reconstruct biome distributions at the last glacial maximum in southern Europe and Africa. Biome reconstructions were made using the objective biomization method applied to pollen counts using a complete list of dryland taxa wherever possible. Consistent and major differences from present‐day biomes are shown. Forest and xerophytic woods/scrub were replaced by steppe, both in the Mediterranean region and in southern Africa, except in south‐western Cape Province where fynbos (xerophytic scrub) persisted. Sites in the tropical highlands, characterized today by evergreen forest, were dominated by steppe and/or xerophytic vegetation (cf. today’s Ericaceous belt and Afroalpine grassland) at the last glacial maximum. Available data from the tropical lowlands are sparse but suggest that the modern tropical rain forest was largely replaced by tropical seasonal forest while the modern seasonal or dry forests were encroached on by savanna or steppe. Montane forest elements descended to lower elevations than today. 相似文献
997.
Mingyue Wang Pranab K Mukherjee Jyotsna Chandra Ali Abdul Lattif Thomas S McCormick Mahmoud A Ghannoum 《BMC microbiology》2008,8(1):31
Background
We have previously shown that supernatant from Candida albicans (CA) culture contains a Secretory Interleukin (IL)-12 Inhibitory Factor (CA-SIIF), which inhibits IL-12 production by human monocytes. However, the effect of CA-SIIF on secretion of other cytokines by monocytes is unknown, and detailed characterization of this factor has not been performed. 相似文献998.
K. Choi M. D. Burow G. Church G. Burow A. H. Paterson C. E. Simpson J. L. Starr 《Journal of nematology》1999,31(3):283-290
Segregation of resistance to Meloidogyne arenaria in six BC₅F₂ peanut breeding populations was examined in greenhouse tests. Chi-square analysis indicated that segregation of resistance was consistent with resistance being conditioned by a single gene in three breeding populations (TP259-3, TP262-3, and TP271-2), whereas two resistance genes may be present in the breeding populations TP259-2, TP263-2, and TP268-3. Nematode development in clonally propagated lines of resistant individuals of TP262-3 and TP263-2 was compared to that of the susceptible cultivar Florunner. Juvenile nematodes readily penetrated roots of all peanut genotypes, but rate of development was slower (P = 0.05) in the resistant genotypes than in Florunner. Host cell necrosis indicative of a hypersensitive response was not consistently observed in resistant genotypes of either population. Three RFLP loci linked to resistance at distances of 4.2 to 11.0 centiMorgans were identified. Resistant and susceptible alleles for RFLP loci R2430E and R2545E were quite distinct and are useful for identifying individuals homozygous for resistance in segregating populations. 相似文献
999.
1000.
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