全文获取类型
收费全文 | 3557篇 |
免费 | 447篇 |
国内免费 | 275篇 |
出版年
2024年 | 2篇 |
2023年 | 93篇 |
2022年 | 47篇 |
2021年 | 91篇 |
2020年 | 167篇 |
2019年 | 177篇 |
2018年 | 135篇 |
2017年 | 178篇 |
2016年 | 149篇 |
2015年 | 149篇 |
2014年 | 220篇 |
2013年 | 212篇 |
2012年 | 151篇 |
2011年 | 165篇 |
2010年 | 145篇 |
2009年 | 210篇 |
2008年 | 225篇 |
2007年 | 195篇 |
2006年 | 178篇 |
2005年 | 166篇 |
2004年 | 160篇 |
2003年 | 126篇 |
2002年 | 120篇 |
2001年 | 98篇 |
2000年 | 75篇 |
1999年 | 75篇 |
1998年 | 94篇 |
1997年 | 68篇 |
1996年 | 47篇 |
1995年 | 47篇 |
1994年 | 31篇 |
1993年 | 34篇 |
1992年 | 34篇 |
1991年 | 35篇 |
1990年 | 31篇 |
1989年 | 18篇 |
1988年 | 17篇 |
1987年 | 19篇 |
1986年 | 19篇 |
1985年 | 12篇 |
1984年 | 14篇 |
1983年 | 4篇 |
1982年 | 6篇 |
1981年 | 12篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1976年 | 8篇 |
1973年 | 2篇 |
排序方式: 共有4279条查询结果,搜索用时 15 毫秒
1.
Latitudinal comparison of altitudinal changes in forest structure,leaf-type,and species richness in humid monsoon Asia 总被引:10,自引:0,他引:10
M. Ohsawa 《Plant Ecology》1995,121(1-2):3-10
A new template for mountain vegetation zonation along latitudinal gradients is proposed for examining geographical pattern of various forest attributes in humid monsoon Asia. The contrasting temperature regime in tropical and temperate mountains, i.e., the former is a non-seasonal, temperature-sum controlled environment, and the latter is a seasonal, low temperature limiting environment, leads to different altitudinal patterns of tree height distribution and species richness. In the tropical mountains, both tree height and species richness decrease steeply, and the tree height often stepwise. The decline of tree height and species diversity in the temperate mountains is far less pronounced except near the forest limit. Both trends are explained by their temperature regime. 相似文献
2.
《Current biology : CB》2020,30(13):2433-2445.e3
Media player
3.
M. Wall L. Putchim G. M. Schmidt C. Jantzen S. Khokiattiwong C. Richter 《Proceedings. Biological sciences / The Royal Society》2015,282(1799)
Tropical scleractinian corals are particularly vulnerable to global warming as elevated sea surface temperatures (SSTs) disrupt the delicate balance between the coral host and their algal endosymbionts, leading to symbiont expulsion, mass bleaching and mortality. While satellite sensing of SST has proved a reliable predictor of coral bleaching at the regional scale, there are large deviations in bleaching severity and mortality on the local scale that are poorly understood. Here, we show that internal waves play a major role in explaining local coral bleaching and mortality patterns in the Andaman Sea. Despite a severe region-wide SST anomaly in May 2010, frequent upslope intrusions of cold sub-pycnocline waters due to breaking large-amplitude internal waves (LAIW) mitigated coral bleaching and mortality in shallow waters. In LAIW-sheltered waters, by contrast, bleaching-susceptible species suffered severe bleaching and total mortality. These findings suggest that LAIW benefit coral reefs during thermal stress and provide local refugia for bleaching-susceptible corals. LAIW are ubiquitous in tropical stratified waters and their swash zones may thus be important conservation areas for the maintenance of coral diversity in a warming climate. Taking LAIW into account can significantly improve coral bleaching predictions and provide a valuable tool for coral reef conservation and management. 相似文献
4.
Because estuaries are dynamic on many temporal scales, it is difficult to differentiate long-term shifts from those resulting from erratic pulse events like severe storms. Diatoms are abundant and diverse in estuaries, and may characterize these dynamics across relevant time scales. The climate of south Florida is subtropical, with dry winters and wet summers that may also harbor tropical storms and cyclones. We investigated whether these repeated seasonal drivers of change in estuaries differ from the influence of tropical cyclones on planktonic and benthic diatom assemblages. Diatom assemblages and environmental parameters were measured in the Charlotte Harbor watershed of southwest Florida to identify differences between the wet and dry seasons and changes that occurred following Tropical Storm Debby in 2012. Indicator taxa that were significantly affiliated with each season and post-storm conditions were identified. Diatom assemblages across the watershed were similar in wet and dry seasons, but differences increased following the tropical storm. The reduction in community dispersion following the storm was pronounced in each of the major drainages of the estuary. There were spatially-distinct responses of diatoms to environmental changes driven by the storm. These results suggest that past storm activity could be detected using diatoms preserved in estuarine sediments using both single indicator-species as well as community approaches. 相似文献
5.
Global species delimitation and phylogeography of the circumtropical ‘sexy shrimp’ Thor amboinensis reveals a cryptic species complex and secondary contact in the Indo‐West Pacific 下载免费PDF全文
6.
7.
Up to now, the development of dendrochronological records from tropical regions in South America has been limited to the lowlands with emphasis in the Amazon basin. In this contribution, we present the first chronology of Cedrela nebulosa, a species that develops in the tropical mountainous regions of South America. We collected samples from trees in Monobamba district in Peru, analysed the anatomical features that determine the growth rings, and processed following the methods commonly used in dendrochronology. The 133-years chronology covering the 1883–2015 period, showed large correlation between series. In order to determine the climatic variables that control tree growth, we performed correlation analyses between tree-growth and local and regional precipitation and temperature records. We found that precipitation triggers tree growth at the beginning of the spring season but temperature seems to be the main control in annual growth. Also, C. nebulosa chronology present coherent variations with Multivariate Enso Index (MEI) and Pacific Ocean sea surface temperatures during summer months. This climate-sensitive tree-ring record indicates good potential for dendroclimatic studies and provides an opportunity to reconstruct climatic variations in montane forests of the tropical Andes. 相似文献
8.
9.
Understanding the mechanisms of resilience of coral reefs to anthropogenic stressors is a critical step toward mitigating their current global decline. Coral–bacteria associations are fundamental to reef health and disease, but direct observations of these interactions remain largely unexplored. Here, we use novel technology, high-speed laser scanning confocal microscopy on live coral (Pocillopora damicornis), to test the hypothesis that corals exert control over the abundance of their associated bacterial communities by releasing (‘shedding'') bacteria from their surface, and that this mechanism can counteract bacterial growth stimulated by organic inputs. We also test the hypothesis that the coral pathogen Vibrio coralliilyticus can evade such a defense mechanism. This first report of direct observation with high-speed confocal microscopy of living coral and its associated bacterial community revealed a layer (3.3–146.8 μm thick) on the coral surface where bacteria were concentrated. The results of two independent experiments showed that the bacterial abundance in this layer was not sensitive to enrichment (5 mg l−1 peptone), and that coral fragments exposed to enrichment released significantly more bacteria from their surfaces than control corals (P<0.01; 35.9±1.4 × 105 cells cm−2 coral versus 1.3±0.5 × 105 cells cm−2 coral). Our results provide direct support to the hypothesis that shedding bacteria may be an important mechanism by which coral-associated bacterial abundances are regulated under organic matter stress. Additionally, the novel ability to watch this ecological behavior in real-time at the microscale opens an unexplored avenue for mechanistic studies of coral–microbe interactions. 相似文献
10.
Raghunath Chatterjee Charles Vinson 《Biochimica et Biophysica Acta (BBA) - Gene Regulatory Mechanisms》2012,1819(7):763-770
CG methylation is an epigenetically inherited chemical modification of DNA found in plants and animals. In mammals it is essential for accurate regulation of gene expression and normal development. Mammalian genomes are depleted for the CG dinucleotide, a result of the chemical deamination of methyl-cytosine in CG resulting in TpG. Most CG dinucleotides are methylated, but ~ 15% are unmethylated. Five percent of CGs cluster into ~ 20,000 regions termed CG islands (CGI) which are generally unmethylated. About half of CGIs are associated with housekeeping genes. In contrast, the gene body, repeats and transposable elements in which CGs are generally methylated. Unraveling the epigenetic machinery operating in normal cells is important for understanding the epigenetic aberrations that are involved in human diseases including cancer. With the advent of high-throughput sequencing technologies, it is possible to identify the CG methylation status of all 30 million unique CGs in the human genome, and monitor differences in distinct cell types during differentiation and development. Here we summarize the present understanding of DNA methylation in normal cells and discuss recent observations that CG methylation can have an effect on tissue specific gene expression. We also discuss how aberrant CG methylation can lead to cancer. This article is part of a Special Issue entitled: Chromatin in time and space. 相似文献