全文获取类型
收费全文 | 11686篇 |
免费 | 2089篇 |
国内免费 | 2647篇 |
专业分类
测绘学 | 668篇 |
大气科学 | 2333篇 |
地球物理 | 3091篇 |
地质学 | 5755篇 |
海洋学 | 1205篇 |
天文学 | 613篇 |
综合类 | 1135篇 |
自然地理 | 1622篇 |
出版年
2024年 | 48篇 |
2023年 | 157篇 |
2022年 | 499篇 |
2021年 | 586篇 |
2020年 | 478篇 |
2019年 | 595篇 |
2018年 | 628篇 |
2017年 | 582篇 |
2016年 | 685篇 |
2015年 | 570篇 |
2014年 | 707篇 |
2013年 | 681篇 |
2012年 | 636篇 |
2011年 | 739篇 |
2010年 | 710篇 |
2009年 | 642篇 |
2008年 | 618篇 |
2007年 | 632篇 |
2006年 | 482篇 |
2005年 | 385篇 |
2004年 | 358篇 |
2003年 | 332篇 |
2002年 | 351篇 |
2001年 | 314篇 |
2000年 | 360篇 |
1999年 | 482篇 |
1998年 | 402篇 |
1997年 | 434篇 |
1996年 | 342篇 |
1995年 | 314篇 |
1994年 | 335篇 |
1993年 | 272篇 |
1992年 | 220篇 |
1991年 | 162篇 |
1990年 | 116篇 |
1989年 | 93篇 |
1988年 | 120篇 |
1987年 | 65篇 |
1986年 | 58篇 |
1985年 | 48篇 |
1984年 | 42篇 |
1983年 | 35篇 |
1982年 | 36篇 |
1981年 | 32篇 |
1980年 | 8篇 |
1979年 | 11篇 |
1978年 | 6篇 |
1975年 | 2篇 |
1958年 | 8篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
441.
Green tides have occurred every year from 2007 to 2014 in the Yellow Sea. Ulva prolifera(Müller) J. Agardh has been identified as the bloom-forming alga,co-occurring with U. intestinalis. We observed distinct strategies for both algal species during green tides. U. prolifera exhibited a high abundance initially and then decreased dramatically,while U. intestinalis persisted throughout. The antioxidant system responses of these two macroalgae were compared in the late phase of a green tide(in-situ) and after laboratory acclimation. Lipid peroxidation and antioxidant system responses differed significantly between the two. Malondialdehyde and hydrogen peroxide contents increased significantly in-situ in U. prolifera,but not in U. intestinalis. In U. prolifera,we observed a significant decrease in total antioxidant ability(T-AOC),antioxidant enzymes(SOD and Apx),and non-enzyme antioxidants(GSH and As A) in-situ. U. intestinalis showed the same pattern of T-AOC and SOD,but its Gpx,Apx,and GSH responses did not differ significantly. The results suggest that U. prolifera was more susceptible than U. intestinalis to the harsh environmental changes during the late phase of a Yellow Sea green tide. The boom and bust strategy exhibited by U. prolifera and the persistence of U. intestinalis can be explained by differences in enzyme activity and antioxidant systems. 相似文献
442.
Seed long-distance dispersal(LDD) events are typically rare, but are important in the population processes that determine large-scale forest changes and the persistence of species in fragmented landscapes. However, previous studies focused on species dispersed via animal-mediated LDD, and ignored those dispersed by wind. The aim of this study was to assess the effects of canopy openness, edge, seed source, and patch tree density on the LDD of seeds by wind in forest. We collected birch seeds, a typical wind-dispersed species, throughout a larch plantation. We then assessed the relationship between birch LDD and each factor that may influence LDD of seeds by wind including distance to edge, canopy openness size, distance to mature forest, and the tree density of the larch plantation. We used univariate linear regression analysis to assess the influence of those factors on birch LDD, and partial correlations to calculate the contribution of each factor to LDD. The results showed that both canopy openness and edge had significant influences on birch LDD. Specifically, a negative relationship was observed between distance to edge and birch LDD, whereas there was a positive correlation between canopy openness size and LDD. In contrast, the distance to the mature forest was not correlated with birch LDD. Our results suggest that patch tree density could potently affect the probability of LDD by wind vectors, which provides novel and revealing insights regarding the effect of fragmentation on wind dynamics. The data also provide compelling evidence for the previously undocumented effect of habitat fragmentation on wind-dispersed organisms. As such, these observations will facilitate reasonable conservation planning, which requires a detailed understanding of the mechanisms by which patch properties hamper the delivery of seeds of wind-dispersed plants to fragmented areas. 相似文献
443.
The Drake Passage is located between the Antarctic Peninsula and Tierra del Fuego in the south of South America. Surface seawater samples were collected at seven sites in the Drake Passage during the austral summer of 2012. The 16S rRNA sequences were analyzed from 187 isolated bacterial strains. Three phyla, 29 genera and 56 species were identified. The three phyla were Actinobacteria, Firmicutes and Proteobacteria; the Proteobacteria included α-Proteobacteria, β-Proteobacteria and γ-Proteobacteria. γ-Proteobacteria, Actinobacteria and Firmicutes were the dominant class or phyla in terms of quantity and species. Gram-positive bacteria (Actinobacteria and Firmicutes) accounted for 57.8% of all types identified. There were nine dominant genera, including Curtobacterium, Staphylococcus, and Halomonas, and 14 dominant species including Curtobacterium flaccumfaciens, Curtobacterium pusillum, and Staphylococcus sciuri. Of the strains identified, 87.2% were catalase positive or weakly positive. 相似文献
444.
445.
446.
447.
448.
449.
450.