收费全文 | 2888篇 |
免费 | 210篇 |
国内免费 | 1篇 |
3099篇 |
2024年 | 3篇 |
2023年 | 31篇 |
2022年 | 27篇 |
2021年 | 81篇 |
2020年 | 72篇 |
2019年 | 67篇 |
2018年 | 129篇 |
2017年 | 126篇 |
2016年 | 141篇 |
2015年 | 196篇 |
2014年 | 198篇 |
2013年 | 249篇 |
2012年 | 286篇 |
2011年 | 256篇 |
2010年 | 128篇 |
2009年 | 107篇 |
2008年 | 186篇 |
2007年 | 131篇 |
2006年 | 134篇 |
2005年 | 111篇 |
2004年 | 90篇 |
2003年 | 79篇 |
2002年 | 67篇 |
2001年 | 25篇 |
2000年 | 28篇 |
1999年 | 23篇 |
1998年 | 7篇 |
1997年 | 7篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 6篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1981年 | 5篇 |
1977年 | 4篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1973年 | 3篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1967年 | 3篇 |
1965年 | 2篇 |
Salicylic acid (SA) is a plant hormone that stimulates the growth and metabolism of plants, also acting as an abiotic elicitor. This study aimed to evaluate the effect of SA on leaf production, leaf area and synthesis of secondary compounds in yarrow plants. The experiments were conducted under field conditions in two consecutive years and f-received SA foliar applications (T1-control; T2-1.0 mmol L−1 applications at 20, 60 and 100 days after planting (DAP) and T3-1.0 mmol L−1 applications at 100 DAP during 3 days). The exogenous application of SA resulted in increases in leaf area (total and specific), number of leaves and leaf mass ratio of yarrow plants, polyphenolic compounds, phenylalanine ammonia-lyase and chalcone synthase enzymes and the antioxidant activity of the plant extract. The HPLC–DAD–MS/MS analysis of phenolic compounds revealed increases in the amounts of quinic acid and rutin. The results of this research lead us to affirm that SA exerted both the hormonal effect on number of leaves and leaf area, and also acted as eliciting substance.
相似文献The Atlantic Forest is one of the most threatened tropical forests in the world, being drastically reduced, fragmented, and disturbed. The drastic process of anthropic occupation and exploitation of this biome has, in many cases, led to the introduction of exotic species, such as the jackfruits (Artocarpus heterophyllus). However, studies on the influence of jackfruits on the native biota are still scarce. Here we investigated the influence of fruit trees on the seed rain and early recruitment of seedlings in native remnants, comparing these patterns with those observed for a native species tapirira (Tapirira guianensis), which similarly to jackfruits, produces many fruits throughout the year, attracting a variety of frugivore species. Seed rain and seedlings observed under the jackfruits were both more abundant and equally rich to the assemblages reported under the native tapirira trees. In both species, co-specifics comprise a large part of the number of seeds (>?70%) and seedlings (>?45%) individuals and, although they attract similar seed assemblages, seedling composition diverge, particularly when co-specifics are excluded. We reported that jackfruits can attract a diverse seed and seedling assemblages, and we find no evidence that the presence of jackfruits negatively affects the arrival and initial recruitment of native plant species in the study area. These results should be analyzed with caution but considered when evaluating costs and benefits of management options to control exotic species.
相似文献Wetlands are among the most diverse environments on the planet and are strongly threatened by human activities. Their restoration and/or mitigation of human impacts, therefore, relies on information that can aid to the management of impacted wetlands so that they return to a (semi-) natural state. We investigate in this study the relationship between dormant stages of zooplankton and clay removal in areas subjected to mining. We evaluate whether a gradual increase in topsoil addition from donor natural wetlands to the sediment of mined wetlands influenced the zooplankton community. Eight wetlands were sampled in the Sinos River floodplain, four natural and four mined. In the laboratory, four field sediment samples were incubated for zooplankton hatching in five treatments comprising sediments from: mined wetlands, natural wetlands, and three treatments containing mined sediments added with low (5%), medium (20%) and high (40%) quantities of sediment from natural wetlands. Hatching consisted of 61 individuals distributed across eight zooplankton taxa. Copepod nauplii were the most abundant (31.1%) followed by Epiphanes sp. (29.5%) and Ovalona glabra (16.4%). While natural wetlands provided 42.6% of the hatched zooplankton, mined wetlands had just 6.5%. Zooplankton richness and abundance were higher in natural wetland sediments compared with mined and added sediment wetlands. To some degree, the sediment soil donation from natural to mined wetlands was considered viable. As long as prior studies are performed to test the size and quality of the dormant banks present in the sediment of candidate donor wetlands, sediment from donor wetlands may aid in the establishment of a more diverse community in disturbed systems.
相似文献