全文获取类型
收费全文 | 55725篇 |
免费 | 4955篇 |
国内免费 | 5942篇 |
出版年
2024年 | 85篇 |
2023年 | 1169篇 |
2022年 | 1019篇 |
2021年 | 1629篇 |
2020年 | 2063篇 |
2019年 | 2451篇 |
2018年 | 2283篇 |
2017年 | 1946篇 |
2016年 | 2021篇 |
2015年 | 1998篇 |
2014年 | 2846篇 |
2013年 | 3639篇 |
2012年 | 2315篇 |
2011年 | 2810篇 |
2010年 | 2914篇 |
2009年 | 2528篇 |
2008年 | 2583篇 |
2007年 | 2822篇 |
2006年 | 2568篇 |
2005年 | 2529篇 |
2004年 | 2368篇 |
2003年 | 2135篇 |
2002年 | 1721篇 |
2001年 | 1505篇 |
2000年 | 1164篇 |
1999年 | 1235篇 |
1998年 | 1072篇 |
1997年 | 999篇 |
1996年 | 995篇 |
1995年 | 949篇 |
1994年 | 896篇 |
1993年 | 878篇 |
1992年 | 816篇 |
1991年 | 674篇 |
1990年 | 635篇 |
1989年 | 589篇 |
1988年 | 594篇 |
1987年 | 401篇 |
1986年 | 410篇 |
1985年 | 397篇 |
1984年 | 391篇 |
1983年 | 196篇 |
1982年 | 314篇 |
1981年 | 238篇 |
1980年 | 212篇 |
1979年 | 144篇 |
1978年 | 119篇 |
1977年 | 101篇 |
1976年 | 94篇 |
1974年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Interaction between atmospheric and pedospheric nitrogen nutrition in spruce (Picea abies L. Karst) seedlings 总被引:6,自引:2,他引:4
The effect of NO2 fumigation on root N uptake and metabolism was investigated in 3-month-old spruce (Picea abics L. Karst) seedlings. In a first experiment, the contribution of NO2 to the plant N budget was measured during a 48 h fumigation with 100mm3m?3 NO2. Plants were pre-treated with various nutrient solutions containing NO2 and NH4+, NO3? only or no nitrogen source for 1 week prior to the beginning of fumigation. Absence of NH4+ in the solution for 6d led to an increased capacity for NO3? uptake, whereas the absence of both ions caused a decrease in the plant N concentration, with no change in NO3? uptake. In fumigated plants, NO2 uptake accounted for 20–40% of NO3? uptake. Root NO3? uptake in plants supplied with NH4+plus NO3? solutions was decreased by NO2 fumigation, whereas it was not significantly altered in the other treatments. In a second experiment, spruce seedlings were grown on a solution containing both NO2 and NH4+ and were fumigated or not with 100mm3m?3 NO2 for 7 weeks. Fumigated plants accumulated less dry matter, especially in the roots. Fluxes of the two N species were estimated from their accumulations in shoots and roots, xylem exudate analysis and 15N labelling. Root NH4+ uptake was approximately three times higher than NO3? uptake. Nitrogen dioxide uptake represented 10–15% of the total N budget of the plants. In control plants, N assimilation occurred mainly in the roots and organic nitrogen was the main form of N transported to the shoot. Phloem transport of organic nitrogen accounted for 17% of its xylem transport. In fumigated plants, neither NO3? nor NH4+ accumulated in the shoot, showing that all the absorbed NO2 was assimilated. Root NO3? reduction was reduced whereas organic nitrogen transport in the phloem increased by a factor of 3 in NO2-fimugated as compared with control plants. The significance of the results for the regulation of whole-plant N utilization is discussed. 相似文献
92.
We investigated the synthesis and translocation of amino compounds in Parasponia, a genus of the Ulmaceae that represents the only non-legumes known to form a root nodule symbiosis with rhizohia. In the xylem sap of P. andersonii we identified asparagine. aspartate. glutamine, glutamated significant quantities of a non-protein amino acid. 4-methylglutamte(2-amino-4-methylpentanedioic acid). This identification was confirmed by two methods, capillary gas chromatography (GC) electron ionization (El) mass spectrometry (MS) and reverse phase high pressure liquid chromatography (HPLC) analysis of derivatized compounds. In leaf, root and nodule samples from P. andersonii and P. parviflora we also identified the related compounds 4-methyleneglutamate and 4-methyleneglulamine. Using 15N2 labelling and GC-Ms analysis of root nodule extracts we followed N2 fixation and ammonia assimilation in P. andersonii root nodules and observed Label initially in glutamine and subsequently in glutamate, suggesting operation of the glutamine synthetase/glutamine:2-oxoglutarate aminotransferase (GS/GOGAT) pathway. Importantly, we observed the incorporation of significant quantities of 15N into 4-methylglutamate in nodules, demonstrating the de nova synthesis of this non protein amino acid and suggesting a role in the translation of N in symbioticParasponia. 相似文献
93.
94.
95.
96.
97.
98.
Understanding how tropical tree phenology (i.e., the timing and amount of seed and leaf production) responds to climate is vital for predicting how climate change may alter ecological functioning of tropical forests. We examined the effects of temperature, rainfall, and photosynthetically active radiation (PAR) on seed phenology of four dominant species and community-level leaf phenology in a montane wet forest on the island of Hawaiʻi using monthly data collected over ~ 6 years. We expected that species phenologies would be better explained by variation in temperature and PAR than rainfall because rainfall at this site is not limiting. The best-fit model for all four species included temperature, rainfall, and PAR. For three species, including two foundational species of Hawaiian forests (Acacia koa and Metrosideros polymorpha), seed production declined with increasing maximum temperatures and increased with rainfall. Relationships with PAR were the most variable across all four species. Community-level leaf litterfall decreased with minimum temperatures, increased with rainfall, and showed a peak at PAR of ~ 400 μmol/m2s−1. There was considerable variation in monthly seed and leaf production not explained by climatic factors, and there was some evidence for a mediating effect of daylength. Thus, the impact of future climate change on this forest will depend on how climate change interacts with other factors such as daylength, biotic, and/or evolutionary constraints. Our results nonetheless provide insight into how climate change may affect different species in unique ways with potential consequences for shifts in species distributions and community composition. 相似文献
99.
Using a modified postmodern perspective, Canada's policy of multiculturalism and the emphasis upon ‘unity within diversity’ are related to the theme of globalization and the development of ‘a new world order’. It is argued that Canada is not unique in its efforts to come to terms with the contradictions and conflicts generated by postindustrialism and the realignment of superpowers. Questions of identity, collective self‐determination and the problematic relation between universalism and particularism, in relation to sovereignty, legitimacy, human rights and participation are explored. 相似文献
100.