全文获取类型
收费全文 | 7061篇 |
免费 | 234篇 |
国内免费 | 236篇 |
出版年
2023年 | 47篇 |
2022年 | 57篇 |
2021年 | 96篇 |
2020年 | 103篇 |
2019年 | 108篇 |
2018年 | 118篇 |
2017年 | 84篇 |
2016年 | 106篇 |
2015年 | 125篇 |
2014年 | 195篇 |
2013年 | 289篇 |
2012年 | 161篇 |
2011年 | 219篇 |
2010年 | 162篇 |
2009年 | 240篇 |
2008年 | 273篇 |
2007年 | 289篇 |
2006年 | 270篇 |
2005年 | 277篇 |
2004年 | 262篇 |
2003年 | 259篇 |
2002年 | 278篇 |
2001年 | 222篇 |
2000年 | 173篇 |
1999年 | 189篇 |
1998年 | 184篇 |
1997年 | 175篇 |
1996年 | 181篇 |
1995年 | 178篇 |
1994年 | 169篇 |
1993年 | 184篇 |
1992年 | 165篇 |
1991年 | 166篇 |
1990年 | 142篇 |
1989年 | 159篇 |
1988年 | 135篇 |
1987年 | 122篇 |
1986年 | 131篇 |
1985年 | 135篇 |
1984年 | 130篇 |
1983年 | 78篇 |
1982年 | 117篇 |
1981年 | 107篇 |
1980年 | 82篇 |
1979年 | 54篇 |
1978年 | 29篇 |
1977年 | 30篇 |
1976年 | 31篇 |
1975年 | 14篇 |
1973年 | 8篇 |
排序方式: 共有7531条查询结果,搜索用时 31 毫秒
1.
2.
The accumulation of heavy metals in Typha latifolia L. grown in a stream carrying secondary effluent
Typha latifolia L. from aquatic plants is widely found throughout Kehli Stream (Elazig, Turkey). This study examined the uptake of some metals by T. latifolia and the transfer from roots to other plant parts. The accumulation of Mn in T. latifolia L. can be suggested as a tolerance strategy due to its transfer factor higher than 1.0. The enrichment coefficients in the leaves of T. latifolia L. were higher than 1.0 for Zn and Mn and often lower than 1.0 for other metals. Similarly, the enrichment coefficients of all metals, except for Cr, in roots of T. latifolia L. were higher than 1.0. This study demonstrated that T. latifolia L. could be considered as either a bio-indicator or a bio-accumulator for sediments and water polluted by metals. 相似文献
3.
4.
《Neuron》2020,105(6):1062-1076.e6
5.
6.
7.
The Orlando Easterly Wetland (OEW), located near Christmas, Florida, USA, is among the longer-lived treatment wetlands in the United States. It was established in the late 1980s to reduce nitrogen and phosphorus concentrations from tertiary treated wastewater bound for the St. Johns River. A goal of 0.07 mg/l total phosphorus concentration has been set by the regulating agency (St. Johns River Water Management District). In order to understand and define the operating conditions for which this target could be met, a systematic study of historic phosphorus uptake was performed using a traditional first-order model. Phosphorus uptake performance is shown to correlate well with hydraulic performance for two parallel upstream cells. The first-order model is enhanced with predictive capabilities that acknowledge the correlation between the phosphorus uptake rate constant and the hydraulic loading rate observed in the system. Inherent limitations with the first-order modeling approach are addressed and uncertainty in model performance is used to bound predictions. 相似文献
8.
《Fungal biology》2020,124(10):903-913
Although water is essential for photosynthetic activation in lichens, rates of vapor uptake and activation in humid air, which likely influence their niche preferences and distribution ranges, are insufficiently known. This study simultaneously quantifies rehydration kinetics and PSII reactivation in sympatric, yet morphologically and functionally distinct cephalolichens (Lobaria amplissima, Lobaria pulmonaria, Lobaria virens). High-temporal resolution monitoring of rehydrating thalli by automatic weighing combined with chlorophyll fluorescence imaging of maximal PSII efficiency (FV/FM) was applied to determine species-specific rates of vapor uptake and photosynthetic activation. The thin and loosely attached growth form of L. pulmonaria rehydrates and reactivates faster in humid air than the thick L. amplissima, with L. virens in between. This flexible hydration strategy is consistent with L. pulmonaria’s wide geographical distribution stretching from rainforests to continental forests. By contrast, the thick and resupinate L. amplissima reactivates slowly in humid air but stores much water when provided in abundance. This prolongs active periods after rain, which could represent an advantage where abundant rain and stem flow alternates with long-lasting drying. Understanding links between morphological traits and functional responses, and their ecological implications for species at risk, is crucial to conservation planning and for modelling populations under various climate scenarios. 相似文献
9.
Concentrations of soluble aluminum (Al) and manganese (Mn) frequently reach phytotoxic levels in acid soils. While dose response relationships for these metals are well documented, the effects of combined exposure have received less attention. We have examined the effect of combinations of Al and Mn on growth and metal accumulation in Vigna unguiculata (L.) Walp. grown in solution culture under conditions of low ionic strength (conductivities typically < 100 µS cm−1 ). The nature of interaction between these metals varied with the specific physiological response, the part of the plant investigated, and the relative amount of stress imposed. Analysis of growth data provided evidence for amelioration of metal toxicity (antagonistic effects), although this effect was dose dependent. Analysis of metal content data provided evidence for antagonistic and synergistic (exacerbation of toxicity) effects, again depending on dose. Analysis of foliar symptoms also provided evidence for antagonisms and synergisms, with the nature of the response dependent on the specific physiological response and specific plant part investigated. In contrast with previous reports, evidence for antagonistic, synergistic, and multiplicative effects on growth, metal uptake, and expression of foliar symptoms have been obtained under physiologically and environmentally relevant conditions. These results suggest a more detailed analysis of the potential for interactions between metals in the environment is required. 相似文献
10.
L. Fontana C. Cravanzola S. Colombatto M. A. Grillo 《Cell biochemistry and function》1996,14(3):173-180
Uptake of polyamines has been investigated extensively in many cells, but not in placenta, where the polyamine– polyamine oxidase system is supposed to have an immunoregulatory function in pregnancy. Due to the importance of the transfer in this tissue, we have started this study. JAR human placental choriocarcinoma cells in monolayer at confluency were used as a model for measuring the key enzymes of polyamine synthesis and interconversion, rate of uptake and efflux, and the polyamine content. Polyamines were taken up by JAR cells and released by an independent mechanism. Ornithine decarboxylase and spermidine acetyltransferase activities and the rate of transport in and out of the cell were much higher than in other cells, such as L1210 cells. However the systems used for uptake and release appear in many respects to be similar to those observed in L1210 cells, but different from others. The uptake appears to be regulated by an inhibitory protein. Moreover, protein kinase C appears to be involved in the process. The efflux also is regulated as in L1210 cells, through control of H+ and Ca2+ concentration. In conclusion, this study shows that, in JAR cells, ornithine decarboxylase and spermidine acetyltransferase activities were much higher than in other cells, and so was the rate of transport in and out of the cells. As a result, a much higher polyamine content was observed. 相似文献