全文获取类型
收费全文 | 7526篇 |
免费 | 657篇 |
国内免费 | 4篇 |
专业分类
8187篇 |
出版年
2023年 | 58篇 |
2022年 | 142篇 |
2021年 | 274篇 |
2020年 | 147篇 |
2019年 | 176篇 |
2018年 | 199篇 |
2017年 | 160篇 |
2016年 | 335篇 |
2015年 | 509篇 |
2014年 | 522篇 |
2013年 | 532篇 |
2012年 | 756篇 |
2011年 | 645篇 |
2010年 | 375篇 |
2009年 | 363篇 |
2008年 | 489篇 |
2007年 | 417篇 |
2006年 | 375篇 |
2005年 | 287篇 |
2004年 | 289篇 |
2003年 | 264篇 |
2002年 | 240篇 |
2001年 | 52篇 |
2000年 | 30篇 |
1999年 | 38篇 |
1998年 | 60篇 |
1997年 | 29篇 |
1996年 | 22篇 |
1995年 | 27篇 |
1994年 | 23篇 |
1993年 | 27篇 |
1992年 | 23篇 |
1991年 | 19篇 |
1990年 | 14篇 |
1989年 | 21篇 |
1988年 | 12篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 12篇 |
1984年 | 15篇 |
1983年 | 24篇 |
1982年 | 8篇 |
1981年 | 16篇 |
1980年 | 13篇 |
1979年 | 11篇 |
1978年 | 15篇 |
1977年 | 10篇 |
1976年 | 9篇 |
1975年 | 9篇 |
1974年 | 8篇 |
排序方式: 共有8187条查询结果,搜索用时 17 毫秒
51.
Does Fire Trigger Seed Germination in the Neotropical Savannas? Experimental Tests with Six Cerrado Species 下载免费PDF全文
Betânia Santos Fichino Julia R. G. Dombroski Vânia R. Pivello Alessandra Fidelis 《Biotropica》2016,48(2):181-187
The Cerrado (Brazilian savanna) is a biodiversity hotspot with a history of fire that goes back as far as 10 million years. Fire has influenced the evolution of several aspects of the vegetation, including reproduction and life cycles. This study tested how fire by‐products such as heat and smoke affect the germination of six species common to two Cerrado open physiognomies: wet grasslands and the campo sujo (grassland with scattered shrubs and dwarf trees). We subjected seeds collected in northern Brazil to heat shock and smoke treatments, both separately and combined, using different temperatures, exposure times, and smoke concentrations in aqueous solutions. High temperatures and smoke did not break seed dormancy nor stimulate germination of the Cerrado study species. However, seeds were not killed by high temperatures, indicating that they are fire‐tolerant. Our findings differed from those of other fire‐prone ecosystems (mostly of Mediterranean vegetation), where fire stimulates germination. Moreover, we provide important information regarding germination strategies of non‐woody Cerrado plants, showing the importance of considering the tolerance of seeds to high temperatures when evaluating fire‐related traits in fire‐prone ecosystems. 相似文献
52.
Prince EK Myers TL Naar J Kubanek J 《Proceedings. Biological sciences / The Royal Society》2008,275(1652):2733-2741
Biotic interactions in the plankton can be both complex and dynamic. Competition among phytoplankton is often chemically mediated, but no studies have considered whether allelopathic compounds are modified by biotic interactions. Here, we show that compounds exuded during Karenia brevis blooms were allelopathic to the cosmopolitan diatom Skeletonema costatum, but that bloom allelopathy varied dramatically among collections and years. We investigated several possible causes of this variability and found that neither bloom density nor concentrations of water-borne brevetoxins correlated with allelopathic potency. However, when we directly tested whether the presence of competing phytoplankton influenced bloom allelopathy, we found that S. costatum reduced the growth-inhibiting effects of bloom exudates, suggesting that S. costatum has a mechanism for undermining K. brevis allelopathy. Additional laboratory experiments indicated that inducible changes to K. brevis allelopathy were restricted to two diatoms among five sensitive phytoplankton species, whereas five other species were constitutively resistant to K. brevis allelopathy. Our results suggest that competitors differ in their responses to phytoplankton allelopathy, with S. costatum exhibiting a previously undescribed method of resistance that may influence community structure and alter bloom dynamics. 相似文献
53.
Víctor Faundes William G. Newman Laura Bernardini Natalie Canham Jill Clayton-Smith Bruno Dallapiccola Sally J. Davies Michelle K. Demos Amy Goldman Harinder Gill Rachel Horton Bronwyn Kerr Dhavendra Kumar Anna Lehman Shane McKee Jenny Morton Michael J. Parker Julia Rankin Siddharth Banka 《American journal of human genetics》2018,102(1):175-187
54.
Julia Beaumont Jonny Geber Natasha Powers Andrew Wilson Julia Lee‐Thorp Janet Montgomery 《American journal of physical anthropology》2013,150(1):87-98
Historical evidence documents mass migration from Ireland to London during the period of the Great Irish Famine of 1845–52. The rural Irish were reliant on a restricted diet based on potatoes but maize, a C4 plant, was imported from the United States of America in 1846–47 to mitigate against Famine. In London, Irish migrants joined a population with a more varied diet. To investigate and characterize their diet, carbon and nitrogen isotope ratios were obtained from bone collagen of 119 and hair keratin of six individuals from Lukin Street cemetery, Tower Hamlets (1843–54), and bone collagen of 20 individuals from the cemetery at Kilkenny Union Workhouse in Ireland (1847–51). A comparison of the results with other contemporaneous English populations suggests that Londoners may have elevated δ15N compared with their contemporaries in other cities. In comparison, the Irish group have lower δ15N. Hair analysis combined with bone collagen allows the reconstruction of perimortem dietary changes. Three children aged 5–15 years from Kilkenny have bone collagen δ13C values that indicate consumption of maize (C4). As maize was only imported into Ireland in quantity from late 1846 and 1847, these results demonstrate relatively rapid bone collagen turnover in children and highlight the importance of age‐related bone turnover rates, and the impact the age of the individual can have on studies of short‐term dietary change or recent migration. Stable light isotope data in this study are consistent with the epigraphic and documentary evidence for the presence of migrants within the London cemetery. Am J Phys Anthropol, 2013. © 2012 Wiley Periodicals, Inc. 相似文献
55.
56.
Pedro M. F. Sousa Marco A. M. Videira Thomas Vorburger Sara T. N. Silva James W. Moir Julia Steuber Ana M. P. Melo 《Archives of microbiology》2013,195(3):211-217
Neisseria meningitidis is a pathogenic bacterium responsible for meningitis. The mechanisms underlying the control of Na+ transmembrane movement, presumably important to pathogenicity, have been barely addressed. To elucidate the function of the components of the Na+ transport system in N. meningitidis, an open reading frame from the genome of this bacterium displaying similarity with the NhaE type of Na+/H+ antiporters was expressed in Escherichia coli and characterized for sodium transport ability. The N. meningitidis antiporter (NmNhaE) was able to complement an E. coli strain devoid of Na+/H+ antiporters (KNabc) respecting the ability to grow in the presence of NaCl and LiCl. Ion transport assays in everted vesicles prepared from KNabc expressing NmNhaE from a plasmid confirmed its ability to translocate Na+ and Li+. Here is presented the characterization of the first NhaE from a pathogen, an important contribution to the comprehension of sodium ion metabolism in this kind of microorganisms. 相似文献
57.
58.
Efficient in vitro regeneration systems for Vaccinium species 总被引:1,自引:0,他引:1
Julia Meiners Melanie Schwab Iris Szankowski 《Plant Cell, Tissue and Organ Culture》2007,89(2-3):169-176
Efficient protocols for shoot regeneration from leaf explants suitable for micropropagation as well as for the development
of transgenic plants were developed for blueberry (Vaccinium corymbosum) and lingonberry (Vaccinium vitis-idaea) cultivars. Nodal segments were used to initiate in vitro shoot cultures of lingonberry cultivar ‘Red Pearl’ and southern
highbush blueberry cultivar ‘Ozarkblue’. In order to develop an optimized regeneration procedure, different types and concentrations
of plant growth regulators were tested to induce adventitious shoot regeneration on excised leaves from micropropagated shoots
of both cultivars. The effect on percentage regeneration and number of shoots per explant was investigated. Results indicated
that zeatin was superior to TDZ and meta-topolin in promoting adventitious shoot formation. A concentration of 20 μM zeatin
was most effective in promoting shoot regeneration in both cultivars, in case of ‘Red Pearl’ along with 1 μM NAA. Shoots were
either allowed to root in vitro on medium containing IBA or NAA or ex vitro in a fog tunnel. IBA was superior to NAA for induction
of root development in vitro in both Vaccinium cultivars. Ex vitro rooting under high humidity was tested with cuttings from mature field-grown plants, from acclimatized
tissue culture derived plants and with unrooted in vitro proliferated shoots planted directly. It was found that in vitro
shoots rooted better under fog than cuttings from the other plant sources and rooting was equivalent to that achieved in vitro. 相似文献
59.
Matthew J Christmas Julia C Jones Anna Olsson Ola Wallerman Ignas Bunikis Marcin Kierczak Valentina Peona Kaitlyn M Whitley Tuuli Larva Alexander Suh Nicole E Miller-Struttmann Jennifer C Geib Matthew T Webster 《Molecular biology and evolution》2021,38(8):3126
Evidence is accumulating that gene flow commonly occurs between recently diverged species, despite the existence of barriers to gene flow in their genomes. However, we still know little about what regions of the genome become barriers to gene flow and how such barriers form. Here, we compare genetic differentiation across the genomes of bumblebee species living in sympatry and allopatry to reveal the potential impact of gene flow during species divergence and uncover genetic barrier loci. We first compared the genomes of the alpine bumblebee Bombus sylvicola and a previously unidentified sister species living in sympatry in the Rocky Mountains, revealing prominent islands of elevated genetic divergence in the genome that colocalize with centromeres and regions of low recombination. This same pattern is observed between the genomes of another pair of closely related species living in allopatry (B. bifarius and B. vancouverensis). Strikingly however, the genomic islands exhibit significantly elevated absolute divergence (dXY) in the sympatric, but not the allopatric, comparison indicating that they contain loci that have acted as barriers to historical gene flow in sympatry. Our results suggest that intrinsic barriers to gene flow between species may often accumulate in regions of low recombination and near centromeres through processes such as genetic hitchhiking, and that divergence in these regions is accentuated in the presence of gene flow. 相似文献
60.
I Bruce M Akhlaq GC Bloomfield E Budd B Cox B Cuenoud P Finan P Gedeck J Hatto JF Hayler D Head T Keller L Kirman C Leblanc DL Grand C McCarthy D O'Connor C Owen MS Oza G Pilgrim NE Press L Sviridenko L Whitehead 《Bioorganic & medicinal chemistry letters》2012,22(17):5445-5450
Using a parallel synthesis approach to target a non-conserved region of the PI3K catalytic domain a pan-PI3K inhibitor 1 was elaborated to provide alpha, delta and gamma isoform selective Class I PI3K inhibitors 21, 24, 26 and 27. The compounds had good cellular activity and were selective against protein kinases and other members of the PI3K superfamily including mTOR and DNA-PK. 相似文献