全文获取类型
收费全文 | 393篇 |
免费 | 3篇 |
专业分类
396篇 |
出版年
2017年 | 3篇 |
2016年 | 1篇 |
2014年 | 6篇 |
2013年 | 10篇 |
2012年 | 10篇 |
2011年 | 21篇 |
2010年 | 27篇 |
2009年 | 27篇 |
2008年 | 20篇 |
2007年 | 24篇 |
2006年 | 15篇 |
2005年 | 21篇 |
2004年 | 11篇 |
2003年 | 9篇 |
2001年 | 5篇 |
2000年 | 2篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 8篇 |
1996年 | 21篇 |
1995年 | 9篇 |
1994年 | 14篇 |
1993年 | 11篇 |
1992年 | 9篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 10篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1972年 | 2篇 |
1971年 | 5篇 |
1969年 | 2篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1947年 | 1篇 |
排序方式: 共有396条查询结果,搜索用时 0 毫秒
131.
132.
The effects of the recent warming trend in many northern temperate lakes on the species composition of spring phytoplankton remain poorly understood, especially because a recent change in nutrients has complicated efforts, and previous studies have defined spring according to the calendar. We analysed data from 1979 to 2004 from Lake Müggelsee (Berlin, Germany), using physical and biological parameters to define the spring period. We show that a change in timing of spring plankton events in warm years led to the paradox of lower mean water temperatures during the growth period, favouring cold-adapted diatoms over cyanobacteria, and within the diatoms, some cold-adapted centric forms over pennate forms. Under high P : Si ratios, the increased time between phytoplankton and cladoceran peaks opened a loophole for filamentous cyanobacteria (Oscillatoriales) in warm years to establish dominance after the diatoms, which are silicate limited. Therefore, the warming trend promotes filamentous cyanobacteria, a well-known nuisance in eutrophic lakes, and surprisingly, cold-adapted diatoms. 相似文献
133.
134.
135.
JAN F. KREUZE ILANIT SAMOLSKI KLEIN MILTON UNTIVEROS LAZARO WILMER J. CUELLAR CHUQUIYURI GABRIELA LAJO MORGAN PATRICIA G. CIPRIANI MEJÍA MARC GHISLAIN JARI P.T. VALKONEN 《Molecular Plant Pathology》2008,9(5):589-598
Sweetpotato chlorotic stunt virus (SPCSV; genus Crinivirus , family Closteroviridae) is one of the most important pathogens of sweetpotato ( Ipomoea batatas L.). It can reduce yields by 50% by itself and cause various synergistic disease complexes when co-infecting with other viruses, including sweetpotato feathery mottle virus (SPFMV; genus Potyvirus , family Potyviridae). Because no sources of true resistance to SPCSV are available in sweetpotato germplasm, a pathogen-derived transgenic resistance strategy was tested as an alternative solution in this study. A Peruvian sweetpotato landrace 'Huachano' was transformed with an intron-spliced hairpin construct targeting the replicase encoding sequences of SPCSV and SPFMV using an improved genetic transformation procedure with reproducible efficiency. Twenty-eight independent transgenic events were obtained in three transformation experiments using a highly virulent Agrobacterium tumefaciens strain and regeneration through embryogenesis. Molecular analysis indicated that all regenerants were transgenic, with 1–7 transgene loci. Accumulation of transgene-specific siRNA was detected in most of them. None of the transgenic events was immune to SPCSV, but ten of the 20 tested transgenic events exhibited mild or no symptoms following infection, and accumulation of SPCSV was significantly reduced. There are few previous reports of RNA silencing-mediated transgenic resistance to viruses of Closteroviridae in cultivated plants. However, the high levels of resistance to accumulation of SPCSV could not prevent development of synergistic sweet potato virus disease in those transgenic plants also infected with SPFMV. 相似文献
136.
JAN HOFFMAN FRENCH 《American anthropologist》2004,106(4):663-674
In this article, I explore issues of authenticity, legal discourse, and local requirements of belonging by considering the recent surge of indigenous recognitions in northeastern Brazil. I investigate how race and ethnicity are implicated in the recognition process in Brazil on the basis of an analysis of a successful struggle for indigenous identity and access to land by a group of mixed-race, visibly, African-descended rural workers. I propose that the debate over mestizaje (ethnoracial and cultural mixing) in the Spanish-speaking countries of Latin America can be reconfigured and clarified by broadening it to include such Brazilian experiences. I argue that the interaction between two processes—law making and indigenous identity formation—is crucial to understanding how the notion of "mixed heritage" is both reinforced and disentangled. As such, this article is an illustration of the role of legal discourse in the constitution of indigenous identities and it introduces northeastern Brazil into the global discussion of law, indigenous rights, and claims to citizenship. 相似文献
137.
Three species of blind Amphipoda are recorded from anchihaline cave waters in the Galapagos Islands. One of these, Galapsiellus leleuporum (Monod, 1970) was previously know from the islands; the male is described for the first time. The other two belong to families with predominantly (though not exclusively) deep-sea members: Valettietta cavernicola sp. nov. (family Lysianassidae; and Antronicippe serrata gen. et sp. nov. (family Pardaliscidae). The genus Valettietta was not previously recorded from caves; it contained two abyssal species in the Atlantic, while a third species was recorded from the Pacific in a vertical haul between 0 and 5300 m. Antronicippe is closely related to Spelaeonicippe , a genus with two species known from anchihaline caves in the Canary Islands and the West Indies. 相似文献
138.
PEETERS KURT M. U.; GEUNS JAN M. C.; VAN LAERE ANDRE J. 《Journal of experimental botany》1993,44(11):1709-1715
The free polyamine content of flag leaves, peduncles, rachis,glumes, and grains of wheat (Triticum aestivum L., cv. Castell)plants, ripening under field conditions, has been investigatedduring three consecutive growing seasons. Putrescine was quantitativelythe most important of all polyamines detected in these organs.Concentrations were highest in the grains, glumes and flag leaves.No correlation was found between polyamine content and the onsetof senescence of flag leaves and other organs. Excised primaryleaves, however, showed a decrease in polyamine content in thedark and also in light/dark cycles, but in the latter case onlyafter an initial increase. Sink removal of otherwise intactwheat plants caused an accumulation of putrescine in flag leavesat the later stages of senescence, whereas removal of all otherleaves was without any significant effect. Putrescine was alsorecovered in phloem-exudate samples collected throughout theperiod of grain development. In both grains and glumes, peakconcentrations of polyamines were found early during seed development. Key words: Triticum aestivum, polyamines, ripening, senescence 相似文献
139.
140.
Emigration and immigration can be important determinants of benthic diatom assemblages in streams 总被引:3,自引:0,他引:3
SUMMARY.
- 1 Interspecific differences in diatom abundances in stream drift (plankton), immigration, and natural benthic assemblages were compared to assess the importance of emigration and immigration in benthic diatom community dynamics. Water samples were collected throughout a 24-h period to measure diel changes in diatom drift abundances and to estimate benthic diatom emigration rates. Immigration was assessed with 24-h colonization of bare tiles.
- 2 Dissimilarity in species composition of drift, immigration, and natural substrate assemblages indicated differential emigration and immigration among diatom species.
- 3 A mathematical model indicated that reproduction by diatoms in the plankton could not account for diel drift peaks and that diel variation in drift was an informative measure of benthic diatom emigration.
- 4 Emigration and immigration of some species constituted substantial proportions of diatom abundances on natural substrata. We conclude that emigration into the drift and immigration onto substrata can be important processes that regulate benthic diatom species composition and standing crop in streams.