全文获取类型
收费全文 | 168篇 |
免费 | 1篇 |
专业分类
169篇 |
出版年
2021年 | 1篇 |
2016年 | 1篇 |
2015年 | 1篇 |
2014年 | 3篇 |
2013年 | 8篇 |
2012年 | 7篇 |
2011年 | 15篇 |
2010年 | 8篇 |
2009年 | 10篇 |
2008年 | 6篇 |
2007年 | 6篇 |
2006年 | 10篇 |
2005年 | 5篇 |
2004年 | 3篇 |
2003年 | 1篇 |
2002年 | 2篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 3篇 |
1998年 | 8篇 |
1997年 | 11篇 |
1996年 | 7篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 1篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1975年 | 1篇 |
1973年 | 2篇 |
1972年 | 3篇 |
1970年 | 1篇 |
1958年 | 2篇 |
1957年 | 2篇 |
1956年 | 1篇 |
排序方式: 共有169条查询结果,搜索用时 0 毫秒
31.
STEFAN SLOVIK 《Global Change Biology》1996,2(5):459-477
Field data on the sulphur and cation budget of growing Norway spruce canopies (Picea abies [L.] Karst.) are summarized. They are used to test a spruce decline model capable of quantifying effects of chronic SO2 pollution on spruce forests. At ambient SO2 concentrations, acute SO2 damage is rare, but exposure to polluted air produces reversible thinning of the canopy structure with a half-time of a few years. Canopy thinning in the spruce decline model is highest (i) at elevated SO2 pollution, (ii) in the mountains, (iii) at unfertilized sites with poor K+, Mg2+ or Zn2+ supply, (iv) at low spruce litter decomposition rates, and (v) acidic, shallow soils at high annual precipitation rates in the field and vice versa. Model application using field data from Würzburg (moderate SO2 pollution, alkaline soils, no spruce decline) and from the Erzgebirge (extreme SO2 pollution, acidic soils in the mountains, massive spruce decline) predicts canopy thinning by 2–11% in Würzburg and by 45–70% in the Erzgebirge. The model also predicts different SO2-tolerance limits for Norway spruce depending on the site elevation and on the nutritional status of the needles. If needle loss of more than 25% (damage class 2) is taken to indicate ‘real damage’ exceeding natural variances, then for optimum soil conditions SO2 tolerance limits range from (27.3 ± 7.4) μg m?3 to (62.6 ± 16.5) μg m?3. For shallow and acidic soils, SO2 tolerance limits range from (22.0 ± 5.5) μg m?3 to (37.4 ± 7.5) μ m?3. These tolerance limits, which are calculated on an ecophysiological data basis for Norway spruce are close to epidemiological SO2-toIerance limits as recommended by the IUFRO, UN-ECE and WHO. The observed statistical regression slope of the plot (damaged spruce trees vs. SO2-pollution) in west Germany is confirmed by modelling (6% error). Model application to other forest trees allows deduction of the observed sequence of SO2-sensitivity: Abies > Picea > Pinus > Fagus > Quercus. Thus, acute phytotoxicity of SO2 seems not to be involved in ‘forest decline’. Chronic SO2-pollution induces massive canopy thinning of Abies alba and Picea abies only at unfavourable sites, where natural stress factors and secondary effects of SO2pollution act together to produce tree decline. 相似文献
32.
HONOR C. PRENTICE STEFAN ANDERSSON EVA MÅNSBY 《Botanical journal of the Linnean Society. Linnean Society of London》2011,166(2):127-148
The perennial herbs Silene vulgaris and S. uniflora are closely related, partially sympatric and interfertile, yet morphologically distinct. We used nuclear (allozyme) and plastid (polymerase chain reaction–restriction fragment length polymorphism) DNA markers to investigate whether these species have a shared history of postglacial colonization and hybridization in Europe, as inferred from large‐scale patterns of geographic variation. The majority of plastid haplotypes and allozyme alleles were widespread and patchily distributed within both species and there was no geographic structure in the distributions of shared allozymes or haplotypes. The mosaic variation is consistent with a scenario in which repeated episodes of interspecific hybridization pre‐dated the largely allopatric range expansion of the two species during the postglacial period. Our overall results are not consistent with a scenario of extensive hybridization and introgression during the postglacial range expansion of the species or within their current areas of sympatry, but we found some evidence for local, postglacial evolution and hybridization in the Baltic region. © 2011 The Linnean Society of London, Botanical Journal of the Linnean Society, 2011, 166 , 127–148. 相似文献
33.
Embryonic and post-embryonic development of the Early Cambrian cnidarian Olivooides 总被引:12,自引:0,他引:12
YUE ZHAO STEFAN BENGTSON 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1999,32(2):181-195
Phosphatized specimens of Olivooides from the Early Cambrian of Shaanxi, China, represent a number of developmental stages. These include cleavage, gastrulation, organogenesis, cuticularization, pre-hatching, post-hatching and subsequent growth. This allows the reconstruction of a nearly full developmental sequence of this animal. Olivooides had large (600-870 μm in diameter), sphaerical eggs, indicating a high yolk content. Development was direct. Thus adult characters were forming already in the embryo, and there was no free larval stage. The embryonic development took place within a smooth protective membrane. Gastrulation probably was by polar ingression, and the blastopore appears to correspond to the aperture of the later stages. An embryonic cuticle formed which carried star-shaped structures, stellae, over the entire surface except for a radially folded non-stellate portion around the future aperture. At a later stage, the stellate cuticle was thrown into folds concentric with the aperture. This radially folded tissue then became more dominant. After hatching, the body assumed the shape of a strongly annulated cone, with the stellate cuticle forming the apical part and the folded cuticle forming a longitudinally striate cuticle around the aperture. Subsequent growth took place through the addition of striate tissue. A pentaradial symmetry of the body is suggested by lateral folds in the apical part. Olivooides is interpreted as a cnidarian, probably closely related to the scyphozoans. The conical test may have housed a polyp similar to the thecate polyps of modern coronate scyphozoans, but, unlike the latter, Olivooides had no visible attachment structures. There is no evidence for or against a free medusa stage. The prevalence of lecithotrophic direct developers in the Neoproterozoic and Cambrian, unless reflecting a preservational bias, casts some doubts on evolutionary models that assume larval planktotrophy to be primitive among metazoans. 相似文献
34.
35.
36.
37.
38.
STEFAN I. CSÖGÖR 《Nature: New biology》1972,238(87):287-288
CHOLESTEROL is found in the blood as a structural component of lipoproteins concerned with the transport of other lipids1. The high resolution nuclear magnetic resonance spectra of high density serum lipoproteins are similar to that observed when lipids are dissolved in organic solvents, or dispersed in water by bile salts or detergents, or in sonicated form. The lipid component in lipoproteins is therefore probably in an extremely fluid condition2. If human serum is mixed with paraffin oil, some of the cholesterol diffuses into the oil without affecting the ultraviolet absorption spectrum of serum proteins. This procedure avoids any protein denaturing action used for cholesterol extraction3–5. It therefore seems that serum cholesterol has two fractions, one strongly bound by lipoprotein structures and the other loosely bound and diffusible in an oil phase. In this article I designate the loosely bound fraction “diffusible”. 相似文献
39.
40.
The experimental silicification of Aquificales and their role in hot spring sinter formation 总被引:2,自引:1,他引:1
STEFAN V. LALONDE KURT O. KONHAUSER ANNA-LOUISE REYSENBACH F. GRANT FERRIS 《Geobiology》2005,3(1):41-52
Archean microfossils provide some of the earliest physical evidence for life on Earth, yet there remains a great deal of uncertainty regarding which micro‐organisms were actually preserved. Because of the limited cellular detail remaining, interpretation of those microfossils has been based solely on size and morphology. This has led to significant controversy surrounding the presence or absence of cyanobacteria as early as 3.5 billion years. Accordingly, there has been an experimental bias towards studying their silicification. Here we report the very first findings on thermophilic bacteria–silica interactions, and investigate how Sulfurihydrogenibium azorense, a representative of the Aquificales often found as prominent members of modern hot spring vent communities, interacts with highly siliceous hydrothermal fluids. We show that adsorption of silica is limited to silica polymers and colloids, and that the magnitude of silica adsorption is dependent on its chemolithoautotrophic pathway. Intriguingly, when S. azorense is grown as a H2‐oxidizer, it responds to increasing silica concentrations by producing a protein‐rich biofilm that may afford the cells protection against cell wall silicification. Although the biofilms of Aquificales could potentially contribute to or accelerate siliceous sinter formation under certain growth conditions, the cells themselves show a low preservation potential and are unlikely to have been preserved in the ancient rock record, despite phylogenetic analyses suggesting that they represent one of the most primordial life forms. 相似文献