全文获取类型
收费全文 | 10700篇 |
免费 | 1096篇 |
国内免费 | 1674篇 |
出版年
2024年 | 39篇 |
2023年 | 235篇 |
2022年 | 229篇 |
2021年 | 362篇 |
2020年 | 469篇 |
2019年 | 499篇 |
2018年 | 424篇 |
2017年 | 454篇 |
2016年 | 460篇 |
2015年 | 446篇 |
2014年 | 501篇 |
2013年 | 723篇 |
2012年 | 519篇 |
2011年 | 502篇 |
2010年 | 469篇 |
2009年 | 530篇 |
2008年 | 584篇 |
2007年 | 660篇 |
2006年 | 618篇 |
2005年 | 563篇 |
2004年 | 485篇 |
2003年 | 485篇 |
2002年 | 443篇 |
2001年 | 390篇 |
2000年 | 363篇 |
1999年 | 280篇 |
1998年 | 235篇 |
1997年 | 238篇 |
1996年 | 173篇 |
1995年 | 151篇 |
1994年 | 126篇 |
1993年 | 127篇 |
1992年 | 101篇 |
1991年 | 81篇 |
1990年 | 90篇 |
1989年 | 60篇 |
1988年 | 61篇 |
1987年 | 52篇 |
1986年 | 30篇 |
1985年 | 41篇 |
1984年 | 35篇 |
1983年 | 18篇 |
1982年 | 34篇 |
1981年 | 22篇 |
1980年 | 14篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1977年 | 10篇 |
1976年 | 9篇 |
1973年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
942.
943.
944.
945.
946.
Population genetic diversity of the clonal self‐incompatible herbaceous plant Linaria vulgaris along an urbanization gradient
下载免费PDF全文
![点击此处可从《Biological journal of the Linnean Society. Linnean Society of London》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Jacek Bartlewicz Katrien Vandepitte Hans Jacquemyn Olivier Honnay 《Biological journal of the Linnean Society. Linnean Society of London》2015,116(3):603-613
How increasing urbanization affects biodiversity is one of the most understudied aspects of global change biology. It is, however, known that it may negatively affect plant population genetic diversity in numerous ways, for example through its negative effects on plant population size, between‐population connectivity, and reproductive success. Therefore, it is important to investigate to what extent different levels of urbanization result in these negative phenomena. Here we used microsatellite markers to investigate urbanization effects on the population genetic structure of 23 populations of the self‐incompatible, partially clonal herb Linaria vulgaris which were sampled across a gradient of urbanization. Clonal diversity as measured by the Pareto‐parameter varied between 1.11 and 2.97 and was negatively correlated to both the degree of urbanization and to population size. Urbanization and population size were not interrelated. The least clonally rich populations also experienced significantly reduced seed set. Irrespective of the degree of urbanization, L. vulgaris populations exhibited strong genetic differentiation (FST = 0.33) and there was no significant correlation between genetic and geographic distances, suggesting low gene flow among populations. In conclusion, we showed that urbanization negatively affected fitness of L. vulgaris populations through decreasing their clonal diversity and reproductive success, an effect that may be exacerbated by the low gene flow between populations. Although the effect was modest, the results could probably be extrapolated to bigger cities where it would be considerably more pronounced. 相似文献
947.
Ryan D. Phillips Rod Peakall Bryony A. Retter Kirke Montgomery Myles H. M. Menz Belinda J. Davis Christine Hayes Graham R. Brown Nigel D. Swarts Kingsley W. Dixon 《Botanical journal of the Linnean Society. Linnean Society of London》2015,179(3):511-525
An increasing diversity of highly specialized pollination systems are being discovered, many of which are likely to be vulnerable to anthropogenic landscape modification. Here, we investigate if a specialized pollination system limits the persistence of Caladenia huegelii (Orchidaceae), an endangered species pollinated by sexual deception of thynnine wasps. Once locally common in part of its geographical range, C. huegelii is now largely restricted to small habitat remnants in urban areas. Pollinator surveys coupled with DNA barcoding detected a single pollinator taxon, a small form of Macrothynnus insignis. Phylogenetic analysis revealed that small M. insignis from within the range of C. huegelii are strongly divergent from other wasp populations, suggesting that some reproductive isolation may exist. Although common in intact landscapes outside the range of C. huegelli, small M. insignis individuals were recorded at only 4% of sites in suitable C. huegelii habitat. Accordingly, reproductive success in C. huegelii was low compared with related Caladenia spp., with 33–60% of populations failing to set fruit in any given year. As such, populations are likely to now persist primarily through individual plant longevity rather than reproduction. Due to the low reproductive success of C. huegelii, ongoing human intervention will almost certainly be needed to sustain the species. Future research will need to focus on optimizing hand pollination to maintain reproduction and high seed fitness. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179 , 511–525. 相似文献
948.
Molecular phylogenetics and complementary geographical distributions of species of the Western Australian land snail genera Plectorhagada Iredale, 1933 and Strepsitaurus Solem, 1997 (Gastropoda: Camaenidae)
下载免费PDF全文
![点击此处可从《Zoological Journal of the Linnean Society》网站下载免费的PDF全文](/ch/ext_images/free.gif)
James P. A. Taylor Michael S. Johnson Sean Stankowski 《Zoological Journal of the Linnean Society》2015,174(2):305-321
The Western Australian camaenid genera Plectorhagada and Strepsitaurus have morphological similarities and mutually exclusive ranges near Cape Range. Sequences of ctyochrome c oxidase subunit I (COI) and 16S mitochondrial DNA (mtDNA) genes confirmed that the two genera are genetically close sister clades. Targeted sampling showed that Strepsitaurus, which is confined to Cape Range, lies within a hole in the distribution of the more broadly distributed Plectorhagada that occurs on the coastal strip surrounding Cape Range. Species of the two genera meet at the transition between the rocky Cape Range and the sandier coastal areas, providing a rare example of the close replacement of genera. Within each genus, mtDNA sequences confirmed the monophyly and genetic distinctness of species, with few exceptions that show the need for additional work, and with the addition of three new species in the Cape Range area. As is typical of Australian camaenids, distributions of congeneric species are mutually exclusive, but in some cases close proximity is associated with contrasting habitats, such as gorge endemics versus the species on top of Cape Range. In sympatry, Strepsitaurus rugus (Cotton, 1951) and Strepsitaurus williami Solem, 1997 are separated by microhabitat. These local associations with habitat indicate that ecological differences, and not simply allopatric divergence, contribute to the lack of sympatry between closely related Australian camaenids. © 2015 The Linnean Society of London 相似文献
949.
为了探索生物质焦对糠醛的吸附脱除特性,利用流化床快速热解制得稻壳焦,研究N2、CO2气氛下高温改性方式对稻壳焦孔隙特征与表面性质的影响,以及稻壳焦对糠醛的吸附脱除特性。采用元素分析、N2等温吸脱附、傅里叶红外、Boehm滴定等方法对稻壳焦的孔隙结构与表面化学特性进行表征。结果表明:原始的稻壳焦残留大量有机基团,孔隙结构较差;经N2和CO2高温改性后,稻壳焦表面的含氧酸性官能团大量分解,碱性官能团增加,比表面积和孔结构得到较好的扩充和优化,稻壳焦与糠醛的π-π色散力作用力增强。综合考虑π-π色散力和表面吸附位点的作用,CO2改性的稻壳焦表现出了最好的吸附效果。 相似文献
950.
Variation in ecological selection pressures has been implicated to explain variation in brain size and architecture in fishes, birds and mammals, but little is known in this respect about amphibians. Likewise, the relative importance of constraint vs. mosaic hypotheses of brain evolution in explaining variation in brain size and architecture remains contentious. Using phylogenetic comparative methods, we studied interspecific variation in brain size and size of different brain parts among 43 Chinese anuran frogs and explored how much of this variation was explainable by variation in ecological factors (viz. habitat type, diet and predation risk). We also evaluated which of the two above‐mentioned hypotheses best explains the observed patterns. Although variation in brain size explained on average 80.5% of the variation in size of different brain parts (supporting the constraint hypothesis), none of the three ecological factors were found to explain variation in overall brain size. However, habitat and diet type explained a significant amount of variation in telencephalon size, as well in three composite measures of brain architecture. Likewise, predation risk explained a significant amount of variation in bulbus olfactorius and optic tecta size. Our results show that evolution of anuran brain accommodates features compatible with both constraint (viz. strong allometry among brain parts) and mosaic (viz. independent size changes in response to ecological factors in certain brain parts) models of brain size evolution. 相似文献