全文获取类型
收费全文 | 2846篇 |
免费 | 216篇 |
国内免费 | 1篇 |
出版年
2023年 | 19篇 |
2022年 | 43篇 |
2021年 | 80篇 |
2020年 | 54篇 |
2019年 | 60篇 |
2018年 | 87篇 |
2017年 | 83篇 |
2016年 | 112篇 |
2015年 | 127篇 |
2014年 | 155篇 |
2013年 | 207篇 |
2012年 | 217篇 |
2011年 | 234篇 |
2010年 | 168篇 |
2009年 | 132篇 |
2008年 | 168篇 |
2007年 | 189篇 |
2006年 | 186篇 |
2005年 | 125篇 |
2004年 | 125篇 |
2003年 | 103篇 |
2002年 | 106篇 |
2001年 | 29篇 |
2000年 | 15篇 |
1999年 | 24篇 |
1998年 | 26篇 |
1997年 | 15篇 |
1996年 | 17篇 |
1995年 | 8篇 |
1994年 | 8篇 |
1993年 | 9篇 |
1992年 | 9篇 |
1991年 | 7篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 3篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1973年 | 3篇 |
1969年 | 3篇 |
1967年 | 3篇 |
排序方式: 共有3063条查询结果,搜索用时 15 毫秒
51.
Steroidal compounds have been utilized as carriers and for modification of physico-chemical properties of model biologically active secondary alcohols - juvenoids. Juvenoids are juvenile hormone analogues - environmentally safe insecticides, possessing significant biological activity towards different arthropods groups in focus on insect pest species. Structure modification of juvenoids plays important role to control the rate of liberation and decomposition of juvenoid in digestive system and can also play important role in the mode of action towards selected insect. This study presents an approach to the synthesis of steroidal monomers and dimers carrying one and two molecules of a juvenoid in their structures. The prepared compounds were tested for their inhibition activity on reproduction of the blowfly Neobellieria (Sarcophaga) bullata. These steroid-juvenoid conjugates showed promising possibilities in synthesis of new unique biochemical insecticides. Preliminary biological test results of prepared compounds are presented. 相似文献
52.
H. River Gates Stephen Yezerinac Abby N. Powell Pavel S. Tomkovich Olga P. Valchuk Richard B. Lanctot 《Journal of Field Ornithology》2013,84(4):389-402
Five subspecies of Dunlins (Calidris alpina) that breed in Beringia are potentially sympatric during the non‐breeding season. Studying their ecology during this period requires techniques to distinguish individuals by subspecies. Our objectives were to determine (1) if five morphometric measures (body mass, culmen, head, tarsus, and wing chord) differed between sexes and among subspecies (C. a. actites, arcticola, kistchinski, pacifica, and sakhalina), and (2) if these differences were sufficient to allow for correct classification of individuals using equations derived from discriminant function analyses. We conducted analyses using morphometric data from 10 Dunlin populations breeding in northern Russia and Alaska, USA. Univariate tests revealed significant differences between sexes in most morphometric traits of all subspecies, and discriminant function equations predicted the sex of individuals with an accuracy of 83–100% for each subspecies. We provide equations to determine sex and subspecies of individuals in mixed subspecies groups, including the (1) Western Alaska group of arcticola and pacifica (known to stage together in western Alaska) and (2) East Asia group of arcticola, actites, kistchinski, and sakhalina (known to winter together in East Asia). Equations that predict the sex of individuals in mixed groups had classification accuracies between 75% and 87%, yielding reliable classification equations. We also provide equations that predict the subspecies of individuals with an accuracy of 22–96% for different mixed subspecies groups. When the sex of individuals can be predetermined, the accuracy of these equations is increased substantially. Investigators are cautioned to consider limitations due to age and feather wear when using these equations during the non‐breeding season. These equations will allow determination of sexual and subspecies segregation in non‐breeding areas, allowing implementation of taxonomic‐specific conservation actions. 相似文献
53.
54.
Olga R. Yamilova Ilya V. Martynov Allison S. Brandvold Irina V. Klimovich Alex H. Balzer Alexander V. Akkuratov Ilya E. Kusnetsov Natalie Stingelin Pavel A. Troshin 《Liver Transplantation》2020,10(7)
In view of a rapid development and increase in efficiency of organic solar cells, reaching their long‐term operational stability represents now one of the main challenges to be addressed on the way toward commercialization of this photovoltaic technology. However, intrinsic degradation pathways occurring in organic solar cells under realistic operational conditions remain poorly understood. The light‐induced dimerization of the fullerene‐based acceptor materials discovered recently is considered to be one of the main causes for burn‐in degradation of organic solar cells. In this work, it is shown that not only the fullerene derivatives but also different types of conjugated polymers and small molecules undergo similar light‐induced crosslinking regardless of their chemical composition and structure. In the case of conjugated polymers, crosslinking of macromolecules leads to a rapid increase in their molecular weight and consequent loss of solubility, which can be revealed in a straightforward way by gel permeation chromatography analysis via a reduction/loss of signal and/or smaller retention times. Results of this work, thus, shift the paradigm of research in the field toward designing a new generation of organic absorbers with enhanced intrinsic photochemical stability in order to reach practically useful operation lifetimes required for successful commercialization of organic photovoltaics. 相似文献
55.
Pavel Pipek Tim M. Blackburn Steven Delean Phillip Cassey aan H. ekerciolu Petr Pyek 《Ecology and evolution》2020,10(9):3944-3953
Invasive alien species are a major threat to biodiversity and human activities, providing a strong incentive to understand the processes by which alien invasion occurs. While it is important to understand the determinants of success at each of several invasion stages—transport, introduction, establishment, and spread—few studies have explored the first of these stages. Here, we quantify and analyze variation in the success of individual animals in surviving the transport stage, based on shipping records of European passerines destined for New Zealand. We mined the original documents of Acclimatisation Societies, established in New Zealand for the purpose of introducing supposedly beneficial alien species, in combination with recently digitized newspaper archives, to produce a unique dataset of 122 ships that carried passerines from Europe to New Zealand between 1850 and 1885. For 37 of these shipments, data on the survival of individual species were available. Using generalized linear mixed models, we explored how survival was related to characteristics of the shipments and the species. We show that species differed greatly in their survival, but none of the tested traits accounted for these differences. Yet, survival increased over time, which mirrors the switch from early haphazard shipments to larger organized shipments. Our results imply that it was the quality of care received by the birds that most affected success at this stage of the invasion process. 相似文献
56.
Magdalena Gajdoov Oldich Sychra Jakub Kreisinger Ondej Sedl
ek Eric Djomo Nana Tom Albrecht Pavel Munclinger 《Ecology and evolution》2020,10(13):6512-6524
Coevolutionary processes that drive the patterns of host–parasite associations can be deduced through congruence analysis of their phylogenies. Feather lice and their avian hosts have previously been used as typical model systems for congruence analysis; however, such analyses are strongly biased toward nonpasserine hosts in the temperate zone. Further, in the Afrotropical region especially, cospeciation studies of lice and birds are entirely missing. This work supplements knowledge of host–parasite associations in lice using cospeciation analysis of feather lice (genus Myrsidea and the Brueelia complex) and their avian hosts in the tropical rainforests of Cameroon. Our analysis revealed a limited number of cospeciation events in both parasite groups. The parasite–host associations in both louse groups were predominantly shaped by host switching. Despite a general dissimilarity in phylogeny for the parasites and hosts, we found significant congruence in host–parasite distance matrices, mainly driven by associations between Brueelia lice and passerine species of the Waxbill (Estrildidae) family, and Myrsidea lice and their Bulbul (Pycnonotidae) host species. As such, our study supports the importance of complex biotic interactions in tropical environments. 相似文献
57.
58.
59.
Falcão Fábio Dodonov Pavel Caselli Christini B. dos Santos Juliana Silveira Faria Deborah 《Biodiversity and Conservation》2021,30(8-9):2545-2564
Biodiversity and Conservation - Tropical forests are being lost and modified at an unprecedented rate, with extant biodiversity increasingly restricted to human-modified landscapes. Resulting... 相似文献
60.
Zuzana Chumová Eliška Záveská Petra Hloušková Jan Ponert Philipp-André Schmidt Martin Čertner Terezie Mandáková Pavel Trávníček 《The Plant journal : for cell and molecular biology》2021,107(2):511-524
Although the evolutionary drivers of genome size change are known, the general patterns and mechanisms of plant genome size evolution are yet to be established. Here we aim to assess the relative importance of proliferation of repetitive DNA, chromosomal variation (including polyploidy), and the type of endoreplication for genome size evolution of the Pleurothallidinae, the most species-rich orchid lineage. Phylogenetic relationships between 341 Pleurothallidinae representatives were refined using a target enrichment hybrid capture combined with high-throughput sequencing approach. Genome size and the type of endoreplication were assessed using flow cytometry supplemented with karyological analysis and low-coverage Illumina sequencing for repeatome analysis on a subset of samples. Data were analyzed using phylogeny-based models. Genome size diversity (0.2–5.1 Gbp) was mostly independent of profound chromosome count variation (2n = 12–90) but tightly linked with the overall content of repetitive DNA elements. Species with partial endoreplication (PE) had significantly greater genome sizes, and genomic repeat content was tightly correlated with the size of the non-endoreplicated part of the genome. In PE species, repetitive DNA is preferentially accumulated in the non-endoreplicated parts of their genomes. Our results demonstrate that proliferation of repetitive DNA elements and PE together shape the patterns of genome size diversity in orchids. 相似文献