全文获取类型
收费全文 | 23286篇 |
免费 | 2105篇 |
国内免费 | 10篇 |
出版年
2023年 | 113篇 |
2022年 | 299篇 |
2021年 | 547篇 |
2020年 | 315篇 |
2019年 | 381篇 |
2018年 | 507篇 |
2017年 | 453篇 |
2016年 | 745篇 |
2015年 | 1118篇 |
2014年 | 1223篇 |
2013年 | 1449篇 |
2012年 | 1834篇 |
2011年 | 1808篇 |
2010年 | 1143篇 |
2009年 | 903篇 |
2008年 | 1365篇 |
2007年 | 1252篇 |
2006年 | 1167篇 |
2005年 | 1108篇 |
2004年 | 1006篇 |
2003年 | 868篇 |
2002年 | 877篇 |
2001年 | 351篇 |
2000年 | 288篇 |
1999年 | 334篇 |
1998年 | 239篇 |
1997年 | 175篇 |
1996年 | 163篇 |
1995年 | 121篇 |
1994年 | 121篇 |
1993年 | 138篇 |
1992年 | 201篇 |
1991年 | 179篇 |
1990年 | 168篇 |
1989年 | 184篇 |
1988年 | 141篇 |
1987年 | 125篇 |
1986年 | 110篇 |
1985年 | 142篇 |
1984年 | 106篇 |
1983年 | 92篇 |
1982年 | 84篇 |
1981年 | 84篇 |
1980年 | 71篇 |
1979年 | 89篇 |
1978年 | 76篇 |
1977年 | 71篇 |
1974年 | 73篇 |
1973年 | 74篇 |
1969年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
The phylogeny of the fungus gnat family Mycetophilidae (Diptera) is reconstructed with a focus on the species‐rich and taxonomically difficult subfamilies Gnoristinae and Mycomyinae. The multigene phylogenetic analyses are based on five nuclear (18S, 28S, CAD, MCS, ITS2) and four mitochondrial (12S, 16S, COI, CytB) gene markers. The analyses strongly support the monophyly of Mycetophilidae and the subfamilies Manotinae, Sciophilinae, Leiinae, and Mycomyinae, although Gnoristinae is paraphyletic with respect to Mycetophilinae. All the genera and groups of genera included are supported as monophyletic, except for Acomoptera Vockeroth, Boletina Staeger, Dziedzickia Johannsen, Ectrepesthoneura Enderlein, and Neoempheria Osten Sacken. Ancestral character state reconstructions were applied to two morphological features present in Gnoristinae and Mycomyinae (i.e. presence of setae on wing membrane and wing vein R4) in order to assess their evolution. The wing vein R4 appears as an unstable character, spread throughout different clades. A dated phylogeny of the family Mycetophilidae showed that most of the subfamilies of Mycetophilidae originated and diversified during the Cretaceous. The youngest subfamilies, originated in the Paleogene, appear to be Mycomyinae and Mycetophilinae. 相似文献
2.
Andrea J Webster Andy Purvis 《Proceedings. Biological sciences / The Royal Society》2002,269(1487):143-149
Many methods are available for estimating ancestral values of continuous characteristics, but little is known about how well these methods perform. Here we compare six methods: linear parsimony, squared-change parsimony, one-parameter maximum likelihood (Brownian motion), two-parameter maximum likelihood (Ornstein-Uhlenbeck process), and independent comparisons with and without branch-length information. We apply these methods to data from 20 morphospecies of Pleistocene planktic Foraminifera in order to estimate ancestral size and shape variables, and compare these estimates with measurements on fossils close to the phylogenetic position of 13 ancestors. No method produced accurate estimates for any variable: estimates were consistently less good as predictors of the observed values than were the averages of the observed values. The two-parameter maximum-likelihood model consistently produces the most accurate size estimates overall. Estimation of ancestral sizes is confounded by an evolutionary trend towards increasing size. Shape showed no trend but was still estimated very poorly: we consider possible reasons. We discuss the implications of our results for the use of estimates of ancestral characteristics. 相似文献
3.
A method is described which allows a clear demonstration of capillaries and muscle fibers in the heart and skeletal muscle of experimental animals. The fluorescent dyes fluorescein isothiocyanate (FITC) and lissamine rhodamine B 200 (RB 200) were conjugated with a protein of high (gamma-globulin) and low (myoglobin) molecular weight, respectively, and were intravitally injected into the vascular system of rats. FITC globulin distributes itself in the intravasal space and RB 200 myoglobin in the extracellular. In histological sections the capillary lumina and the borderlines of the muscular fibers can be clearly identified and quantitatively evaluated because of the selective fluorescence in the respective structures. 相似文献
4.
5.
The aquatic larvae of the genus Chironomus (Diptera, Insecta) contain at least 12 different hemoglobin (Hb) variants in their hemolymph. In the present study we have analysed the structure and part of the nucleotide sequence of a Hb gene cluster cloned from the genomic DNA of Chironomus thummi piger. The cluster contains probably 6 different genes, separated by intergenic regions of various lengths. The nucleotide sequence of three putative Hb genes including the intergenic regions is presented. The inferred amino-acid sequences show clearly that two of these putative genes code for subvariants of the Hb variant VIIB. The third gene codes for a so far unknown Hb protein. As known already for other chironomid Hb genes, there are no intron sequences present in the coding regions. 相似文献
6.
Genome-wide analysis of the SET DOMAIN GROUP family in grapevine 总被引:1,自引:0,他引:1
The SET DOMAIN GROUP (SDG) proteins represent an evolutionarily-conserved family of epigenetic regulators present in eukaryotes
and are putative candidates for the catalysis of lysine methylation in histones. Plant genomes analyses of this family have
been performed in arabidopsis, maize, and rice and functional studies have shown that SDG genes are involved in the control
of plant development. In this work, we describe the identification and structural characterization of SDG genes in the Vitis vinifera genome. This analysis revealed the presence of 33 putative SDG genes that can be grouped into different classes, as it has
been previously described for plants. In addition to the SET domain, the proteins identified possessed other domains in the
different classes. As part of our study regarding the growth and development of grapevine, we selected eight genes and their
expression levels were analyzed in representative vegetative and reproductive organs of this species. The selected genes showed
different patterns of expression during inflorescence and fruit development, suggesting that they participate in these processes.
Furthermore, we showed that the expression of selected SDGs changes during viral infection, using as a model Grapevine Leafroll
Associated Virus 3-infected symptomatic grapevine leaves and fruits. Our results suggest that developmental changes caused
by this virus could be the result of alterations in SDG expression. 相似文献
7.
E Ortega Rincón J M Marchena J J García A Schmidt T Schulz I Malpica A B Rodríguez C Barriga H Michna H L?tzerich 《Journal of applied physiology》2001,91(3):1067-1072
Flow cytometer measurements were made of the basal variations in peripheral blood functional monocytes and granulocytes over the course of a training season (January to November) of a cycling team. Parallel determinations were made of plasma concentration of catecholamines (chromatography) and cortisol (RIA) in a search for neuroendocrine markers. The results showed the greatest phagocytic capacity to occur in the central months (March, May, and July), coinciding with the greatest number and highest level of competitive events with good correlation with a peak in epinephrine during these months (r(2) = 0.998 for monocytes and r(2) = 0.674 for granulocytes). No good correlations were found between phagocytosis and norepinephrine or cortisol. The highest values for phagocytosis and epinephrine concentration were found in May. These results suggest that blood epinephrine concentration could be a good neuroendocrine marker of sportspeople's phagocytic response. 相似文献
8.
Summary A male fetus of a pregnancy known to be at risk for X-linked mental retardation with fragile site Xq27 was found to be affected by demonstrating the marker X-chromosome in five of 180 (2.8%) of metaphases derived from amniocytes cultured in medium 199. The results were confirmed in fetal lymphocytes (25 of 86 metaphases, i.e. 29%), and fetal fibroblasts (five of 100 metaphases when cultured in medium 199, and 14 of 100 after exposure to methotrexate for 43 h).This work was supported in part by a research grant from the Deutsche Forschungsgemeinschaft 相似文献
9.
10.