全文获取类型
收费全文 | 24688篇 |
免费 | 2110篇 |
国内免费 | 13篇 |
出版年
2023年 | 93篇 |
2022年 | 219篇 |
2021年 | 420篇 |
2020年 | 257篇 |
2019年 | 343篇 |
2018年 | 425篇 |
2017年 | 383篇 |
2016年 | 604篇 |
2015年 | 1036篇 |
2014年 | 1205篇 |
2013年 | 1490篇 |
2012年 | 1720篇 |
2011年 | 1595篇 |
2010年 | 1052篇 |
2009年 | 994篇 |
2008年 | 1334篇 |
2007年 | 1255篇 |
2006年 | 1236篇 |
2005年 | 1219篇 |
2004年 | 1150篇 |
2003年 | 1020篇 |
2002年 | 1048篇 |
2001年 | 393篇 |
2000年 | 353篇 |
1999年 | 367篇 |
1998年 | 333篇 |
1997年 | 265篇 |
1996年 | 272篇 |
1995年 | 256篇 |
1994年 | 263篇 |
1993年 | 268篇 |
1992年 | 305篇 |
1991年 | 248篇 |
1990年 | 204篇 |
1989年 | 230篇 |
1988年 | 223篇 |
1987年 | 184篇 |
1986年 | 160篇 |
1985年 | 175篇 |
1984年 | 196篇 |
1983年 | 160篇 |
1982年 | 189篇 |
1981年 | 147篇 |
1980年 | 140篇 |
1979年 | 141篇 |
1978年 | 124篇 |
1977年 | 112篇 |
1976年 | 94篇 |
1975年 | 87篇 |
1973年 | 106篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
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
4.
Temporal variability of top-down forces and their role in host choice evolution of phytophagous arthropods 总被引:1,自引:0,他引:1
The role of top-down forces in host choice evolution of phytophagous arthropods is the subject of a vividly animated debate. Empirical evidence for the evolutionary role of top-down forces comes from studies showing that phytophagous arthropods prefer hosts that entail enemy-free space. The aim of this paper is to draw the attention of plant–arthropod researchers to the potentially, temporally variable nature of third trophic level effects. We show that this aspect is largely neglected in studies on enemy-free space, despite the fact that relative enemy impact varies seasonally among plants in at least some studies. We conclude that rigorous testing of the enemy-free space hypothesis can only be performed when within and between season variation in higher trophic level effects is taken into account. 相似文献
5.
Thomas J Hughes 《BMJ (Clinical research ed.)》1980,281(6239):562-563
6.
7.
8.
K M Jan 《Biorheology》1986,23(2):91-98
Red blood cell (RBC) aggregation in heparin-saline solution was quantified by microscopic observation. The adsorption isotherms of heparin onto normal and neuraminidase-treated RBC surfaces were determined by radioactive heparin labeled with 125I-Bolton-Hunter Reagent. RBC aggregation by heparin requires the presence of sialic acids at cell surface and was enhanced by reduction of ionic strength of the suspending medium. Adsorption of heparin onto RBC surface was increased by removal of sialic acids. These findings not only serve to elucidate the basic mechanism of cell-cell interaction mediated by negatively charged macromolecules, but also provide experimental evidence for the possible conformational change of macromolecules at the charged surface. 相似文献
9.
Avian hepatic T3 generation by 5'-monodeiodination: characterization of two enzymatic pathways and the effects of goitrogens 总被引:1,自引:0,他引:1
F M McNabb L J Lyons T E Hughes 《Comparative biochemistry and physiology. A, Comparative physiology》1986,85(2):249-255
The enzymatic nature of 5'-monodeiodination (5'-D) in avian liver homogenates was demonstrated by abolishment of activity by iopanoic acid (IOP). T3 production from T4 was dependent on enzyme and substrate concentrations, incubation time, incubation temperature, and pH. Two pathways of 5'-D activity were present in avian liver and exhibited characteristics similar to those described in mammalian tissues. Type II activity was identified as propylthiouracil (PTU)-insensitive activity. Type I (PTU-sensitive) was determined by difference between Total and Type II. Km values were 1.58 microM T4 for Total activity and 0.90 nM T4 for Type II, corresponding to the characteristics of the mammalian pathways. The effects of goitrogens on avian hepatic 5'-D were equivalent to those reported for the mammalian enzyme. 相似文献
10.
Responses in pigmentation and anti-oxidant expression in Arctic Daphnia along gradients of DOC and UV exposure 总被引:1,自引:0,他引:1
Responses in carapace melanization and expression of the majoranti-oxidant catalase (CAT) and glutathione transferase (GST)in Arctic Daphnia were assessed in enclosures along a gradientof dissolved organic carbon (DOC). This gradient was createdby adding freeze-dried humic matter to 2 m3 UV-transparent enclosures,yielding final nominal concentrations of 1, 2.5, 5 and 10 mgC l-1. The UV attenuation was strongly affected by additionsof DOC, and attenuation coefficients at 320 nm increased from3.0 in the control to approximately 3.5 and 11.0 m-1 in the1 and 10 mg DOC treatments respectively. Most Daphnia showedpronounced carapace melanization, and the absorbance of short-waveradiation through the carapace was strongly related to the degreeof melanization. Nevertheless, the different UV climate in theenclosures did not cause any short-term adaptation in Daphniapigmentation over a 3 week period. The levels of CAT and GSTwere assessed over time in the control and at 10 mg DOC. Theseenzymes displayed opposite patterns, with somewhat lower activitiesof CAT at low DOC (control) relative to 10 mg DOC, while theopposite was found for GST. There was also a significant negativecorrelation between CAT and solar irradiation for GST in bothbags, while no effects were found for GST. 相似文献