全文获取类型
收费全文 | 99322篇 |
免费 | 5028篇 |
国内免费 | 5125篇 |
出版年
2023年 | 1421篇 |
2022年 | 1590篇 |
2021年 | 3471篇 |
2020年 | 2313篇 |
2019年 | 2555篇 |
2018年 | 2273篇 |
2017年 | 1979篇 |
2016年 | 2681篇 |
2015年 | 4647篇 |
2014年 | 8054篇 |
2013年 | 8152篇 |
2012年 | 6093篇 |
2011年 | 6847篇 |
2010年 | 5072篇 |
2009年 | 5080篇 |
2008年 | 4941篇 |
2007年 | 5300篇 |
2006年 | 3777篇 |
2005年 | 3343篇 |
2004年 | 2477篇 |
2003年 | 2182篇 |
2002年 | 1936篇 |
2001年 | 1473篇 |
2000年 | 1321篇 |
1999年 | 1299篇 |
1998年 | 1095篇 |
1997年 | 942篇 |
1996年 | 946篇 |
1995年 | 1083篇 |
1994年 | 1021篇 |
1993年 | 983篇 |
1992年 | 901篇 |
1991年 | 851篇 |
1990年 | 707篇 |
1989年 | 720篇 |
1988年 | 656篇 |
1987年 | 578篇 |
1986年 | 492篇 |
1985年 | 785篇 |
1984年 | 1119篇 |
1983年 | 719篇 |
1982年 | 916篇 |
1981年 | 900篇 |
1980年 | 642篇 |
1979年 | 655篇 |
1978年 | 413篇 |
1977年 | 436篇 |
1976年 | 391篇 |
1974年 | 271篇 |
1973年 | 268篇 |
排序方式: 共有10000条查询结果,搜索用时 25 毫秒
61.
FRANÇOIS MUNOZ THIERRY PAILLER INGRID KOTTKE CÉDRIC GONNEAU MARC‐ANDRÉ SELOSSE 《Molecular ecology》2012,21(20):5098-5109
Characterizing the architecture of bipartite networks is increasingly used as a framework to study biotic interactions within their ecological context and to assess the extent to which evolutionary constraint shape them. Orchid mycorrhizal symbioses are particularly interesting as they are viewed as more beneficial for plants than for fungi, a situation expected to result in an asymmetry of biological constraint. This study addressed the architecture and phylogenetic constraint in these associations in tropical context. We identified a bipartite network including 73 orchid species and 95 taxonomic units of mycorrhizal fungi across the natural habitats of Reunion Island. Unlike some recent evidence for nestedness in mycorrhizal symbioses, we found a highly modular architecture that largely reflected an ecological barrier between epiphytic and terrestrial subnetworks. By testing for phylogenetic signal, the overall signal was stronger for both partners in the epiphytic subnetwork. Moreover, in the subnetwork of epiphytic angraecoid orchids, the signal in orchid phylogeny was stronger than the signal in fungal phylogeny. Epiphytic associations are therefore more conservative and may co‐evolve more than terrestrial ones. We suggest that such tighter phylogenetic specialization may have been driven by stressful life conditions in the epiphytic niches. In addition to paralleling recent insights into mycorrhizal networks, this study furthermore provides support for epiphytism as a major factor affecting ecological assemblage and evolutionary constraint in tropical mycorrhizal symbioses. 相似文献
62.
Bird specimens collected by 19th century explorer and ornithologist Prince Maximilian of Wied-Neuwied form one of the foundation collections of the American Museum of Natural History in New York. However, parts of his collection remained in Germany and came to the Museum Wiesbaden. Since Wied described numerous new species without designating types, some of these specimens might be type material. Here we present a catalog of the 30 Wiesbaden specimens associated with him and discuss their potential type status. We conclude that 17 individuals in 11 species are potential type specimens that should be considered in future taxonomic work. 相似文献
63.
The gorgoniid Eugorgia is exclusively an eastern Pacific genus. It has a wide geographic and bathymetric range of distribution, found from California to Perú and extends down to 65 m deep. Two new species are herein described. The morphological characters were analyzed and illustrated by light and scanning electron microscopy. Eugorgia beebei
sp. n. can be distinguished by its white, ascending, sparse colony growth. Eugorgia mutabilis
sp. n. can be distinguished by its white colony that changes color after collection, and the conspicuous sharp-crested disc sclerites. From a morphological point of view the new species are related to the daniana-group, the rubens-group and the siedenburgae-group of Eugorgia; their affiliations, and the proposal of a new group are discussed. These new species increases the number of species in the genus to 15, and contribute to the knowledge of the eastern Pacific octocoral biodiversity. 相似文献
64.
65.
Influence of ionic strength and pH on the interaction between high-affinity heparin and antithrombin
Birgitta Nordenman Ingemar Björk 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,672(3):227-238
Binding constants for the binding of high-affinity heparin to antithrombin at different ionic strengths were determined by fluorescence titrations and were also estimated from dissociation curves of the heparin-antithrombin complex. These curves were monitored by near-ultraviolet circular dichroism or fluorescence. The dependence of the binding constant on the activity of NaCl suggested that maximally 5–6 charged groups are directly involved in the interaction between the two macromolecules. Major pH-dependent changes of the interaction, as evident by changes of the spectroscopic properties of the complex between the molecules, were found to occur below pH 5.5 and above pH 8.5. The acid change, which was irreversible, was most likely caused by an irreversible conformational change of antithrombin. At alkaline pH, however, the gross conformation of antithrombin was stable up to pH 12, while the affinity of high-affinity heparin for antithrombin began to decrease markedly at pH 8.5. The dissociation curve, which was reversible, had a midpoint around pH 9.5. This is compatible with the loss of affinity being caused by either a local conformational change, by ionization of tyrosine or by titration of one or more amino groups. 相似文献
66.
Hitoshi Sawada Hideyoshi Yokosawa Shin-Ichi Ishii Motonori Hoshi 《Molecular reproduction and development》1982,5(3):291-301
The presence of a protease has been demonstrated in sperm of the solitary ascidian, Halocynthia roretzi, by using t-butyloxycarbonyl-L-Val-L-Pro-L-Arg-4-methylcoumaryl-7-amide (Boc-Val-Pro-Arg-MCA) and other arginyl or lysyl MCA derivatives as substrates. Several properties of the enzyme were investigated in a crude extract. The activity had a pH optimum near 8.0 and was enhanced by the addition of CaCl2. The Km value of 87μM was determined for Boc-Val-Pro-Arg-MCA under the optimal conditions. An apparent molecular weight was estimated to be 35,000 by gel filtration. The enzyme was inhibited with diisopropyl fluorophosphate, leupeptin, antipain, p-aminobenzamidine, Val-Pro-Arg-CH2Cl, and soybean trypsin inhibitor, but scarcely inhibited with chymostatin, elastatinal, p-chloromercuribenzoic acid, tosyl-Lys-CH2Cl, and tosyl-Phe-CH2Cl. Boc-Val-Pro-Arg-MCA, the most susceptible of the substrates examined, showed the most effective inhibition against fertilization of ascidian eggs. Thus, this enzyme in ascidian sperm extract has features closely similar to mammalian acrosin [EC 3.4.21.10], and we conclude that the enzyme is involved in fertilization as one of the lysins. 相似文献
67.
68.
69.
70.