全文获取类型
收费全文 | 4226篇 |
免费 | 281篇 |
国内免费 | 3篇 |
出版年
2023年 | 22篇 |
2022年 | 54篇 |
2021年 | 96篇 |
2020年 | 65篇 |
2019年 | 74篇 |
2018年 | 125篇 |
2017年 | 94篇 |
2016年 | 174篇 |
2015年 | 220篇 |
2014年 | 218篇 |
2013年 | 330篇 |
2012年 | 351篇 |
2011年 | 334篇 |
2010年 | 227篇 |
2009年 | 150篇 |
2008年 | 250篇 |
2007年 | 242篇 |
2006年 | 224篇 |
2005年 | 201篇 |
2004年 | 176篇 |
2003年 | 158篇 |
2002年 | 158篇 |
2001年 | 35篇 |
2000年 | 24篇 |
1999年 | 34篇 |
1998年 | 36篇 |
1997年 | 32篇 |
1996年 | 28篇 |
1995年 | 28篇 |
1994年 | 22篇 |
1993年 | 23篇 |
1992年 | 35篇 |
1991年 | 24篇 |
1990年 | 19篇 |
1989年 | 19篇 |
1988年 | 19篇 |
1987年 | 17篇 |
1986年 | 19篇 |
1985年 | 10篇 |
1984年 | 10篇 |
1983年 | 13篇 |
1982年 | 16篇 |
1981年 | 9篇 |
1980年 | 5篇 |
1979年 | 14篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1975年 | 12篇 |
1974年 | 8篇 |
1965年 | 4篇 |
排序方式: 共有4510条查询结果,搜索用时 15 毫秒
91.
Nematodes are very common in the deep sea and are an important component of deep-sea hydrothermal vent communities. In early 2006, the eruption of the underwater volcano at 9°50’N East Pacific Rise wiped out almost the entire faunal communities of the area. This provided us with the opportunity to study nematode primary succession at vents as well as on adjacent seafloor basalt. Nematode abundance and richness were extremely low at all studied sites in late 2006 and 2007, and increased only slightly in 2009. Interestingly, the most abundant species during early succession were also prominent in this area prior to the eruption. Our results show that nematodes are extremely influenced by volcanic eruptions and need a long period of time to colonize the lava-flooded area in greater numbers and richness. We hypothesize that low food availability on the young bare basalt and harsh environmental conditions at early succession vent sites might hinder a more successful nematode establishment. In addition to the newly established active vent sites we also studied an inactive vent site that was not directly hit by the eruption but whose vent fluid had ceased after the eruption. At this inactive and older vent, diversity was also relatively low but was higher than at the younger, newly established sites. In addition to the ecological analyses, we here describe the two most abundant species found at inactive vents, namely Neochromadora aff. poecilosoma De Mann 1893 and Linhomoeus caudipapillosus sp. n. 相似文献
92.
Jan Plue Pieter De Frenne Kamal Acharya Jrg Brunet Olivier Chabrerie Guillaume Decocq Martin Diekmann Bente J. Graae Thilo Heinken Martin Hermy Annette Kolb Isgard Lemke Jaan Liira Tobias Naaf Anna Shevtsova Kris Verheyen Monika Wulf Sara A. O. Cousins 《Global Ecology and Biogeography》2013,22(10):1106-1117
93.
94.
95.
Chris J. Brauer Peter J. Unmack Michael P. Hammer Mark Adams Luciano B. Beheregaray 《PloS one》2013,8(12)
Habitat fragmentation caused by human activities alters metapopulation dynamics and decreases biological connectivity through reduced migration and gene flow, leading to lowered levels of population genetic diversity and to local extinctions. The threatened Yarra pygmy perch, Nannoperca obscura, is a poor disperser found in small, isolated populations in wetlands and streams of southeastern Australia. Modifications to natural flow regimes in anthropogenically-impacted river systems have recently reduced the amount of habitat for this species and likely further limited its opportunity to disperse. We employed highly resolving microsatellite DNA markers to assess genetic variation, population structure and the spatial scale that dispersal takes place across the distribution of this freshwater fish and used this information to identify conservation units for management. The levels of genetic variation found for N. obscura are amongst the lowest reported for a fish species (mean heterozygosity of 0.318 and mean allelic richness of 1.92). We identified very strong population genetic structure, nil to little evidence of recent migration among demes and a minimum of 11 units for conservation management, hierarchically nested within four major genetic lineages. A combination of spatial analytical methods revealed hierarchical genetic structure corresponding with catchment boundaries and also demonstrated significant isolation by riverine distance. Our findings have implications for the national recovery plan of this species by demonstrating that N. obscura populations should be managed at a catchment level and highlighting the need to restore habitat and avoid further alteration of the natural hydrology. 相似文献
96.
Claudia Siegl Patricia Hamminger Herbert Jank Uwe Ahting Benedikt Bader Adrian Danek Allison Gregory Monika Hartig Susan Hayflick Andreas Hermann Holger Prokisch Esther M. Sammler Zuhal Yapici Rainer Prohaska Ulrich Salzer 《PloS one》2013,8(10)
Neuroacanthocytosis (NA) refers to a group of heterogenous, rare genetic disorders, namely chorea acanthocytosis (ChAc), McLeod syndrome (MLS), Huntington’s disease-like 2 (HDL2) and pantothenate kinase associated neurodegeneration (PKAN), that mainly affect the basal ganglia and are associated with similar neurological symptoms. PKAN is also assigned to a group of rare neurodegenerative diseases, known as NBIA (neurodegeneration with brain iron accumulation), associated with iron accumulation in the basal ganglia and progressive movement disorder. Acanthocytosis, the occurrence of misshaped erythrocytes with thorny protrusions, is frequently observed in ChAc and MLS patients but less prevalent in PKAN (about 10%) and HDL2 patients. The pathological factors that lead to the formation of the acanthocytic red blood cell shape are currently unknown. The aim of this study was to determine whether NA/NBIA acanthocytes differ in their functionality from normal erythrocytes. Several flow-cytometry-based assays were applied to test the physiological responses of the plasma membrane, namely drug-induced endocytosis, phosphatidylserine exposure and calcium uptake upon treatment with lysophosphatidic acid. ChAc red cell samples clearly showed a reduced response in drug-induced endovesiculation, lysophosphatidic acid-induced phosphatidylserine exposure, and calcium uptake. Impaired responses were also observed in acanthocyte-positive NBIA (PKAN) red cells but not in patient cells without shape abnormalities. These data suggest an “acanthocytic state” of the red cell where alterations in functional and interdependent membrane properties arise together with an acanthocytic cell shape. Further elucidation of the aberrant molecular mechanisms that cause this acanthocytic state may possibly help to evaluate the pathological pathways leading to neurodegeneration. 相似文献
97.
98.
Synaptic clustering on dendritic branches enhances plasticity, input integration and neuronal firing. However, the mechanisms guiding axons to cluster synapses at appropriate sites along dendritic branches are poorly understood. We searched for such a mechanism by investigating the structural overlap between dendritic branches and axons in a simplified model of neuronal networks - the hippocampal cell culture. Using newly developed software, we converted images of meshes of overlapping axonal and dendrites into topological maps of intersections, enabling quantitative study of overlapping neuritic geometry at the resolution of single dendritic branch-to-branch and axon-to-branch crossings. Among dendro-dendritic crossing configurations, it was revealed that the orientations through which dendritic branches cross is a regulated attribute. While crossing angle distribution among branches thinner than 1 µm appeared to be random, dendritic branches 1 µm or wider showed a preference for crossing each other at angle ranges of either 50°–70° or 80°–90°. It was then found that the dendro-dendritic crossings themselves, as well as their selective angles, both affected the path of axonal growth. Axons displayed 4 fold stronger tendency to traverse within 2 µm of dendro-dendritic intersections than at farther distances, probably to minimize wiring length. Moreover, almost 70% of the 50°–70° dendro-denritic crossings were traversed by axons from the obtuse angle’s zone, whereas only 15% traversed through the acute angle’s zone. By contrast, axons showed no orientation restriction when traversing 80°–90° crossings. When such traverse behavior was repeated by many axons, they converged in the vicinity of dendro-dendritic intersections, thereby clustering their synaptic connections. Thus, the vicinity of dendritic branch-to-branch crossings appears to be a regulated structure used by axons as a target for efficient wiring and as a preferred site for synaptic clustering. This synaptic clustering mechanism may enhance synaptic co-activity and plasticity. 相似文献
99.
The structure of a two-dimensional film formed by adsorbed polymer chains was studied by means of Monte Carlo simulations. The polymer chains were represented by linear sequences of lattice beads and positions of these beads were restricted to vertices of a two-dimensional square lattice. Two different Monte Carlo methods were employed to determine the properties of the model system. The first was the random sequential adsorption (RSA) and the second one was based on Monte Carlo simulations with a Verdier-Stockmayer sampling algorithm. The methodology concerning the determination of the percolation thresholds for an infinite chain system was discussed. The influence of the chain length on both thresholds was presented and discussed. It was shown that the RSA method gave considerably lower thresholds for longer chains. This behavior can be explained by a different pool of chain conformations used in the calculations in both methods under consideration. Figure
The percolation cluster (in red) in the system consisting of long flexible chains 相似文献
100.
Marcus Bär Joachim Klaer Lothar Weinhardt Regan G. Wilks Stefan Krause Monika Blum Wanli Yang Clemens Heske Hans‐Werner Schock 《Liver Transplantation》2013,3(6):777-781
The surface properties of CuInS2 (CIS) thin‐film solar cell absorbers are investigated by a combination of electron and soft X‐ray spectroscopies. Spatially separated regions of varying colors are observed and identified to be dominated by either CuS or Cu2S surface phases. After their removal by KCN etching, the samples cannot be distinguished by eye and the CIS surface is found to be Cu‐deficient in both regions. However, a significantly more pronounced off‐stoichiometry in the region initially covered by Cu2S can be identified. In this region, the resulting surface band gap is also significantly larger than the EgSurf of the initially CuS‐terminated region. Such variations may represent a hidden parameter which, if overlooked, induces irreproducibility and thus prevents systematic optimization efforts. 相似文献