全文获取类型
收费全文 | 150510篇 |
免费 | 5951篇 |
国内免费 | 904篇 |
专业分类
157365篇 |
出版年
2023年 | 243篇 |
2022年 | 505篇 |
2021年 | 1005篇 |
2020年 | 632篇 |
2019年 | 808篇 |
2018年 | 12620篇 |
2017年 | 11325篇 |
2016年 | 8787篇 |
2015年 | 3009篇 |
2014年 | 2971篇 |
2013年 | 3431篇 |
2012年 | 8327篇 |
2011年 | 16621篇 |
2010年 | 14313篇 |
2009年 | 10381篇 |
2008年 | 12923篇 |
2007年 | 14492篇 |
2006年 | 3497篇 |
2005年 | 3579篇 |
2004年 | 3966篇 |
2003年 | 3768篇 |
2002年 | 3453篇 |
2001年 | 1121篇 |
2000年 | 868篇 |
1999年 | 835篇 |
1998年 | 861篇 |
1997年 | 609篇 |
1996年 | 521篇 |
1995年 | 470篇 |
1994年 | 503篇 |
1993年 | 523篇 |
1992年 | 642篇 |
1991年 | 536篇 |
1990年 | 493篇 |
1989年 | 497篇 |
1988年 | 457篇 |
1987年 | 421篇 |
1986年 | 403篇 |
1985年 | 377篇 |
1984年 | 441篇 |
1983年 | 415篇 |
1982年 | 421篇 |
1981年 | 382篇 |
1980年 | 396篇 |
1979年 | 310篇 |
1978年 | 299篇 |
1977年 | 263篇 |
1976年 | 257篇 |
1972年 | 403篇 |
1971年 | 437篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Mei Li Zhong Wei Jianing Wang Alexandre Jousset Ville‐Petri Friman Yangchun Xu Qirong Shen Thomas Pommier 《Ecology letters》2019,22(1):149-158
While several studies have established a positive correlation between community diversity and invasion resistance, it is less clear how species interactions within resident communities shape this process. Here, we experimentally tested how antagonistic and facilitative pairwise interactions within resident model microbial communities predict invasion by the plant–pathogenic bacterium Ralstonia solanacearum. We found that facilitative resident community interactions promoted and antagonistic interactions suppressed invasions both in the lab and in the tomato plant rhizosphere. Crucially, pairwise interactions reliably explained observed invasion outcomes also in multispecies communities, and mechanistically, this was linked to direct inhibition of the invader by antagonistic communities (antibiosis), and to a lesser degree by resource competition between members of the resident community and the invader. Together, our findings suggest that the type and strength of pairwise interactions can reliably predict the outcome of invasions in more complex multispecies communities. 相似文献
992.
Jenny Anne Glikman Jerry J. Vaske Alistair J. Bath Paolo Ciucci Luigi Boitani 《European Journal of Wildlife Research》2012,58(1):295-302
This article examines the combined influence of cognitions (i.e., impact beliefs) and affect (i.e., feelings) on normative
beliefs (i.e., support for management options) about wolves and brown bears. Data were obtained from stratified random face-to-face
interviews (n = 1,611). The survey was conducted in the Abruzzo Lazio and Molise National Park (central Italy), where people have a long
history of coexistence with large carnivores. Knowledge was hypothesized to moderate the relationships of beliefs and feelings
on support for management actions. Path analyses supported the role of affect in mediating perceived impact beliefs and support
for the protection of large carnivores. Knowledge moderated these relationships in the case of wolves but not brown bears.
Residents of the national park had more knowledge about bears than wolves, which might partly explain both the stronger effect
that knowledge had on the affective component and its lack of a moderating effect on the bear model. Overall, our findings
show the positive attitude of residents toward large carnivores and support the idea of affect being more important than cognition
in predicting normative beliefs. 相似文献
993.
Robo4 maintains vessel integrity and inhibits angiogenesis by interacting with UNC5B 总被引:1,自引:0,他引:1
Koch AW Mathivet T Larrivée B Tong RK Kowalski J Pibouin-Fragner L Bouvrée K Stawicki S Nicholes K Rathore N Scales SJ Luis E del Toro R Freitas C Bréant C Michaud A Corvol P Thomas JL Wu Y Peale F Watts RJ Tessier-Lavigne M Bagri A Eichmann A 《Developmental cell》2011,20(1):33-46
Robo4 is an endothelial cell-specific member of the Roundabout axon guidance receptor family. To identify Robo4 binding partners, we performed a protein-protein interaction screen with the Robo4 extracellular domain. We find that Robo4 specifically binds to UNC5B, a vascular Netrin receptor, revealing unexpected interactions between two endothelial guidance receptors. We show that Robo4 maintains vessel integrity by activating UNC5B, which inhibits signaling downstream of vascular endothelial growth factor (VEGF). Function-blocking monoclonal antibodies against Robo4 and UNC5B increase angiogenesis and disrupt vessel integrity. Soluble Robo4 protein inhibits VEGF-induced vessel permeability and rescues barrier defects in Robo4(-/-) mice, but not in mice treated with anti-UNC5B. Thus, Robo4-UNC5B signaling maintains vascular integrity by counteracting VEGF signaling in endothelial cells, identifying a novel function of guidance receptor interactions in the vasculature. 相似文献
994.
William J. Sutherland Rosalind Aveling Thomas M. Brooks Mick Clout Lynn V. Dicks Liz Fellman Erica Fleishman David W. Gibbons Brandon Keim Fiona Lickorish Kathryn A. Monk Diana Mortimer Lloyd S. Peck Jules Pretty Johan Rockström Jon Paul Rodríguez Rebecca K. Smith Mark D. Spalding Femke H. Tonneijck Andrew R. Watkinson 《Trends in ecology & evolution》2014
995.
Vanessa B. Fortes Júlio César Bicca-Marques 《International journal of primatology》2008,29(3):717-722
We located 4 brown howlers (1 adult male, 2 adult females, and 1 juvenile male) showing abnormally lighter pelage in 3 social
groups comprising 5, 6, and 9 individuals in a 20 ha-forest fragment in the State of Rio Grande do Sul, Brazil. Two additional
groups composed only of normally colored individuals also live in the fragment, which is isolated from nearby fragments by
267–1009 m. They were the only brown howlers with abnormal pelage color out of a total of 386 individuals belonging to 67
groups in 21 fragments in the 5876-ha study area. The isolation of the forest fragment, its high howler density (2.2 individuals⁄ha),
and large group size (8.8 ± 2.4 individuals) may decrease the likelihood of successful immigration into the population, leading
to an increased probability of inbreeding that may facilitate the expression of rare alleles. 相似文献
996.
Takashi Haramura 《Journal of Ethology》2007,25(1):3-7
Microhabitat selection is particularly important to increase the survival rate and reproductive success of animals inhabiting
heterogeneous environments. I investigated microhabitat selection of Buergeria
japonica tadpoles inhabiting the stream in a coastal area to reveal how animals select their appropriate habitat under heterogeneous
and unstable environments on the subtropical Okinawa Island of Japan. Tadpoles are sensitive to subtle environmental changes,
and the mouths of streams in coastal areas that have intense environmental fluctuations such as desiccation and sudden changes
in current velocity would be risky habitat for tadpoles. Tadpoles of B. japonica can inhabit both lotic and lentic systems. Field observations showed that, among six physical factors (water depth, water
temperature, salinity, pH, current velocity, and substrate), current velocity and water temperature were key factors in microhabitat
selection by tadpoles. It is likely that tadpoles stay at low current velocity sites to reduce the probability of being washed
out to the sea by a sudden squall and that selection of warmer sites would accelerate development of tadpoles so as to escape
the heterogeneous aquatic habitat sooner. 相似文献
997.
Random network models have been a popular tool for investigating cortical network dynamics. On the scale of roughly a cubic millimeter of cortex, containing about 100,000 neurons, cortical anatomy suggests a more realistic architecture. In this locally connected random network, the connection probability decreases in a Gaussian fashion with the distance between neurons. Here we present three main results from a simulation study of the activity dynamics in such networks. First, for a broad range of parameters these dynamics exhibit a stationary state of asynchronous network activity with irregular single-neuron spiking. This state can be used as a realistic model of ongoing network activity. Parametric dependence of this state and the nature of the network dynamics in other regimes are described. Second, a synchronous excitatory stimulus to a fraction of the neurons results in a strong activity response that easily dominates the network dynamics. And third, due to that activity response an embedding of a divergent-convergent feed-forward subnetwork (as in synfire chains) does not naturally lead to a stable propagation of synchronous activity in the subnetwork; this is in contrast to our earlier findings in isolated subnetworks of that type. Possible mechanisms for stabilizing the interplay of volleys of synchronous spikes and network dynamics by specific learning rules or generalizations of the subnetworks are discussed. 相似文献
998.
A significant macrophage and T-cell infiltrate commonly occurs in inflammatory joint conditions such as rheumatoid arthritis
that have significant bone destruction. Cytokines produced by activated macrophages and T cells are implicated in arthritis
pathogenesis and are involved in osteoclast-mediated bone resorption. The scope of the present review is to analyze current
knowledge and to provide a better understanding of how macrophage-derived factors promote the differentiation of a novel T-helper
subset (Th17) that promotes osteoclast formation and activation. 相似文献
999.
Histones are wrapped around by genomic DNA to form nucleosomes which are the basic units of chromatin. In eukaryotes histones undergo various covalent modifications such as methylation, phosphorylation, acetylation, ubiquitination and ribosylation. Histone modifications play a fundamental role in the epigenetic regulation of gene expression in multicellular eukaryotes. Histone methylation is one of the most important modifications occurring on Lysine (K) and Arginine (R) residues of histones, dynamically regulated by histone methyltransferases and demethylases. Identifications of such histone modification enzymes and to study how they work are the most fundamental questions needs to be answered. Uncovering the regulation and functions of the various histone methylation enzymes will help us to better understand the epigenetic code. This review summarizes the regulation of histone methyltransferases activity, the recruitment of methyltransferases and the distribution patterns and function of histone methylations. 相似文献
1000.
Independence of stem and leaf hydraulic traits in six Euphorbiaceae tree species with contrasting leaf phenology 总被引:1,自引:0,他引:1
Hydraulic traits and hydraulic-related structural properties were examined in three deciduous (Hevea brasiliensis, Macaranga denticulate, and Bischofia javanica) and three evergreen (Drypetes indica, Aleurites moluccana, and Codiaeum variegatum) Euphorbiaceae tree species from a seasonally tropical forest in south-western China. Xylem water potential at 50% loss of
stem hydraulic conductivity (P50stem) was more negative in the evergreen tree, but leaf water potential at 50% loss of leaf hydraulic conductivity (P50leaf) did not function as P50stem did. Furthermore, P50stem was more negative than P50leaf in the evergreen tree; contrarily, this pattern was not observed in the deciduous tree. Leaf hydraulic conductivity overlapped
considerably, but stem hydraulic conductivity diverged between the evergreen and deciduous tree. Correspondingly, structural
properties of leaves overlapped substantially; however, structural properties of stem diverged markedly. Consequently, leaf
and stem hydraulic traits were closely correlated with leaf and stem structural properties, respectively. Additionally, stem
hydraulic efficiency was significantly correlated with stem hydraulic resistance to embolism; nevertheless, such a hydraulic
pattern was not found in leaf hydraulics. Thus, these results suggest: (1) that the evergreen and deciduous tree mainly diverge
in stem hydraulics, but not in leaf hydraulics, (2) that regardless of leaf or stem, their hydraulic traits result primarily
from structural properties, and not from leaf phenology, (3) that leaves are more vulnerable to drought-induced embolism than
stem in the evergreen tree, but not always in the deciduous tree and (4) that there exists a trade-off between hydraulic efficiency
and safety for stem hydraulics, but not for leaf hydraulics. 相似文献