全文获取类型
收费全文 | 52677篇 |
免费 | 4534篇 |
国内免费 | 34篇 |
专业分类
57245篇 |
出版年
2023年 | 237篇 |
2022年 | 355篇 |
2021年 | 722篇 |
2020年 | 501篇 |
2019年 | 578篇 |
2018年 | 895篇 |
2017年 | 810篇 |
2016年 | 1414篇 |
2015年 | 2286篇 |
2014年 | 2381篇 |
2013年 | 3087篇 |
2012年 | 3857篇 |
2011年 | 3787篇 |
2010年 | 2438篇 |
2009年 | 2008篇 |
2008年 | 3091篇 |
2007年 | 3121篇 |
2006年 | 2844篇 |
2005年 | 2830篇 |
2004年 | 2783篇 |
2003年 | 2582篇 |
2002年 | 2534篇 |
2001年 | 610篇 |
2000年 | 496篇 |
1999年 | 571篇 |
1998年 | 665篇 |
1997年 | 500篇 |
1996年 | 453篇 |
1995年 | 443篇 |
1994年 | 391篇 |
1993年 | 382篇 |
1992年 | 433篇 |
1991年 | 377篇 |
1990年 | 306篇 |
1989年 | 300篇 |
1988年 | 306篇 |
1987年 | 269篇 |
1986年 | 260篇 |
1985年 | 343篇 |
1984年 | 360篇 |
1983年 | 305篇 |
1982年 | 387篇 |
1981年 | 304篇 |
1980年 | 278篇 |
1979年 | 217篇 |
1978年 | 244篇 |
1977年 | 228篇 |
1976年 | 219篇 |
1974年 | 207篇 |
1973年 | 217篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
82.
83.
84.
Body size may be more important than species identity in determining species interactions and community structure. However, co‐occurrence of organisms has commonly been analysed from a taxonomic perspective and the body size is rarely taken into account. On six sampling occasions, we analysed patterns of killifish co‐occurrences in nestedness (tendency for less rich communities to be subsamples of the richest), checkerboard structure (tendency for species segregation), and modularity (tendency for groups to co‐occur more frequently than random expectation) in a pond metacommunity located in Uruguay. We contrasted co‐occurrence patterns among species and body size‐classes (individuals from different species were combined into size categories). The analysis was performed at two spatial scales: ponds (communities) and sample units within ponds. Observed nestedness was frequently smaller than the null expectation, with significantly greater deviations for body size‐classes than for species, and for sample units than for communities. At the sample unit level, individuals tended to segregate (i.e. clump into a checkerboard pattern) to a larger extent by body size rather than by taxonomy. Modularity was rarely detected, but nevertheless indicated a level of taxonomic organization not evident in nestedness or checkerboard indices. Identification of the spatial scale and organization at which ecological forces determine community structure is a basic requirement for advancement of robust theory. In our study system, these ecological forces probably structured the community by body sizes of interacting organisms rather than by species identities. 相似文献
85.
86.
Azospirillum strains isolated from the roots and rhizosphere of some plants growing in West Bengal were subjected to qualitative and quantitative
evaluation for poly-3-hydroxybutyrate (PHB) production. Out of the total 49 isolates, 13 (26%) were confirmed as PHB producers
according to staining and chemical assay methods. The majority of these strains belonged toAzospirillum brasilense butA. amazonense andA. lipoferum were also present. When grown in the presence of NH4Cl in the medium, the PHB content of the strains ranged from 1 to 14% of cell dry mass. The identity of the PHB extracted
fromAzospirillum strain 24P-N-72 was confirmed by the characteristic UV and IR absorption peaks at 235 nm and 1730 cm−1, respectively. 相似文献
87.
R A Seder M Plaut S Barbieri J Urban F D Finkelman W E Paul 《Journal of immunology (Baltimore, Md. : 1950)》1991,147(3):903-909
Non-B, non-T cells from spleen and bone marrow cells produce IL-4 in response to cross-linkage of high affinity receptors for Fc epsilon R or Fc gamma RII, and to treatment with calcium ionophores. Cells bearing high affinity Fc epsilon R constituted 1 to 2% of non-B, non-T cells of spleen and of total bone marrow cells from naive donors. In mice whose immune systems had been polyclonally activated by injection with anti-IgD antibodies or had been infected with Nippostrongylus brasiliensis larvae, the frequency of Fc epsilon R+ cells in splenic non-B, non-T cells was also 1 to 2% but in bone marrow from anti-IgD-injected mice donors the frequency was approximately 5%. Cell sorting experiments revealed that all of the capacity to produce IL-4 in response to immobilized IgE or IgG2a or to ionomycin was found in the Fc epsilon R+ fraction. Among the Fc epsilon R+ spleen cells from naive donors, the frequency of IL-4-producing cells was 1/20 to 1/40 whereas in mice that had been injected with anti-IgD or infected with N. brasiliensis, the frequency of IL-4 producing cells in the Fc epsilon R+ population was approximately 1/5. 相似文献
88.
Sabrina Locatelli Kurt A. McKean Paul R. Sesink Clee Mary Katherine Gonder 《International journal of primatology》2014,35(2):349-375
Examining how pathogens cross species boundaries, spread within species, and persist within their hosts is an essential part of understanding the factors that underpin the evolution of virulence and host resistance. Here, we review current knowledge about the genetic diversity, molecular epidemiology, prevalence, and pathogenicity of simian immunodeficiency viruses (SIVs). SIVs have crossed species boundaries from simian hosts to humans on at least 12 separate occasions, one of which led to the global HIV–AIDS crisis. Though SIVs infect a wide range of primates, scientists have only recently begun to describe the natural history of SIV infection in their natural hosts. Several new studies reveal how both viral and host factors are responsible for the transmission to, and adaptation in, new hosts. These studies also suggest that the spread of the virus may be affected by host-specific traits, including social structure, mating system and demographic history. These studies challenge the traditional view that SIV is relatively benign in its natural host, and instead suggest that a highly dynamic relationship exists between SIV and its simian hosts. 相似文献
89.
Moisés Rodríguez-Ma?ero Paul Schurmann Miguel Valderrábano 《Indian pacing and electrophysiology journal》2015,15(6):303-304
A perceived distinctive feature of cryoablation is the stability (cryoadherence) of the catheter tip during cold temperatures at the desired location, even during tachycardia. We report the case report of a young patient with a parahisian accessory pathway where stability of the ablation catheter was not achieved despite using the cryocatheter with a steerable sheath. Ultimately, stability at the desired location was achieved robotically by means of Hansen system (Hansen Medical, Mountain View, CA, USA). 相似文献
90.
Lisa Rizzetto Gloria Giovannini Michael Bromley Paul Bowyer Luigina Romani Duccio Cavalieri 《PloS one》2013,8(2)
For over a century microbiologists and immunologist have categorized microorganisms as pathogenic or non-pathogenic species or genera. This definition, clearly relevant at the strain and species level for most bacteria, where differences in virulence between strains of a particular species are well known, has never been probed at the strain level in fungal species. Here, we tested the immune reactivity and the pathogenic potential of a collection of strains from Aspergillus spp, a fungus that is generally considered pathogenic in immuno-compromised hosts. Our results show a wide strain-dependent variation of the immune response elicited indicating that different isolates possess diverse virulence and infectivity. Thus, the definition of markers of inflammation or pathogenicity cannot be generalized. The profound understanding of the molecular mechanisms subtending the different immune responses will result solely from the comparative study of strains with extremely diverse properties. 相似文献