首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1582篇
  免费   116篇
  国内免费   33篇
  2024年   5篇
  2023年   28篇
  2022年   37篇
  2021年   49篇
  2020年   54篇
  2019年   68篇
  2018年   61篇
  2017年   52篇
  2016年   46篇
  2015年   42篇
  2014年   66篇
  2013年   137篇
  2012年   50篇
  2011年   68篇
  2010年   62篇
  2009年   61篇
  2008年   80篇
  2007年   83篇
  2006年   72篇
  2005年   65篇
  2004年   64篇
  2003年   51篇
  2002年   48篇
  2001年   48篇
  2000年   35篇
  1999年   26篇
  1998年   28篇
  1997年   21篇
  1996年   27篇
  1995年   13篇
  1994年   22篇
  1993年   13篇
  1992年   16篇
  1991年   9篇
  1990年   12篇
  1989年   7篇
  1988年   10篇
  1987年   11篇
  1986年   14篇
  1985年   11篇
  1984年   10篇
  1983年   4篇
  1982年   9篇
  1981年   5篇
  1980年   5篇
  1979年   5篇
  1978年   4篇
  1975年   5篇
  1974年   5篇
  1973年   2篇
排序方式: 共有1731条查询结果,搜索用时 15 毫秒
991.
Candelabrum gorgonian corals (genus Eunicea Lamouroux, 1816 ), with 15 valid species, comprise the most diverse and abundant group of octocorals in Caribbean coral reefs. The systematics of Eunicea was estimated based on 17 discrete morphological characters, mostly from sclerite morphology. Club sclerites, found on the surface of gorgonian tissue, provided the largest number of informative characters. A maximum parsimony analysis corroborated the monophyletic status of the group and the two internal groups (subgenera Eunicea s.s. and Euniceopsis Verrill). Differentiation based on the distribution and architecture of modules (e.g. polyps and calyces), as well as colony size and fecundity in females, was consistent amongst sister species (e.g. the new species and their closest morphological sister species) but not within species [e.g. amongst Eunicea succinea (Pallas) morphotypes], suggesting species boundaries for this group. Candelabrum gorgonian corals seem to be distributed indistinctively throughout the Caribbean province, where habitat requirements are met. One new species widely distributed in the Caribbean was described ( Eunicea tayrona sp. nov. ) using electron microscopy. Eunicea tayrona is externally similar to Eunicea fusca but there are numerous morphological and ecological differences between them. The morphotypes and phenotypic plasticity from Eunicea clavigera Bayer, Eunicea calyculata (Ellis & Solander), and Eunicea tourneforti Milne Edwards & Haime still deserve further study and might clarify the presence of additional new Eunicea species.  相似文献   
992.
993.
The primary source of the annual austral spring mid‐tropospheric ozone maxima over the tropical South Atlantic has generally been assumed to be biomass burning. However, ozone precursor emissions from biogenic, lightning, and anthropogenic sources in subequatorial Africa before and during the ozone peak are shown to be comparable, if not greater, in magnitude to regional biomass burning production. Moreover, an investigation of the spatial and temporal characteristics of these ozone precursor sources (i.e. vegetative and microbial activity, lightning‐induced generation, and anthropogenic emissions) suggests that these alternative sources can potentially make a substantial contribution to the seasonal ozone peak. This argument is supported by the practical limitations of atmospheric transport available to regionally produced ozone and ozone precursors.  相似文献   
994.
995.

Background

Despite evidence of an association between variants at the apolipoprotein L1 gene (APOL1) locus and a spectrum of related kidney diseases, underlying biological mechanisms remain unknown. An earlier preliminary study published by our group showed that an APOL1 variant (rs73885319) modified the association between high-density lipoprotein cholesterol (HDLC) and estimated glomerular filtration rate (eGFR) in African Americans. To further understand this relationship, we evaluated the interaction in two additional large cohorts of African Americans for a total of 3,592 unrelated individuals from the Howard University Family Study (HUFS), the Natural History of APOL1-Associated Nephropathy Study (NHAAN), and the Atherosclerosis Risk in Communities Study (ARIC). The association between HDLC and eGFR was determined using linear mixed models, and the interaction between rs73885319 genotype and HDLC was evaluated using a multiplicative term.

Results

Among individuals homozygous for the risk genotype, a strong inverse HDLC-eGFR association was observed, with a positive association in others (p for the interaction of the rs73885319 × HDLC =0.0001). The interaction was similar in HUFS and NHAAN, and attenuated in ARIC. Given that ARIC participants were older, we investigated an age effect; age was a significant modifier of the observed interaction. When older individuals were excluded, the interaction in ARIC was similar to that in the other studies.

Conclusions

Based on these findings, it is clear that the relationship between HDLC and eGFR is strongly influenced by the APOL1 rs73885319 kidney risk genotype. Moreover, the degree to which this variant modifies the association may depend on the age of the individual. More detailed physiological studies are warranted to understand how rs73885319 may affect the relationship between HDLC and eGFR in individuals with and without disease and across the lifespan.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1645-7) contains supplementary material, which is available to authorized users.  相似文献   
996.
997.
998.
African swine fever virus(ASFV) infects domestic pigs and European wild boars with strong, hemorrhagic and high mortality. The primary cellular targets of ASFV is the porcine macrophages. Up to now, no commercial vaccine or effective treatment available to control the disease. In this study, three recombinant Saccharomyces cerevisiae(S. cerevisiae) strains expressing fused ASFV proteins-porcine Ig heavy chains were constructed and the immunogenicity of the S. cerevisiae-vectored cocktail ASFV feeding vaccine was further evaluated. To be specific, the P30-Fcc and P54-Fca fusion proteins displaying on surface of S. cerevisiae cells were produced by fusing the Fc fragment of porcine immunoglobulin Ig G1 or IgA1 with p30 or p54 gene of ASFV respectively. The recombinant P30-Fcc and P54-Fca fusion proteins expressed by S. cerevisiae were verified by Western blotting, flow cytometry and immunofluorescence assay.Porcine immunoglobulin Fc fragment fused P30/P54 proteins elicited P30/P54-specific antibody production and induced higher mucosal immunity in swine. The absorption and phagocytosis of recombinant S. cerevisiae strains in IPEC-J2 cells or porcine alveolar macrophage(PAM) cells were significantly enhanced, too. Here, we introduce a kind of cheap and safe oral S. cerevisiae-vectored vaccine, which could activate the specific mucosal immunity for controlling ASFV infection.  相似文献   
999.
Isotopic values of two Caribbean sharpnose shark Rhizoprionodon porosus litters (Poey, 1861) with two and three embryos and one litter of 11 smalltail shark Carcharhinus porosus embryos showed enriched 15N and 13C compared to their mothers. In R. porosus, embryonic isotope values were 3.06 ± 0.07‰ and 0.69 ± 0.15‰ greater than their mothers' for δ15N and δ13C, respectively, whereas in C. porosus, δ15N and δ13C were 1.79 ± 0.09‰ and 1.31 ± 0.17‰ greater in embryos than their mothers.  相似文献   
1000.
The relationship between Homo habilis and early African Homo erectus has been contentious because H. habilis was hypothesized to be an evolutionary stage between Australopithecus and H. erectus, more than a half‐century ago. Recent work re‐dating key African early Homo localities and the discovery of new fossils in East Africa and Georgia provide the opportunity for a productive re‐evaluation of this topic. Here, we test the hypothesis that the cranial sample from East Africa and Georgia represents a single evolutionary lineage of Homo spanning the approximately 1.9–1.5 Mya time period, consisting of specimens attributed to H. habilis and H. erectus. To address issues of small sample sizes in each time period, and uneven representation of cranial data, we developed a novel nonparametric randomization technique based on the variance in an index of pairwise difference from a broad set of fossil comparisons. We fail to reject the hypothesis of a single lineage this period by identifying a strong, time‐dependent pattern of variation throughout the sequence. These results suggest the need for a reappraisal of fossil evidence from other regions within this time period and highlight the critical nature of the Plio‐Pleistocene boundary for understanding the early evolution of the genus Homo.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号