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   78600篇
  免费   19274篇
  国内免费   6851篇
  104725篇
  2024年   194篇
  2023年   832篇
  2022年   1888篇
  2021年   3266篇
  2020年   3977篇
  2019年   5988篇
  2018年   5866篇
  2017年   5631篇
  2016年   6208篇
  2015年   7216篇
  2014年   7733篇
  2013年   8236篇
  2012年   6987篇
  2011年   6228篇
  2010年   5906篇
  2009年   4497篇
  2008年   3866篇
  2007年   3144篇
  2006年   2636篇
  2005年   2420篇
  2004年   2064篇
  2003年   1957篇
  2002年   1738篇
  2001年   1225篇
  2000年   976篇
  1999年   885篇
  1998年   528篇
  1997年   398篇
  1996年   366篇
  1995年   282篇
  1994年   267篇
  1993年   187篇
  1992年   203篇
  1991年   181篇
  1990年   153篇
  1989年   118篇
  1988年   99篇
  1987年   73篇
  1986年   67篇
  1985年   79篇
  1984年   26篇
  1983年   28篇
  1982年   22篇
  1981年   17篇
  1980年   7篇
  1979年   7篇
  1975年   5篇
  1969年   4篇
  1965年   4篇
  1950年   4篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
ABSTRACT Bacteria grow on avian eggshells and thus can potentially cause diseases in developing embryos. Little is known about culturable bacteria colonizing avian eggshells in free‐living birds, with most studies restricted to poultry. Our objective was to examine the culturable bacterial array growing on eggshells during incubation that could negatively affect hatching success of Pied Flycatchers (Ficedula hypoleuca) in a temperate montane habitat in central Spain. Cloacal culturable bacteria of females were also analyzed because bacteria can be vertically transmitted from females to eggs. We used fecal samples as surrogates of cloacal samples due to the small size of sampled birds. We found that eggshells and female cloacae of Pied Flycatchers harbored 24 and 40 bacterial families and species, respectively, but only a few in each clutch and each cloaca. Rod‐shaped gram‐negative bacteria and bacteria in the family Pseudomonadaceae were the most common bacteria on eggshells during early and late incubation and in female cloacae. Although based on small sample sizes, we found that females with rod‐shaped gram‐negative bacteria in their cloacae laid eggs that also had these bacteria, providing possible evidence for vertical transmission. We found no evidence for vertical transmission of Pseudomonadaceae, suggesting a possible environmental source for these bacteria. The prevalence of bacterial morphological types and major taxonomical categories on eggshells did not vary from early to late stages of incubation, providing support for the hypothesis that incubation may have bacteriostatic effects on bacterial proliferation on eggshells. Despite being primary egg invaders in poultry, we detected no effects of culturable Pseudomonadaceae or Pseudomonas luteola on hatching success. Our study represents the first to examine the culturable bacteria growing on the eggshells of a wild bird in a temperate habitat and additional studies based on culture‐independent techniques are required to confirm our results.  相似文献   
102.
Interleukin (IL)-33 is a recently described member of the IL-1 family and has been shown to induce production of T helper type 2 cytokines. In this study, an anti-IL-33 antibody was evaluated against pulmonary inflammation in mice sensitized and challenged with ovalbumin. The anti-IL-33 or a control antibody (150 μg/mouse) was given intraperitoneally as five doses before the sensitization and antigen challenge. Treatment with anti-IL-33 significantly reduced serum IgE secretion, the numbers of eosinophils and lymphocytes, and concentrations of IL-4, IL-5, and IL-13 in bronchoalveolar lavage fluid compared with administration of a control antibody. Histological examination of lung tissue demonstrated that anti-IL-33 significantly inhibited allergen-induced lung eosinophilic inflammation and mucus hypersecretion. Our data demonstrate for the first time that anti-IL-33 antibody can prevent the development of asthma in a mouse model and indicate that blockade of IL-33 may be a new therapeutic strategy for allergic asthma.  相似文献   
103.
104.
We present a cladistic analysis of the subtribe Megalostomina, a Neotropical group of ‘case‐bearer’ leaf beetles. A comparative study of the external and internal adult morphology of Clytrini was undertaken. New characters are described for the subtribe Megalostomina, from the internal sac of aedeagus, which provide a useful phylogenetic signal. More than 180 photographs illustrating the most important characters (74 characters and their respective states) used in the cladistic analysis are provided. The cladistic analysis of 57 terminal taxa and 95 characters was undertaken, under equal weights, and also using implied weights as a means to down‐weight homoplasious characters. We test the monophyly and explore intergeneric relationships of the subtribe Megalostomina, and reconstruct the relationships among the species of Megalostomis Chevrolat. The 42 species recognized can be assigned either to a group mostly containing species of North and Central America, or to a larger one of mostly South American species. Support is low, and the formal naming of groups is deferred pending a revision of all Megalostomina. We confirm the subgenera of Megalostomis of previous classifications are unnatural, and the following changes in the generic classification of the subtribe Megalostomina are proposed: Coleorozena Moldenke syn.n. of Coscinoptera Lacordaire; Coleothorpa Moldenke syn.n. of Coscinoptera Lacordaire; and Euryscopa (Coleoguerina) Moldenke syn.n. of Coscinoptera Lacordaire. Furthermore, six formerly recognized subgenera of Megalostomis are considered junior synonyms of Megalostomis Chevrolat: Megalostomis (Minturnia) Lacordaire syn.n. ; Megalostomis (Heterostomis) Lacordaire syn.n. ; Megalostomis (Scaphigenia) Lacordaire syn.n. ; Megalostomis (Snellingia) Moldenke syn.n. ; Megalostomis (Coleobyersa) Moldenke syn.n. ; and Megalostomis (Pygidiocarina) Moldenke syn.n. Thus, no subgenera are recognized within Megalostomis. Previous hypotheses on Clytrini biogeography were revisited in the light of new biogeographic and phylogenetic knowledge. We hypothesize an origin of Clytrini in tropical/subtropical Gondwana, when South America, Africa, Madagascar and India were connected. Changes in the configuration of the tectonic plates in the Cenozoic allowed the dispersal of Clytrina to the Palaearctic and Nearctic regions, and dispersion of Babiina and Megalostomina through the Nearctic region.  相似文献   
105.
106.
Apiomerini (Reduviidae: Harpactorinae) collect plant resins with their forelegs and use these sticky substances for prey capture or maternal care. These behaviors have not been described in detail and morphological structures involved in resin gathering, transfer, and storage remain virtually undocumented. We here describe these behaviors in Apiomerus flaviventris and document the involved structures. To place them in a comparative context, we describe and document leg and abdominal structures in 14 additional species of Apiomerini that represent all but one of the 12 recent genera in the tribe. Based on these morphological data in combination with the behavioral observations on A. flaviventris, we infer behavioral and functional hypotheses for the remaining genera within the tribe Apiomerini. Setal abdominal patches for resin storage are associated with maternal care so far only documented for species of Apiomerus. Based on the occurrence of these patches in several other genera, we propose that maternal care is widespread within the tribe. Ventral abdominal glands are widespread within female Apiomerini. We propose that their products may prevent hardening of stored resins thus providing long‐term supply for egg coating. Judging from the diverse setal types and arrangements on the front legs, we predict six different behavioral patterns of resin gathering within the tribe. J. Morphol., 2011. © 2010 Wiley‐Liss, Inc.  相似文献   
107.
Insulin-like growth factor 1 (IGF-1) stimulates α-secretase processing of amyloid precursor protein (APP) and decreases Aβ production. Little is known about the relationship between IGF-1 and β-site amyloid precursor protein cleaving enzyme 1 (BACE-1), the protease essential for the production of β-amyloid peptides (Aβ). Here, we investigated the effect of IGF-1 on BACE-1 in PC12 cells. Quantitative polymerase chain reaction analysis and western blot showed that treatment of cells with IGF-1 significantly decreased the levels of BACE-1 mRNA and protein. Furthermore, IGF-1 increased the phosphorylation of Akt and ERK1/2. The presence of the phosphatidylinositol 3-kinase (PI3-K) inhibitor LY294002 and the mitogen-activated protein kinase kinases (MEK) inhibitor PD98059 blocked the effect of IGF-1 on BACE-1. Our data indicated that IGF-1-induced reduction of BACE-1 might involve the PI3-K/Akt and MAPK/ERK1/2 signaling pathways.  相似文献   
108.
Integrating phylogenetic data into macroecological studies of biodiversity patterns may complement the information provided by present‐day spatial patterns. In the present study, we used range map data for all Geonoma (Arecaceae) species to assess whether Geonoma species composition forms spatially coherent floristic clusters. We then evaluated the extent to which the spatial variation in species composition reflects present‐day environmental variation vs. nonenvironmental spatial effects, as expected if the pattern reflects historical biogeography. We also examined the degree of geographic structure in the Geonoma phylogeny. Finally, we used a dated phylogeny to assess whether species richness within the floristic clusters was constrained by a specific historical biogeographic driver, namely time‐for‐diversification. A cluster analysis identified six spatially coherent floristic clusters, four of which were used to reveal a significant geographic phylogenetic structure. Variation partitioning analysis showed that 56 percent of the variation in species composition could be explained by spatial variables alone, consistent with historical factors having played a major role in generating the Geonoma diversity pattern. To test for a time‐for‐diversification effect, we correlated four different species richness measures with the diversification time of the earliest large lineage that is characteristic of each cluster. In support of this hypothesis, we found that geographic areas with higher richness contained older radiations. We conclude that current geographic diversity patterns in Geonoma reflect the present‐day climate, but to a larger extent are related to nonenvironmental spatial constraints linked to colonization time, dispersal limitation, and geological history, followed by within‐area evolutionary diversification. Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp .  相似文献   
109.
110.
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