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   372篇
  免费   30篇
  402篇
  2021年   7篇
  2019年   7篇
  2018年   4篇
  2016年   4篇
  2015年   15篇
  2014年   18篇
  2013年   16篇
  2012年   16篇
  2011年   16篇
  2010年   9篇
  2009年   11篇
  2008年   16篇
  2007年   13篇
  2006年   14篇
  2005年   17篇
  2004年   10篇
  2003年   10篇
  2002年   13篇
  2001年   12篇
  2000年   6篇
  1999年   6篇
  1998年   6篇
  1997年   5篇
  1996年   4篇
  1994年   4篇
  1993年   6篇
  1992年   4篇
  1991年   3篇
  1990年   7篇
  1989年   16篇
  1988年   6篇
  1987年   6篇
  1985年   6篇
  1984年   3篇
  1983年   4篇
  1982年   5篇
  1981年   3篇
  1980年   4篇
  1979年   5篇
  1976年   3篇
  1975年   4篇
  1973年   3篇
  1972年   3篇
  1970年   2篇
  1951年   2篇
  1938年   2篇
  1912年   2篇
  1911年   3篇
  1908年   3篇
  1906年   2篇
排序方式: 共有402条查询结果,搜索用时 0 毫秒
71.
Social networks of stakeholders are necessary to foster public support for classical biological control for nature. Drawing from recent scholarship in policy-relevant social science fields, this article describes two key concepts that can improve science communication strategies to support invasive species management and biocontrol: lay public risk perception, and public engagement with science. This article then recommends a fundamental communication strategy: construct public trust in invasive species control efforts using public engagement processes that link trustworthy messengers and appropriate messages with the public. It draws examples from biocontrol projects that used pathogens as the natural enemy of choice, but more broadly seeks to inform efforts to engage the public about the use of classical biocontrol agents in nature conservation efforts.  相似文献   
72.
73.

Background  

The influenza A virus is an important infectious cause of morbidity and mortality in humans and was responsible for 3 pandemics in the 20th century. As the replication of the influenza virus is based on its host's machinery, codon usage of its viral genes might be subject to host selection pressures, especially after interspecies transmission. A better understanding of viral evolution and host adaptive responses might help control this disease.  相似文献   
74.
A central region of the beta2 integrin subunit, RN (residues D300 to C459), was replaced by the equivalent sequences from beta1 and beta7 to give the chimeras beta2RN1 and beta2RN7. Whilst the former construct failed to form heterodimer at the cell surface with alphaL, the later of these could be expressed together with the alphaL subunit to form a variant LFA-1. Based on recent modelling work, the RN region consists of two parts, one is the C-terminal end of the putative A-domain (RB, residues D300 to A359), and the other the mid-region (BN, residues Y360 to C459). Chimeras exchanging the two component regions were made. Of the four resultant chimeras, only the beta2RB1 chimera failed to support LFA-1 expression. Thus the beta1 specific residues of this region affect the interaction with the alphaL subunit. Whereas the alphaL/beta2RB7 LFA-1 variant is wildtype like with respect to ICAM-1 adhesion, the alphaLbeta2BN1 and alphaLbeta2BN7, as well as the alphaLbeta2RN7, variants are more adhesive than the wildtype. These results suggest that an authentic beta2 mid-region is, in part, required for maintaining the LFA-1 in a resting state.  相似文献   
75.
76.
Saccharomyces cerevisiae is used to provide fundamental understanding of eukaryotic genetics, gene product function, and cellular biological processes. Saccharomyces Genome Database (SGD) has been supporting the yeast research community since 1993, serving as its de facto hub. Over the years, SGD has maintained the genetic nomenclature, chromosome maps, and functional annotation, and developed various tools and methods for analysis and curation of a variety of emerging data types. More recently, SGD and six other model organism focused knowledgebases have come together to create the Alliance of Genome Resources to develop sustainable genome information resources that promote and support the use of various model organisms to understand the genetic and genomic bases of human biology and disease. Here we describe recent activities at SGD, including the latest reference genome annotation update, the development of a curation system for mutant alleles, and new pages addressing homology across model organisms as well as the use of yeast to study human disease.  相似文献   
77.
[Purpose] To determine whether physical activity (PA), primarily the recommended 60 minutes of moderate-to-vigorous PA, is associated with gut bacterial microbiota in 10-year-old children.[Methods] The Block Physical Activity Screener, which provides minutes/day PA variables, was used to determine whether the child met the PA recommendations. 16S rRNA sequencing was performed on stool samples from the children to profile the composition of their gut bacterial microbiota. Differences in alpha diversity metrics (richness, Pielou’s evenness, and Faith’s phylogenetic diversity) by PA were determined using linear regression, whereas beta diversity (unweighted and weighted UniFrac) relationships were assessed using PERMANOVA. Taxon relative abundance differentials were determined using DESeq2.[Results] The analytic sample included 321 children with both PA and 16S rRNA sequencing data (mean age [SD] =10.2 [0.8] years; 54.2% male; 62.9% African American), where 189 (58.9%) met the PA recommendations. After adjusting for covariates, meeting the PA recommendations as well as minutes/day PA variables were not significantly associated with gut richness, evenness, or diversity (p ≥ 0.19). However, meeting the PA recommendations (weighted UniFrac R2 = 0.014, p = 0.001) was significantly associated with distinct gut bacterial composition. These compositional differences were partly characterized by increased abundance of Megamonas and Anaerovorax as well as specific Christensenellaceae_R-7_group taxa in children with higher PA.[Conclusion] Children who met the recommendations of PA had altered gut microbiota compositions. Whether this translates to a reduced risk of obesity or associated metabolic diseases is still unclear.  相似文献   
78.
We measured 10 characters of pollinaria, anthers, and alar fissures from the flowers of seven clones of common milkweed Asclepias syriaca L. to test whether significant differences existed among them. Strong among-clone differences existed for most of these characters. However, these differences were not correlated with each other, except for width of the alar fissure and width of the translator arm. Thus, these floral structures vary mostly in shape, rather than size per se. Few correlations among characters occurred at a within-clone level. Clones growing in the open and shade differed in half of the characters measured, but habitat did not substantially change the among-clone size patterns observed. The alar fissure–translator arm relationship might promote differential uptake of pollinia; however, A. syriaca does not exhibit a clear separation into donor and recipient clones.  相似文献   
79.
Summary Compound eyes of larval and first postlarval grass shrimp (Palaemonetes pugio Holthuis) were studied with light and electron microscopy following adaptation to darkness or bright light. Larvae have well-developed apposition eyes, including 3 main types of accessory screening and reflecting pigments and a fourth class of putatively reflective granules recently described in adult shrimps. Rhabdoms contain orthogonally layered microvilli, and by the last larval stage, 8 retinular cells. Ocular accessory pigments in both light- and dark-adapted larvae are distributed much like those of light-adapted adults, but the distal mass of reflecting pigment is concentrated dorsally in larvae and ventrally in adults. Since larvae swim upside-down, reflecting pigment is oriented downward in all developmental stages and may function for countershading. Light and dark adaptational migrations of all 3 major accessory pigments commence abruptly at metamorphosis to the first postlarva. Upon dark adaptation in postlarvae, superposition optics remain impossible because (1) distal screening pigment migrates only slightly, (2) no clear zone has developed, and (3) the crystalline cones remain circular in cross section. Nevertheless, a slight improvement in photon catch is expected due to extensive redistributions of reflecting pigment and retinular cell screening pigment granules.
  相似文献   
80.
Summary We compared the patch-choice performances of an ambush predator, the crab spider Misumena vatia (Thomisidae) hunting on common milkweed Asclepias syriaca (Asclepiadaceae) umbles, with two stochastic rule-of-thumb simulation models: one that employed a threshold giving-up time and one that assumed a fixed probability of moving. Adult female Misumena were placed on milkweed plants with three umbels, each with markedly different numbers of flower-seeking prey. Using a variety of visitation regimes derived from observed visitation patterns of insect prey, we found that decreases in among-umbel variance in visitation rates or increases in overall mean visitation rates reduced the clarity of the optimum (the difference in the yield obtained as foraging behavior changes), both locally and globally. Yield profiles from both models were extremely flat or jagged over a wide range of prey visitation regimes; thus, differences between optimal and next-best strategies differed only modestly over large parts of the foraging landscape. Although optimal yields from fixed probability simulations were one-third to one-half those obtained from threshold simulations, spiders appear to depart umbels in accordance with the fixed probability rule.  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