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   474篇
  免费   30篇
  504篇
  2023年   2篇
  2022年   10篇
  2021年   13篇
  2020年   12篇
  2019年   16篇
  2018年   19篇
  2017年   14篇
  2016年   17篇
  2015年   33篇
  2014年   30篇
  2013年   41篇
  2012年   49篇
  2011年   37篇
  2010年   27篇
  2009年   18篇
  2008年   24篇
  2007年   16篇
  2006年   17篇
  2005年   18篇
  2004年   16篇
  2003年   19篇
  2002年   12篇
  2001年   3篇
  2000年   1篇
  1999年   1篇
  1998年   6篇
  1997年   2篇
  1996年   1篇
  1993年   2篇
  1992年   1篇
  1990年   1篇
  1987年   1篇
  1986年   1篇
  1985年   4篇
  1984年   3篇
  1983年   2篇
  1981年   3篇
  1980年   1篇
  1979年   1篇
  1977年   1篇
  1975年   1篇
  1974年   1篇
  1971年   1篇
  1970年   1篇
  1969年   1篇
  1964年   2篇
  1963年   1篇
  1958年   1篇
排序方式: 共有504条查询结果,搜索用时 15 毫秒
471.
Potential Management of Chamaedorea seifrizii (Palmae), a Non-timber Forest Product from the Tropical Forest of Calakmul, Southeast Mexico. Leaves and seeds of Chamaedorea (xate) palms are important non-timber forest products (NTFPs). In the Calakmul region (Yucatan Peninsula) of Mexico, several communities have sporadically collected and sold seeds of C. seifrizii since 1980. However, harvesting has intensified recently, raising concerns about overexploitation. To evaluate the economic potential of leaf and seed exploitation in the area, we collected information on abundance, population patterns, and leaf and seed stocks in Ejido Conhuas, a community within the Calakmul Biosphere Reserve (CBR). Then we combined these data with current market values and hypothetical management regimes obtained from the literature for leaves and seeds. Conducting a quantitative analysis of 43 0.1ha plots with differences in forest and soil type, we assessed the abundance of C. seifrizzi in the area. We also conducted interviews to estimate the importance of xate in the local economy. We found C. seifrizii density to be highly variable, with a mean (±SE) of 295 (±35), with forest type being the most influential factor. Population structures differ between forest types, with healthy populations in medium and lower forest. We found a mean density of leaves of harvestable size of 3,750 (±380) leaves ha-1, while seed production was 1.5 (±0.3) kg/ha-1 of fresh seeds. Assuming sustainable harvest rates of 30–50% for leaves and 80% for seeds, one hectare of forest could generate USD 7.0–15.9/ha-1. Considering the number of households (102) and 10% of the total area managed each year (5,700 ha), this harvest could generate a household income of USD 391–838 annually. At the moment, xate trade represents a minor component in the economy of the community, but given the area’s extensive forest (>57,000 ha), the resource abundance, and the low human population, we believe the NTFPs derived from C. seifrizii have a potentially great economic impact in the area.  相似文献   
472.
Pelobates fuscus is a fossorial amphibian that inhabits much of the European plain areas. To unveil traces of expansion and contraction events of the species' range, we sequenced 702 bp of the mitochondrial cytochrome b gene. To infer the population history we applied phylogeographical methods, such as nested clade phylogeographical analysis (NCPA), and used summary statistics to analyse population structure under a neutral model of evolution. Populations were assigned to different drainage systems and we tested hypotheses of explicit refugial models using information from analysis of molecular variance, nucleotide diversity, effective population size estimation, NCPA, mismatch distribution and Bayesian dating. Coalescent simulations were used as post hoc tests for plausibility of derived or a priori assumed biogeographical hypotheses. Our combination of all approaches enabled the reconstruction of the colonization history and phylogeography of P. fuscus and confirmed a previous assumption of the existence of two major genetic lineages within P. fuscus. Using the Afro-European vicariance of Pelobates cultripes and Pelobates varaldii and applying Bayesian dating we estimated the divergence of these phylogeographical lineages to the Pliocene. We suggest the existence of three different glacial refugia: (i) the area between the Caspian and Black Seas as the origin for the expansion of the 'eastern lineage'; (ii) the Danube system as a centre of diversity for part of the 'western lineage'; (iii) the Po Valley, the largest centre of genetic variability. This fits the hypothesis that climatic fluctuation was a key event for differentiation processes in P. fuscus.  相似文献   
473.
Monitoring ecological restoration has been historically dependent on traditional inventory methods based on detailed information obtained from field plots. New paradigms are now needed to successfully achieve restoration as a large‐scale, long‐lasting transformative process. Fortunately, advances in technology now allow for unprecedented shifts in the way restoration has been planned, implemented, and monitored. Here, we describe our vision on how the use of new technologies by a new generation of restoration ecologists may revolutionize restoration monitoring in the coming years. The success of the many ambitious restoration programs planned for the coming decade will rely on effective monitoring, which is an essential component of adaptive management and accountability. The development of new remote sensing approaches and their application to a restoration context open new avenues for expanding our capacity to assess restoration performance over unprecedented spatial and temporal scales. A new generation of scientists, which have a background in remote sensing but are getting more and more involved with restoration, will certainly play a key role for making large‐scale restoration monitoring a viable human endeavor in the coming decade—the United Nations' decade on ecosystem restoration.  相似文献   
474.
475.
476.
477.
TNFα-matured dendritic cells (DCs) pulsed with tumor antigens are being evaluated as cancer vaccines. It has been shown that DCs produce IL12 during a limited time span and subsequently enter a stage of IL12 exhaustion. If DCs are generated ex vivo, the patient could receive IL12-exhausted DCs which may be detrimental for stimulating anti-tumor Th1 responses. Furthermore, many cancer patients exhibit a cytokine profile skewed toward IL10 and TGFβ. This immunological profile, called the Tr1/Th3 response, is associated with the presence of regulatory T-cells. Tr1/Th3 responses potently inhibit DC maturation, thereby regulating Th1 responses. In the present study, we produced genetically engineered DCs that continuously express Th1-related cytokines such as IL12, and resist negative signals from Tr1/Th3-dominated bladder carcinoma cells. Human immature DCs were genetically engineered by adenoviral vectors to express CD40L, or were treated with TNFα as a positive control for maturation. The expression of different Th1/Th3 and inflammatory cytokines was monitored. IL12 and IFNγ were expressed by CD40L-engineered DCs, while TNFα-matured DCs lacked IFNγ and exhibited low IL12 expression. The addition of recombinant IL10 to genetically engineered DCs did not abolish their Th1 profile. Likewise, coculture with tumor cell lines expressing TGFβ with or without recombinant IL10 did not revert to the engineered DCs. We further demonstrate that the resistance of CD40L-expressing DCs to TGFβ and IL10 may be due to decreased levels of TGFβ and IL10 receptors. Thus, CD40L-engineered DCs are robust Th1-promoting ones that are resistant to Tr1/Th3-signaling via IL10 and TGFβ.  相似文献   
478.
European Americans are often treated as a homogeneous group, but in fact form a structured population due to historical immigration of diverse source populations. Discerning the ancestry of European Americans genotyped in association studies is important in order to prevent false-positive or false-negative associations due to population stratification and to identify genetic variants whose contribution to disease risk differs across European ancestries. Here, we investigate empirical patterns of population structure in European Americans, analyzing 4,198 samples from four genome-wide association studies to show that components roughly corresponding to northwest European, southeast European, and Ashkenazi Jewish ancestry are the main sources of European American population structure. Building on this insight, we constructed a panel of 300 validated markers that are highly informative for distinguishing these ancestries. We demonstrate that this panel of markers can be used to correct for stratification in association studies that do not generate dense genotype data.  相似文献   
479.
The presence of Enterococcus faecalis in root canal teeth affected by primary and secondary periapical lesions was studied using polymerase chain reaction (PCR) assays. The association between presence of E. faecalis with clinical signs of apical lesions was assessed to evaluate a possible relationship between clinical findings. Microbial samples were obtained from healthy patients affected by different periapical lesions, 79 teeth with primary periapical lesion and 23 with secondary periapical lesion. For each tooth, clinical symptoms and X-ray appearance were examined. E. faecalis was detected in 6 of 79 samples with primary lesion (7.6%), and in 9 of 23 with secondary lesion (39.1%). Suggested association was found between E. faecalis and secondary apical lesions. As regard specific signs and symptoms E. faecalis was more associated with asymptomatic lesions (all p<0.05) than with symptomatic apical lesions. The study confirms the high presence of E. faecalis in secondary apical lesions. However, its effective role in endodontic pathogenesis such as bone periapical lesions needs to be clarified.  相似文献   
480.
Nanoscale secondary ion MS (NanoSIMS) imaging makes it possible to visualize stable isotope-labeled lipids in cells and tissues at 50 nm lateral resolution. Here we report the use of NanoSIMS imaging to visualize lipids in mouse cells and tissues. After administering stable isotope-labeled fatty acids to mice by gavage, NanoSIMS imaging allowed us to visualize neutral lipids in cytosolic lipid droplets in intestinal enterocytes, chylomicrons at the basolateral surface of enterocytes, and lipid droplets in cardiomyocytes and adipocytes. After an injection of stable isotope-enriched triglyceride-rich lipoproteins (TRLs), NanoSIMS imaging documented delivery of lipids to cytosolic lipid droplets in parenchymal cells. Using a combination of backscattered electron (BSE) and NanoSIMS imaging, it was possible to correlate the chemical data provided by NanoSIMS with high-resolution BSE images of cell morphology. This combined imaging approach allowed us to visualize stable isotope-enriched TRLs along the luminal face of heart capillaries and the lipids within heart capillary endothelial cells. We also observed examples of TRLs within the subendothelial spaces of heart capillaries. NanoSIMS imaging provided evidence of defective transport of lipids from the plasma LPs to adipocytes and cardiomyocytes in mice deficient in glycosylphosphatidylinositol-anchored HDL binding protein 1.  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