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   556篇
  免费   37篇
  2024年   1篇
  2023年   5篇
  2022年   6篇
  2021年   18篇
  2020年   15篇
  2019年   18篇
  2018年   32篇
  2017年   21篇
  2016年   24篇
  2015年   32篇
  2014年   38篇
  2013年   47篇
  2012年   50篇
  2011年   38篇
  2010年   31篇
  2009年   25篇
  2008年   30篇
  2007年   19篇
  2006年   19篇
  2005年   16篇
  2004年   16篇
  2003年   15篇
  2002年   9篇
  2001年   10篇
  2000年   9篇
  1999年   11篇
  1998年   6篇
  1997年   7篇
  1996年   3篇
  1995年   3篇
  1994年   2篇
  1992年   1篇
  1990年   2篇
  1989年   2篇
  1988年   2篇
  1987年   1篇
  1986年   1篇
  1985年   1篇
  1983年   1篇
  1982年   1篇
  1980年   1篇
  1970年   1篇
  1969年   1篇
  1968年   1篇
  1967年   1篇
排序方式: 共有593条查询结果,搜索用时 78 毫秒
71.
The digestive system of P. interpunctella was characterized during its larval development to determination of carbohydrases using disaccharides (sucrose and maltose) and polysaccharides (starch and inulin) as substrate. At 6(th) instar larval, Invertase>alpha-amylase> maltase activities peaks were observed. Invertase was fractionated with acetone and isolated. The Invertase was 485.5 fold purified by Sephacryl S-200 and DEAE-Sephadex. Its kinetic parameters were K(m) of 6.6 mM, V(max) of 0.48, pH optimum of 5.5 and temperature optimum of 30 degrees C. This enzyme was activated by CaCl(2) and inhibited by EDTA. When analyzed by SDS-PAGE it showed one band of M(r) 34 kDa. The understanding of the digestive system of P. interpunctella could be a key step in the design of bioinsecticides.  相似文献   
72.
Here we focus on factor analysis from a best practices point of view, by investigating the factor structure of neuropsychological tests and using the results obtained to illustrate on choosing a reasonable solution. The sample (n=1051 individuals) was randomly divided into two groups: one for exploratory factor analysis (EFA) and principal component analysis (PCA), to investigate the number of factors underlying the neurocognitive variables; the second to test the “best fit” model via confirmatory factor analysis (CFA). For the exploratory step, three extraction (maximum likelihood, principal axis factoring and principal components) and two rotation (orthogonal and oblique) methods were used. The analysis methodology allowed exploring how different cognitive/psychological tests correlated/discriminated between dimensions, indicating that to capture latent structures in similar sample sizes and measures, with approximately normal data distribution, reflective models with oblimin rotation might prove the most adequate.  相似文献   
73.
The sexual development of Atlantoraja platana from the south-west Atlantic Ocean was analysed. According to the reproductive variables considered for both sexes, males were found to mature at 620 mm and females at 715 mm total length.  相似文献   
74.
Most of the hematophagous insects act as disease vectors, including Aedes aegypti, responsible for transmitting some of the most critical arboviruses globally, such as Dengue. The use of repellents based on natural products is a promising alternative for personal protection compared to industrial chemical repellents. In this study, the repellent effect of essential oils extracted from Lippia thymoides, Lippia alba, Cymbopogon winterianus, and Eucalyptus globulus leaves was evaluated. Essential oils used showed repellent activity against Ae. aegypti in laboratory bioassays, obtaining protection rates above 70 % from 3.75 mg/mL and higher concentration for all analyzed oils. GC/MS identified 57 constituents, which were used in the ligand-based pharmacophore model to expose compounds with requirements for repellents that modulate mosquitoes behavior through odorant-binding protein 1 Ae. aegypti. Ligand-based pharmacophore model approach results suggested that repellent activity from C. winterianus, L. alba, and L. thymoides essential oils’ metabolites is related to Citronelal (QFIT=26.77), Citronelol (QFIT=11.29), Citronelol acetate (QFIT=52.22) and Geranil acetate (QFIT=10.28) with synergistic or individual activity. E. globulus essential oil's repellent activity is associated with Ledol (0.94 %; QFIT=41.95). Molecular docking was applied to understand the binding mode and affinity of the essential oils’ data set at the protein binding site. According to molecular docking, Citronelol (ChemPLP=60.98) and geranyl acetate (ChemPLP=60.55) were the best-classified compounds compared to the others and they can be explored to develop new repellents.  相似文献   
75.
76.
Understanding how the spatial distribution of ecological resources shapes species’ diversity and abundance in human‐modified landscapes is a central theme in conservation biology. However, studies often disregard that such patterns may vary over time, thereby potentially missing critical environmental constraints to species persistence. This may be particularly important in highly mobile species such as bats, which are able to track temporal variations in spatial resource distribution. Here we test the hypothesis that bats in Mediterranean landscapes are strongly affected by the progressive reduction in water availability during the seasonal summer drought. We analyzed the effects of landscape composition and structure on bat diversity and activity, during pregnancy, lactation, and postlactation periods, and identified the most influential variables within and across periods. Water bodies showed the strongest positive effect on bats, followed by riparian habitats and areas with steeper (>30%) slopes. However, while during pregnancy, there were only small landscape effects, these increased during lactation and postlactation, highlighting a progressively stronger association with water habitats during the summer drought. The spatial projection of habitat models showed that the landscape distribution of bat diversity and activity hotspots changed markedly over time. During pregnancy, the spatial pattern of hotspot distribution was weakly defined, while during lactation and particularly postlactation, there was a concentration of hotspots along permanently flowing watercourses. Our study highlights that permanently flowing watercourses are critical for bat conservation in Mediterranean landscapes, calling for measures to counteract their ongoing degradation due in particular to climate change, water abstraction and damming. More generally, our study underlines the importance of considering the temporal dimension in habitat selection studies, without which there is the risk of overlooking the importance of habitats that are key for species persistence only at certain times of the year.  相似文献   
77.
Francisella noatunensis subsp. orientalis (FNO) is an important emerging pathogen associated with disease outbreaks in farm-raised Nile tilapia. FNO genetic diversity using PCR-based typing, no intra-species discrimination was achieved among isolates/strains from different countries, thus demonstrating a clonal behaviour pattern. In this study, we aimed to evaluate the population structure of FNO isolates by comparing whole-genome sequencing data. The analysis of recombination showed that Brazilian isolates group formed a clonal population; whereas other lineages are also supported by this analysis for isolates from foreign countries. The whole-genome multilocus sequence typing (wgMLST) analysis showed varying numbers of dissimilar alleles, suggesting that the Brazilian clonal population are in expansion. Each Brazilian isolate could be identified as a single node by high-resolution gene-by-gene approach, presenting slight genetic differences associated to mutational events. The common ancestry node suggests a single entry into the country before 2012, and the rapid dissemination of this infectious agent may be linked to market sales of infected fingerlings.  相似文献   
78.
Bananas are one of the most important fruits in tropical and subtropical regions worldwide. Each year, banana plantations expand, but the areas available are mostly dry lands. The establishment of strategies for obtaining drought tolerant cultivars depends on understanding of biological responses at genetic, molecular and biochemical levels. Proteomics is a powerful tool for functional characterization of the response of plants to abiotic stress and little is known about drought tolerance in Musa spp. Therefore, the aim of this study was to identify proteins related to drought tolerance in two contrasting banana genotypes, Prata Anã and BRS Tropical, susceptible and tolerant to drought, respectively. Proteins were extracted from rhizomes of bananas grown under greenhouse conditions with control, irrigated and water deficit regimes. The differential protein expression pattern was established by two-dimensional (2-D) electrophoresis and spots analyzed in nano Q-Tof Micro UPLC. Twenty-three differentially expressed proteins were found in the tolerant genotype (BRS Tropical) under water deficit, with proteins involved in metabolism, defense and transport. Proteins were classified according to known function and biosynthetic pathways. Signaling proteins in response to water stress, especially for the biological function of growth and development of plants cells, were also encountered, whereas heat shock proteins played a significant role. This is the first report of proteomic analysis for drought tolerance in ‘Pome’ and ‘Silk-type’ bananas containing the ‘B’ genome. Our work provides insights into Musa spp. response to drought and data for further studies regarding molecular mechanisms, which determine how Musa spp. cells better overcome environmental perturbations.  相似文献   
79.
We describe and illustrate two species of Manihot that occur in Caatinga (semiarid) vegetation in Brazil and redescribe and lectotypify M. quinquefolia Pohl, which was only known from a single collection made by J. E. B. Pohl in 1827. Manihot elongata P.Carvalho & M.Martins is widely distributed and Manihot alterniflora P.Carvalho & M.Martins is endangered because of its small populations and restricted area of occurrence. We establish M. quinquefolia as the only species of Manihot in the Caatinga with compound leaves. An identification key is provided for the 13 species of Manihot present in the Caatinga.  相似文献   
80.
Marmosets (genus Callithrix) are a diverse group of platyrrhine primates with 13-15 purported taxa, many of them considered endangered. Morphological analyses constitute most of the basis for recognition of these forms as distinct taxa. The purpose of this study was to provide a molecular view, based on mitochondrial control region sequences, of the evolutionary history of the marmosets, concomitant with a molecular phylogenetic perspective on species diversity within the group. An additional purpose was to provide the first comparative examination of a complete New World monkey control region sequence with those of other mammals. The phylogenetic analyses provide convincing support for a split between the Atlantic forest and Amazonian marmosets, with the inclusion of the pygmy marmoset (Cebuella pygmaea) at the base of the Amazonian clade. The earliest branch of the Atlantic forest group was C. aurita. In the Amazonian group, the analyses do not support the recognition of C. humeralifer and the recently described C mauesi as distinct taxa. They do, however, support a clear distinction between C. argentata and a strongly supported mixed clade of C. humeralifer and C. mauesi. In the Atlantic forest group, the phylogenetic tree suggests mixing between C. penicillata, C. kuhli, and possibly C. jacchus. Most of the sequence features characteristic of other mammal control regions were also evident in marmosets, with the exception that conserved sequence blocks (CSBs) 2 and 3 were not clearly identifiable. Tandem repeat units often associated with heteroplasmy in a variety of other mammals were not evident in the marmoset sequences.   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