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   241篇
  免费   5篇
  2022年   3篇
  2021年   4篇
  2019年   2篇
  2017年   2篇
  2016年   3篇
  2015年   6篇
  2014年   8篇
  2013年   7篇
  2012年   11篇
  2011年   21篇
  2010年   7篇
  2009年   17篇
  2008年   14篇
  2007年   19篇
  2006年   18篇
  2005年   10篇
  2004年   7篇
  2003年   3篇
  2002年   5篇
  2001年   3篇
  2000年   3篇
  1999年   2篇
  1996年   2篇
  1994年   2篇
  1992年   1篇
  1991年   7篇
  1989年   2篇
  1988年   1篇
  1987年   1篇
  1985年   1篇
  1984年   1篇
  1983年   1篇
  1981年   6篇
  1980年   6篇
  1979年   7篇
  1978年   2篇
  1977年   1篇
  1976年   2篇
  1975年   1篇
  1974年   1篇
  1973年   1篇
  1972年   1篇
  1971年   1篇
  1970年   2篇
  1969年   2篇
  1968年   2篇
  1966年   2篇
  1958年   2篇
  1957年   5篇
  1956年   3篇
排序方式: 共有246条查询结果,搜索用时 593 毫秒
201.
202.
This article examines how alternative rural funding scenarios might influence the pattern of functional land types in mountain areas. The study aims were to explore the use of stakeholders to predict landscape change and to provide a future policy context for other papers in the Carbomont program. EU rural funding policies could have a strong influence on land use and landscapes in mountain areas. At eight sites across Europe, groups of local stakeholders were asked to compare the possible effects of three contrasting funding scenarios over an imagined period of 20 years on (1) the importance of the main land-use sectors; (2) the areas of the main land functional land types; and (3) the management of individual land types. Stakeholders also listed their interests in the area to help define the perspective of the group. The protocols used were ranking and scoring procedures that permitted quantification of changes and of the degree of consensus within the group. The scenarios were (1) continuation of current rural funding (status quo), (2) rapid reduction of farm income support (reduce support), and (3) increasing rural diversification funding (diversification). The eight countries sampled included five established EU members (UK, Germany, Austria, Italy, Spain), two new accession members (Czeck Republic and Slovakia), and Switzerland. There were predicted to be widespread reductions in the importance of the agricultural sector across Europe and increases in the transport, built environment, and tourism sectors. In general, the status quo scenario was perceived to be unsatisfactory in various respects, reduce support was worse, but diversification offered opportunities for conservation and development of mountain communities and land use. Changes in the areas of land types would mainly involve loss of arable and grazing land and increases in scrub, and settlements. Some elements of the landscape such as most forests, mountain tops, and wetlands would, however, be little affected by any of the scenarios. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   
203.
The ultrastructure of two gastric cryptosporidia, Cryptosporidium muris from experimentally infected rodents (Mastomys natalensis) and Cryptosporidium sp. 'toad' from naturally infected toads (Duttaphrynus melanostictus), was studied using electron microscopy. Observations presented herein allowed us to map ultrastructural aspects of the cryptosporidian invasion process and the origin of a parasitophorous sac. Invading parasites attach to the host cell, followed by gradual envelopment, with the host's cell membrane folds, eventually forming the parasitophorous sac. Cryptosporidian developmental stages remain epicellular during the entire life cycle. The parasite development is illustrated in detail using high resolution field emission scanning electron microscopy. This provides a new insight into the ultrastructural detail of host-parasite interactions and species-specific differences manifested in frequency of detachment of the parasitophorous sac, radial folds of the parasitophorous sac and stem-formation of the parasitised host cell.  相似文献   
204.
The relative importance of interference and exploitative competition from Daphnia was tested in contrast to known role of Cyclops vicinus predation, influencing the rotifer community in Římov Reservoir during spring period. The abundances of five dominant rotifer species and their birth and death rates were analysed, together with changes in chl‐a concentration, HNF, bacteria biomass and the abundances of predator and competitor species, from high frequency sampling. Development of loricated species (Keratella cochlearis, K. quadrata and Kellicottia longispina) was driven predominantly by food availability. The dominant soft‐bodied species Polyarthra spp. and Synchaeta lakowitziana were affected predominantly through C. vicinus predation. The role of mechanical interference from D. galeata, although may be important in a limited periods was overclouded by predation and exploitative competition in general. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
205.
The oxidative stress hypothesis of aging suggests that accumulation of oxidative damage is a key factor of the alterations in physiological function during aging. We studied age-related sensitivity to oxidative modifications of proteins and lipids of cardiac sarcoplasmic reticulum (SR) isolated from 6-, 15- and 26-month-old rats. Oxidative stress was generated in vitro by exposing SR vesicles to 0.1 mmol/l FeSO4/EDTA + 1 mmol/l H2O2 at 37 degrees C for 60 min. In all groups, oxidative stress was associated with decreased membrane surface hydrophobicity, as detected by 1-anilino-8-naphthalenesulfonate as a probe. Structural changes in SR membranes were accompanied by degradation of tryptophan and significant accumulation of protein dityrosines, protein conjugates with lipid peroxidation products, conjugated dienes and thiobarbituric acid reactive substances. The sensitivity to oxidative damage was most pronounced in SR of 26-month-old rat. Our results indicate that aging and oxidative stress are associated with accumulation of oxidatively damaged proteins and lipids and these changes could contribute to cardiovascular injury.  相似文献   
206.
207.
Cytosine methylation at CpG dinucleotides is a critical epigenetic modification of mammalian genomes. CpG binding protein (CGBP) exhibits a unique DNA-binding specificity for unmethylated CpG motifs and is essential for early murine development. Embryonic stem cell lines deficient for CGBP were generated to further examine CGBP function. CGBP(-)(/)(-) cells are viable but show an increased rate of apoptosis and are unable to achieve in vitro differentiation following removal of leukemia inhibitory factor from the growth media. Instead, CGBP(-)(/)(-) embryonic stem cells remain undifferentiated as revealed by persistent expression of the pluripotent markers Oct4 and alkaline phosphatase. CGBP(-)(/)(-) cells exhibit a 60 to 80% decrease in global cytosine methylation, including hypo-methylation of repetitive elements, single-copy genes, and imprinted genes. Total DNA methyltransferase activity is reduced by 30 to 60% in CGBP(-)(/)(-) cells, and expression of the maintenance DNA methyltransferase 1 protein is similarly reduced. However, de novo DNA methyltransferase activity is normal. Nearly all aspects of the pleiotropic CGBP(-)(/)(-) phenotype are rescued by introduction of a CGBP expression vector. Hence, CGBP is essential for normal epigenetic modification of the genome by cytosine methylation and for cellular differentiation, consistent with the requirement for CGBP during early mammalian development.  相似文献   
208.
Soluble species of phosphorus, sulfur, selenium and eight metals (Mn, Fe, Co, Ni, Cu, Zn, Mo and Cd) in soybean flour and common white bean seeds were investigated by size exclusion chromatography (SEC) and inductively coupled plasma mass spectrometry (ICP-MS). Samples were extracted by 0.02 mol l(-1) Tris-HCI buffer solution (pH 7.5). Fractionation of sample extracts by preparative scale SEC was accomplished using a Fractogel EMD BioSEC column (600 x 16 mm) and 0.02 mol l(-1) Tris-HCl buffer solution (pH 7.5) as mobile phase (flow rate: 2 ml min(-1)). A 2-ml sample was injected. Contents of elements in chromatographic fractions were determined by AAS, ICP-AES and ICP-MS. The elution profiles of P, Fe, Co, Ni, Cu, Zn and Mo in both samples were similar. Main species of Co, Ni, Cu, Zn and Mo were found in the low molecular weight region (2-5 kDa), whereas Fe is predominantly bound to high molecular weight compounds (180 kDa). The dominant phosphorus fraction was detected in the medium molecular weight region (10-30 kDa) and the other fraction in the low molecular weight region. Isotachophoretic analysis of chromatographic fractions revealed that the main phosphorus compound in the medium molecular weight region is phytic acid. SEC on Superdex 75 and Superdex Peptide columns (300 x 10 mm) was performed in on-line hyphenation with ICP-MS. The same mobile phase was used with a flow rate of 0.5 ml min(-1); volume of injected sample was 200 microl. Element specific chromatograms were obtained by continuous nebulization of effluent into ICP-mass spectrometer measuring intensities of 47(PO)+ and 48(SO)+ oxide ions and 55Mn, 57Fe, 59Co, 62Ni, 65Cu, 66Zn, 82Se, 95Mo and 114Cd nuclides. Chromatographic profiles of elements are generally analogous to those obtained with a Fractogel column, but better chromatographic resolution of separated species was achieved so that slight differences between samples were revealed. Estimated molecular weights of major phosphorus species in soybean flour and common white bean seed extracts are 6 and 3.6 kDa, respectively, whereas those of minor phosphorus species in both samples are 0.7 kDa. Traces of phosphorus were also detected in the high molecular weight region (130 kDa). Chromatograms of P, Ni, Cu, Zn and Mo compounds in both extracts are similar but not identical. Molecular weights of major Cu and Zn species are approximately 1 and 0.4 kDa for soybean flour and white bean seeds, respectively. In cases of Mn, Fe, Co and Se, the element profiles of soybean flour and white bean seed extracts are significantly different.  相似文献   
209.
The relative importance of and changes in resource limitation of herbivorous rotifers were assessed during the clear-water phase in the Rímov Reservoir, Czech Republic, using in situ manipulative experiments. Resource limitation was tested experimentally as the difference in population growth rate (Δr) among various experimental treatments on four occasions. The reservoir community of rotifers was exposed to three treatments: (i) control, (ii) diluted and (iii) diluted and fertilized. Significant responses to these experimental manipulations were shown by Synchaeta spp., Polyarthra spp. and Keratella cochlearis. Growth rate was usually highest during the spring rotifer maximum and decreased during the clear water phase. The highest intensity of food limitation (expressed as ‚Chlorophyll-a’ limitation) was found in Synchaeta spp. K. cochlearis had low food limitation during the spring peak, high food limitation during the second experiment and low food limitation, again, during the later experiment. In contrast, Polyarthra spp. had the same Chlorophyll-a limitation throughout the whole experimental period. Linear regression was used to estimate the relative proportion of Δr variability explained by Chlorophyll-a concentration and rotifer density in all of the experiments. Chlorophyll-a concentration explained 89, 97 and 92% of the resource limitation in Synchaeta spp., Polyarthra spp. and K. cochlearis, respectively. The proportion of variability explained by rotifer density-dependent factors was lower: 60% for Synchaeta spp. and 68 % for Polyarthra spp.  相似文献   
210.
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