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   137篇
  免费   2篇
  2023年   1篇
  2021年   4篇
  2020年   1篇
  2019年   2篇
  2018年   4篇
  2016年   6篇
  2015年   9篇
  2014年   6篇
  2013年   12篇
  2012年   10篇
  2011年   7篇
  2010年   4篇
  2009年   5篇
  2008年   11篇
  2007年   9篇
  2006年   13篇
  2005年   4篇
  2004年   8篇
  2002年   2篇
  2001年   2篇
  1999年   1篇
  1997年   1篇
  1996年   2篇
  1995年   1篇
  1994年   1篇
  1993年   1篇
  1991年   1篇
  1990年   2篇
  1982年   1篇
  1979年   1篇
  1973年   1篇
  1972年   1篇
  1968年   4篇
  1966年   1篇
排序方式: 共有139条查询结果,搜索用时 421 毫秒
81.
Aim We aimed to describe the large‐scale patterns in population density of roe deer Caprelous capreolus in Europe and to determine the factors shaping variation in their abundance. Location Europe. Methods We collated data on roe deer population density from 72 localities spanning 25° latitude and 48° longitude and analysed them in relation to a range of environmental factors: vegetation productivity (approximated by the fraction of photosynthetically active radiation) and forest cover as proxies for food supply, winter severity, summer drought and presence or absence of large predators (wolf, Canis lupus, and Eurasian lynx, Lynx lynx), hunter harvest and a competitor (red deer, Cervus elaphus). Results Roe deer abundance increased with the overall productivity of vegetation cover and with lower forest cover (sparser forest cover means that a higher proportion of overall plant productivity is allocated to ground vegetation and thus is available to roe deer). The effect of large predators was relatively weak in highly productive environments and in regions with mild climate, but increased markedly in regions with low vegetation productivity and harsh winters. Other potentially limiting factors (hunting, summer drought and competition with red deer) had no significant impact on roe deer abundance. Main conclusions The analyses revealed the combined effect of bottom‐up and top‐down control on roe deer: on a biogeographical scale, population abundance of roe deer has been shaped by food‐related factors and large predators, with additive effects of the two species of predators. The results have implications for management of roe deer populations in Europe. First, an increase in roe deer abundance can be expected as environmental productivity increases due to climate change. Secondly, recovery plans for large carnivores should take environmental productivity and winter severity into account when predicting their impact on prey.  相似文献   
82.
In the experiment with water from the hypereutrophic Lake Frederiksborg Slotso (Denmark) sampled during the autumn peak of Microcystis growth, the quantity and production of free-living and cyanobacteria-associated heterotrophic bacteria were determined, as well as the extracellular enzymatic (aminopeptidase) activity. The functional diversities of associated and free-living bacterial communities were additionally compared using BIOLOG GN microplates to reveal the possible export of Microcystis-attached bacteria into ambient water. It has been shown that the cell size, production values, and growth rates of associated bacteria were less than those of free-living bacteria. At the same time, the potential aminopeptidase activity of associated bacteria was always higher than that of free-living bacteria. The experimental results have shown significant compositional differences in the structure of bacterial communities from different habitats.  相似文献   
83.
Pezizomycotina is the largest subphylum of Ascomycota and includes the vast majority of filamentous, ascoma-producing species. Here we report the results from weighted parsimony, maximum likelihood and Bayesian phylogenetic analyses of five nuclear loci (SSU rDNA, LSU rDNA, RPB1, RPB2 and EF-lalpha) from 191 taxa. Nine of the 10 Pezizomycotina classes currently recognized were represented in the sampling. These data strongly supported the monophyly of Pezizomycotina, Arthoniomycetes, Eurotiomycetes, Orbiliomycetes and Sordariomycetes. Pezizomycetes and Dothideomycetes also were resolved as monophyletic but not strongly supported by the data. Lecanoromycetes was resolved as paraphyletic in parsimony analyses but monophyletic in maximum likelihood and Bayesian analyses. Leotiomycetes was polyphyletic due to exclusion of Geoglossaceae. The two most basal classes of Pezizomycotina were Orbiliomycetes and Pezizomycetes, both of which comprise species that produce apothecial ascomata. The seven remaining classes formed a monophyletic group that corresponds to Leotiomyceta. Within Leotiomyceta, the supraclass clades of Leotiomycetes s.s. plus Sordariomycetes and Arthoniomycetes plus Dothideomycetes were resolved with moderate support.  相似文献   
84.
85.
Two new steroid glycosides: distolasteroside D6, (24S)-24-O-(beta-D-xylopyranosyl)-5alpha-cholestane-3beta,6alpha,8,15beta,16beta,24-hexaol, and distolasteroside D7. (22E,24R)-24-O-(beta-D-xylopyranosyl)-5alpha-cholest-22-ene-3beta,6alpha,8,15beta,24-pentaol were isolated along with the previously known distolasterosides D1, D2, and D3, echinasteroside C, and (25S)-5alpha-cholestane-3beta,4beta,6alpha,7alpha,8,15alpha,16beta,26-octaol from the Far Eastern starfish Distolasterias nipon. The structures of new compounds were elucidated by NMR spectroscopy and MALDI TOF mass spectrometry. Like neurotrophins, distolasterosides D1, D2, and D3 were shown to induce neuroblast differentiation in a mouse neuroblastoma C 1300 cell culture.  相似文献   
86.
Comparative analysis of results of clinical and laboratory evaluation of patients with chronic nasal or nasal sinuses' diseases (chronic rhinitis or maxillary sinusitis) associated or not associated with Chlamydia infection was performed. It was shown that in patients infected with Chlamydia, along with unidirectional changes typical for all patients irrespective from presence or absence of Chlamydia, the features of immune response against these infectious agents take place.  相似文献   
87.
The chemical structure of lipid A from the marine gamma-proteobacterium Pseudoalteromonas haloplanktis ATCC 14393T, a main product of lipopolysaccharide hydrolysis (1% AcOH), was determined using chemical methods and NMR spectroscopy. The lipid A was shown to be beta-1,6-glucosaminobiose 1,4'-diphosphate acylated with two (R)-3-hydroxyalkanoic acid residues at C3 and C3' and amidated with one (R)-3-hydroxydodecanoyl and one (R)-3-dodecanoyloxydodecanoyl residue at N2 and N2', respectively.  相似文献   
88.
A novel protein structure alignment technique has been developed reducing much of the secondary and tertiary structure to a sequential representation greatly accelerating many structural computations, including alignment. Constructed from incidence relations in the Delaunay tetrahedralization, alignments of the sequential representation describe structural similarities that cannot be expressed with rigid-body superposition and complement existing techniques minimizing root-mean-squared distance through superposition. Restricting to the largest substructure superimposable by a single rigid-body transformation determines an alignment suitable for root-mean-squared distance comparisons and visualization. Restricted alignments of a test set of histones and histone-like proteins determined superpositions nearly identical to those produced by the established structure alignment routines of DaliLite and ProSup. Alignment of three, increasingly complex proteins: ferredoxin, cytidine deaminase, and carbamoyl phosphate synthetase, to themselves, demonstrated previously identified regions of self-similarity. All-against-all similarity index comparisons performed on a test set of 45 class I and class II aminoacyl-tRNA synthetases closely reproduced the results of established distance matrix methods while requiring 1/16 the time. Principal component analysis of pairwise tetrahedral decomposition similarity of 2300 molecular dynamics snapshots of tryptophanyl-tRNA synthetase revealed discrete microstates within the trajectory consistent with experimental results. The method produces results with sufficient efficiency for large-scale multiple structure alignment and is well suited to genomic and evolutionary investigations where no geometric model of similarity is known a priori.  相似文献   
89.
The chemical structure of a novel lipid A, the major component of the lipopolysaccharide from the marine gamma-proteobacterium Marinomonas vaga ATCC 27119(T), was determined by compositional analysis, NMR spectroscopy, and MS. It was found to be beta-1,6-glucosaminobiose 1-phosphate acylated with (R)-3-[dodecanoyl(dodecenoyl)oxy]decanoic acid [C10 : 0 (3O-C12 : 0 [3O-C12 : 1])] or (R)-3-(decanoyloxy)decanoic acid [C10 : 0 (3O-C10 : 0)], (R)-3-hydroxydecanoic acid [C10 : 0 (3OH)], and (R)-3-[(R)-3-hydroxydecanoyloxy]decanoic acid (C10 : 0 [3O-[C10 : 0 (3OH)]]) at the 2, 3, and 2' positions, respectively. It showed low lethal toxicity, which is probably related to specific structural attributes. The absence of a fatty acid at the 3' position and a phosphoryl group at the 4' position and also the presence of an amide-linked (R)-3-hydroxyalkanoic acid that is further O-acylated with another (R)-3-hydroxyalkanoic acid, distinguish M. vaga lipid A from other such molecules.  相似文献   
90.
Regional distribution of the brain steady potential level (SPL) was shown to depend on the predominant clinical symptom in parkinsonian patients. Those with prevailing bradykinesia revealed a statistically significant decrease in the SPL in the frontal area as compared with the healthy subjects or the patients with prevailing tremor.  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