全文获取类型
收费全文 | 350篇 |
免费 | 31篇 |
国内免费 | 1篇 |
专业分类
382篇 |
出版年
2022年 | 4篇 |
2021年 | 8篇 |
2019年 | 5篇 |
2018年 | 10篇 |
2017年 | 3篇 |
2016年 | 6篇 |
2015年 | 14篇 |
2014年 | 13篇 |
2013年 | 15篇 |
2012年 | 21篇 |
2011年 | 15篇 |
2010年 | 13篇 |
2009年 | 13篇 |
2008年 | 17篇 |
2007年 | 21篇 |
2006年 | 12篇 |
2005年 | 6篇 |
2004年 | 11篇 |
2003年 | 12篇 |
2002年 | 9篇 |
2001年 | 13篇 |
2000年 | 9篇 |
1999年 | 6篇 |
1998年 | 11篇 |
1997年 | 8篇 |
1996年 | 2篇 |
1995年 | 4篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1973年 | 4篇 |
1969年 | 2篇 |
1965年 | 1篇 |
1950年 | 1篇 |
1923年 | 1篇 |
排序方式: 共有382条查询结果,搜索用时 0 毫秒
121.
Effects of Petroleum Hydrocarbons on Plant Litter Microbiota in an Arctic Lake 总被引:3,自引:7,他引:3 下载免费PDF全文
The effects of petroleum hydrocarbons on the microbial community associated with decomposing Carex leaf litter colonized in Toolik Lake, Alaska, were examined. Microbial metabolic activity, measured as the rate of acetate incorporation into lipid, did not vary significantly from controls over a 12-h period after exposure of colonized Carex litter to 3.0 ml of Prudhoe Bay crude oil, diesel fuel, or toluene per liter. ATP levels of the microbiota became elevated within 2 h after the exposure of the litter to diesel fuel or toluene, but returned to control levels within 4 to 8 h. ATP levels of samples exposed to Prudhoe Bay crude oil did not vary from control levels. Mineralization of specifically labeled 14C-[lignin]-lignocellulose and 14C-[cellulose]-lignocellulose by Toolik Lake sediments, after the addition of 2% (vol/vol) Prudhoe Bay crude oil, motor oil, diesel fuel, gasoline, n-hexane, or toluene, was examined after 21 days of incubation at 10°C. Diesel fuel, motor oil, gasoline, and toluene inhibited 14C-[lignin]-lignocellulose mineralization by 58, 67, 67, and 86%, respectively. Hexane-treated samples displayed an increase in the rate of 14C-[lignin]-lignocellulose mineralization of 33%. 14C-[cellulose]-lignocellulose mineralization was inhibited by the addition of motor oil or toluene by 27 and 64%, respectively, whereas diesel fuel-treated samples showed a 17% increase in mineralization rate. Mineralization of the labeled lignin component of lignocellulose appeared to be more sensitive to hydrocarbon perturbations than was the labeled cellulose component. 相似文献
122.
AJ Durston 《Current Genomics》2012,13(4):265-266
123.
Elizabeth A. Hamman Scott A. McKinley Adrian C. Stier Craig W. Osenberg 《Theoretical Ecology》2018,11(1):111-127
Variation in the density of organisms among habitat patches is often attributed to variation in inherent patch properties. For example, higher quality patches might have higher densities because they attract more colonists or confer better post-colonization survival. However, variation in occupant density can also be driven by landscape configuration if neighboring patches draw potential colonists away from the focal habitat (a phenomenon we call propagule redirection). Here, we develop and analyze a stochastic model to quantify the role of landscape configuration and propagule redirection on occupant density patterns. We model a system with a dispersive larval stage and a sedentary adult stage. The model includes sensing and decision-making in the colonization stage and density-dependent mortality (a proxy for patch quality) in the post-colonization stage. We demonstrate that spatial variation in colonization is retained when the supply of colonists is not too high, post-colonization density-dependent survival is not too strong, and colonization events are not too frequent. Using a reef fish system, we show that the spatial variation produced by propagule redirection is comparable to spatial variation expected when patch quality varies. Thus, variation in density arising from the spatial patterning of otherwise identical habitat can play an important role in shaping long-term spatial patterns of organisms occupying patchy habitats. Propagule redirection is a potentially powerful mechanism by which landscape configuration can drive variation in occupant densities, and may therefore offer new insights into how populations may shift as landscapes change in response to natural and anthropogenic forces. 相似文献
124.
125.
Jason R. Buck Eliot T. McKinley Allie Fu Ty W. Abel Reid C. Thompson Lola Chambless Jennifer M. Watchmaker James P. Harty Michael K. Cooper H. Charles Manning 《PloS one》2015,10(10)
Current positron emission tomography (PET) imaging biomarkers for detection of infiltrating gliomas are limited. Translocator protein (TSPO) is a novel and promising biomarker for glioma PET imaging. To validate TSPO as a potential target for molecular imaging of glioma, TSPO expression was assayed in a tumor microarray containing 37 high-grade (III, IV) gliomas. TSPO staining was detected in all tumor specimens. Subsequently, PET imaging was performed with an aryloxyanilide-based TSPO ligand, [18F]PBR06, in primary orthotopic xenograft models of WHO grade III and IV gliomas. Selective uptake of [18F]PBR06 in engrafted tumor was measured. Furthermore, PET imaging with [18F]PBR06 demonstrated infiltrative glioma growth that was undetectable by traditional magnetic resonance imaging (MRI). Preliminary PET with [18F]PBR06 demonstrated a preferential tumor-to-normal background ratio in comparison to 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG). These results suggest that TSPO PET imaging with such high-affinity radiotracers may represent a novel strategy to characterize distinct molecular features of glioma growth, as well as better define the extent of glioma infiltration for therapeutic purposes. 相似文献
126.
Stomata regulate gas exchange and their closure in response to pathogens may, in some cases, contribute to resistance. However, in the cereal mildew and rust systems, stomatal closure follows establishment of compatible infections. In incompatible systems, expression of major (R) gene controlled hypersensitive responses (HR), causes drastic, permanent stomatal dysfunction: stomata become locked open following powdery mildew attack and locked shut following rust attack. Thus, stomatal locking can be a hitherto unsuspected negative consequence of R gene resistance that carries a physiological cost affecting plant performance.Key Words: stomata, rust, mildew, hypersensitive response, stomatal lock-up 相似文献
127.
Cerebral osmoregulation of renal sodium excretion--a response analogous to thirst and vasopressin release 总被引:2,自引:0,他引:2
M J McKinley D A Denton J P Coghlan R B Harvey J G McDougall M Rundgren B A Scoggins R S Weisinger 《Canadian journal of physiology and pharmacology》1987,65(8):1724-1729
Studies in sheep have shown that renal excretion of sodium may be under osmoregulatory control. When sheep become dehydrated, or are infused intravenously with hypertonic saline, they increase renal Na excretion in addition to secreting vasopressin and developing a thirst. These natriuretic, antidiuretic, and dipsogenic responses to dehydration and hypertonicity can be greatly reduced by lowering the cerebrospinal fluid NaCl concentration or by prior ablation of tissue in the anterior wall of the third ventricle. Lowering of cerebrospinal fluid NaCl concentration also prevents postprandial natriuresis which normally occurs in association with a postprandial increase in plasma Na concentration and tonicity. We propose that there is a cerebral osmoregulatory control of Na excretion which may interact with volume influences from the cardiovascular system to regulate renal Na output. The effector mechanism from brain to kidney mediating such cerebral control of Na excretion is probably hormonal. 相似文献
128.
Nucleotide sequence analysis of the lemur beta-globin gene family: evidence for major rate fluctuations in globin polypeptide evolution 总被引:1,自引:0,他引:1
Lemur beta-related globin genes have been isolated and sequenced. Orthology
of prosimian and human epsilon-, gamma-, and beta-related globin genes was
established by dot-matrix analysis. All of these lemur globin genes
potentially encode functional beta-related globin polypeptides, though
precisely when the gamma-globin gene is expressed remains unknown. The
organization of the 18-kb brown lemur beta-globin gene cluster (5'
epsilon-gamma-[psi eta-delta]-beta 3') is consistent with its evolution by
contraction via unequal crossing-over from the putative ancestral mammalian
beta-globin gene cluster (5' epsilon-gamma- eta-delta-beta 3'). The dwarf
lemur nonadult globin genes are arranged as in the brown lemur. Similar
levels of synonymous (silent) nucleotide substitutions and noncoding DNA
sequence differences have accumulated between species in all of these
genes, suggesting a uniform rate of noncoding DNA divergence throughout
primate beta-globin gene clusters. These differences are comparable with
those observed in the nonfunctional psi eta pseudogene and have therefore
accumulated at the presumably maximal neutral rate. In contrast,
nonsynonymous (replacement) nucleotide substitutions show a significant
heterogeneity in distribution for both the same gene in different lineages
and different genes in the same lineage. These major fluctuations in
replacement but not silent substitution rates cannot be attributed to
changes in mutation rate, suggesting that changes in the rate of globin
polypeptide evolution in primates is not governed solely by variable
mutation rates.
相似文献
129.
Laurin AJ Mueller Karl G Kugler Michael Netzer Armin Graber Matthias Dehmer 《Biology direct》2011,6(1):1-15
Background
Chromosomal orthologs can reveal the shared ancestral gene set and their evolutionary trends. Additionally, physico-chemical properties of encoded proteins could provide information about functional adaptation and ecological niche requirements.Results
We analyzed 7080 genes (five groups of 1416 orthologs each) from Rhizobiales species (S. meliloti, R. etli, and M. loti, plant symbionts; A. tumefaciens, a plant pathogen; and B. melitensis, an animal pathogen). We evaluated their phylogenetic relationships and observed three main topologies. The first, with closer association of R. etli to A. tumefaciens; the second with R. etli closer to S. meliloti; and the third with A. tumefaciens and S. meliloti as the closest pair. This was not unusual, given the close relatedness of these three species. We calculated the synonymous (dS) and nonsynonymous (dN) substitution rates of these orthologs, and found that informational and metabolic functions showed relatively low dN rates; in contrast, genes from hypothetical functions and cellular processes showed high dN rates. An alternative measure of sequence variability, percentage of changes by species, was used to evaluate the most specific proportion of amino acid residues from alignments. When dN was compared with that measure a high correlation was obtained, revealing that much of evolutive information was extracted with the percentage of changes by species at the amino acid level. By analyzing the sequence variability of orthologs with a set of five properties (polarity, electrostatic charge, formation of secondary structures, molecular volume, and amino acid composition), we found that physico-chemical characteristics of proteins correlated with specific functional roles, and association of species did not follow their typical phylogeny, probably reflecting more adaptation to their life styles and niche preferences. In addition, orthologs with low dN rates had residues with more positive values of polarity, volume and electrostatic charge.Conclusions
These findings revealed that even when orthologs perform the same function in each genomic background, their sequences reveal important evolutionary tendencies and differences related to adaptation. This article was reviewed by: Dr. Purificación López-García, Prof. Jeffrey Townsend (nominated by Dr. J. Peter Gogarten), and Ms. Olga Kamneva. 相似文献130.
Lei Duan Li‐Na Han Bin‐Bin Liu Artem Leostrin AJ Harris Lin Wang Emine Arslan Kuddisi Ertuğrul Mikhail Knyazev Elena Hantemirova Jun Wen Hong‐Feng Chen 《植物分类学报:英文版》2023,61(1):22-41
The liquorice tribe Glycyrrhizeae is a leguminous herbaceous group of plants comprised of the genera Glycyrrhiza and Glycyrrhizopsis. Some Glycyrrhiza taxa contain glycyrrhizin, a pharmacologically significant sweet substance that also has applications in crafting industrial materials. Here, we utilized an expanded taxon sampling of Glycyrrhizeae to reconstruct the phylogenetic relationships in the tribe based on genome skimming data, including whole chloroplast genomes, nuclear ribosomal DNA, and low-copy nuclear DNA. We also launched machine learning analysis (MLA) for one species pair with controversial taxonomic boundary. The integrated results indicated Glycyrrhizopsis should be split from Glycyrrhiza, while the former genus Meristotropis should be treated as part of Glycyrrhiza. Glycyrrhizopsis includes two species, Glycyrrhizopsis asymmetrica and Glycyrrhizopsis flavescens, and we recognize 13 species in Glycyrrhiza: Glycyrrhiza acanthocarpa, Glycyrrhiza astragalina, Glycyrrhiza bucharica, Glycyrrhiza echinata, Glycyrrhiza foetida, Glycyrrhiza glabra, Glycyrrhiza gontscharovii, Glycyrrhiza lepidota, Glycyrrhiza macedonica, Glycyrrhiza pallidiflora, Glycyrrhiza squamulosa, Glycyrrhiza triphylla, and Glycyrrhiza yunnanensis. We propose a broader G. glabra that includes former Glycyrrhiza aspera, G. glabra s.s., Glycyrrhiza inflata, and Glycyrrhiza uralensis, and represents the glycyrrhizin-contained medicinal group. Our ancestral state inferences show the ancestor of Glycyrrhiza lacked glycyrrhizin, and the presence of glycyrrhizin evolved twice within Glycyrrhiza during the last one million years. Our integrative phylogenomics-MLA study not only provides new insights into long-standing taxonomic controversies of Glycyrrhizeae, but also represents a useful approach for future taxonomic studies on other plant taxa. 相似文献