全文获取类型
收费全文 | 222篇 |
免费 | 28篇 |
专业分类
250篇 |
出版年
2023年 | 2篇 |
2022年 | 2篇 |
2021年 | 7篇 |
2019年 | 2篇 |
2018年 | 2篇 |
2017年 | 4篇 |
2016年 | 3篇 |
2015年 | 3篇 |
2014年 | 5篇 |
2013年 | 8篇 |
2012年 | 15篇 |
2011年 | 18篇 |
2010年 | 14篇 |
2009年 | 14篇 |
2008年 | 11篇 |
2007年 | 13篇 |
2006年 | 8篇 |
2005年 | 11篇 |
2004年 | 13篇 |
2003年 | 6篇 |
2002年 | 9篇 |
2001年 | 8篇 |
2000年 | 7篇 |
1999年 | 9篇 |
1998年 | 4篇 |
1997年 | 6篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1992年 | 4篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 9篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1984年 | 2篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1967年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有250条查询结果,搜索用时 15 毫秒
51.
Sex and rank in competitive brood hierarchies influence stress levels in nestlings of a sexually dimorphic bird 总被引:1,自引:0,他引:1
GUILLERMO BLANCO OSCAR FRÍAS JAVIER MARTÍNEZ JESÚS A. LEMUS RUBEN MERINO BEGOÑA JIMÉNEZ 《Biological journal of the Linnean Society. Linnean Society of London》2006,88(3):383-390
Studies of sibling competition within brood hierarchies have rarely assessed simultaneously the effects of sex and rank in the brood hierarchy on traits other than offspring mortality and differential growth. We studied the expression of heat-shock proteins (Hsps) to assess the physiological stress response to different combinations of sex and position within competitive brood hierarchies in the black kite Milvus migrans (Bodd.), a sexually dimorphic raptor showing facultative siblicide. Senior males showed higher stress levels than did senior females and younger siblings of each sex as revealed by Hsp60 values. The analysis of Hsp70 levels indicated that nestlings from broods in which the senior chick was a male showed higher stress levels than did nestlings from broods in which the senior chick was a female. In addition, levels of Hsp60 were related negatively to nutritional condition expressed as levels of plasmatic albumin. This suggests that the sex of senior chicks may be key in determining their stress level and that of their siblings, which is probably associated with sibling competition by fighting within brood hierarchies. The comparatively higher stress levels of senior males (and their siblings) may be a consequence of their ability to exploit their potential advantage from being the head start while avoiding a possible competitive disadvantage from being the smaller sex, independent of environmental conditions determining the probability of brood reduction. Differential stress levels depending on sex and rank in the brood hierarchy may be a consequence of parental control of offspring behaviour through differential resource allocation (e.g. yolk androgens) or it may reflect adaptations of particular chicks (senior males) to enhance their competitive ability within brood hierarchies. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 383–390. 相似文献
52.
53.
As part of a clinical-pathologic project, older people completed a standard odor identification test at study entry. During a mean of 3.5 years of observation, 201 people died and underwent brain autopsy and neuropathologic examination (6 with a history of Parkinson's disease were excluded). Lewy bodies were identified with antibodies to alpha-synuclein and classified as nigral, limbic, or neocortical based on their distribution in 6 brain regions. Plaques and tangles in 5 regions were summarized with a previously established composite measure, and neuron loss in the substantia nigra was rated on 6-point scale. Odor identification scores ranged from 0 to 12 correct (mean = 8.0, standard deviation = 2.6). On neuropathologic examination, 26 persons had Lewy bodies (13 neocortical, 9 limbic, and 4 nigral). In an analysis adjusted for age, sex, education, and time from olfactory testing to death, limbic (estimate = -2.47, standard error [SE] = 0.73, P < 0.001) and neocortical (estimate = -4.36, SE = 0.63, P < 0.001) Lewy body subgroups were associated with impaired olfaction. Results were comparable in analyses that controlled for dementia or parkinsonism during the study or postmortem measures of plaques and tangles or nigral cell loss. A final set of analyses suggested that impaired olfactory performance may aid detection of underlying Lewy body disease. The findings indicate that Lewy body disease impairs late life olfactory function even in otherwise asymptomatic individuals. 相似文献
54.
Role of external calcium in homeostasis of intracellular pH in the cyanobacterium Anabaena sp. strain PCC7120 exposed to low pH 总被引:1,自引:0,他引:1
The role of external Ca2+ in the homeostasis of intracellular pH (pHi) of Anabaena sp. strain PCC7120 in response to a decrease in the external pH (pHex) has been studied in cell suspensions. Increase in cytoplasmic pH after acid shock is dependent on the presence of Ca2+ in the medium. The observed Ca2+ -mediated alkalization of the cytoplasm depends on the extent of the shift in external pH. Acid pH shifts resulted in an increased permeability of the cytoplasmic membrane to protons, which could be reversed by increasing the concentration of Ca2+ in the medium. Thus, the ability of Ca2+ to increase cytoplasmic pH might be correlated with an inhibition of net proton uptake by increasing concentrations of external Ca2+ under these conditions. This combined response resulted in the generation and maintenance of a larger pH gradient (ΔpH) at acid external pH values. All Ca2+ channel blockers tested, such as verapamil and LaCl3 , inhibited the observed Ca2+ -mediated response. On the other hand, the Ca ionophore calcimycin (compound A23187) was agonistic, and stimulated both cytoplasmic alkalization and inhibition of net proton uptake. The protonophorous uncoupler carbonylcyanide m -chlorophenyl hydrazone, inhibited this Ca2+ -mediated response, whereas monensin, an inhibitor of the Na+ /H+ antiporter, had no significant effect. The results of the present study suggest that an influx of Ca2+ from the extracellular space is required for the regulation of cytoplasmic pH in Anabaena sp. strain PCC7120 exposed to low external pH values. 相似文献
55.
56.
Poplar and shrub willow energy crops in the United States: field trial results from the multiyear regional feedstock partnership and yield potential maps based on the PRISM‐ELM model 下载免费PDF全文
Timothy A. Volk Bill Berguson Christopher Daly Michael D. Halbleib Raymond Miller Timothy G. Rials Lawrence P. Abrahamson Dan Buchman Marylin Buford Michael W. Cunningham Mark Eisenbies Eric S. Fabio Karl Hallen Justin Heavey Gregg A Johnson Yulia A Kuzovkina Bo Liu Bernie Mcmahon Randy Rousseau Shun Shi Richard Shuren Lawrence B. Smart Glen Stanosz Brain Stanton Bryce Stokes Jeff Wright 《Global Change Biology Bioenergy》2018,10(10):735-751
To increase the understanding of poplar and willow perennial woody crops and facilitate their deployment for the production of biofuels, bioproducts, and bioenergy, there is a need for broadscale yield maps. For national analysis of woody and herbaceous crops production potential, biomass feedstock yield maps should be developed using a common framework. This study developed willow and poplar potential yield maps by combining data from a network of willow and poplar field trials and the modeling power of PRISM‐ELM. Yields of the top three willow cultivars across 17 sites ranged from 3.60 to 14.6 Mg ha?1 yr?1 dry weight, while the yields from 17 poplar trials ranged from 7.5 to 15.2 Mg ha?1 yr?1. Relationships between the environmental suitability estimates from the PRISM‐ELM model and results from field trials had an R2 of 0.60 for poplar and 0.81 for willow. The resulting potential yield maps reflected the range of poplar and willow yields that have been reported in the literature. Poplar covered a larger geographic range than willow, which likely reflects the poplar breeding efforts that have occurred for many more decades using genotypes from a broader range of environments than willow. While the field trial data sets used to develop these models represent the most complete information at the time, there is a need to expand and improve the model by monitoring trials over multiple cutting cycles and across a broader range of environmental gradients. Despite some limitations, the results of these models represent a dramatic improvement in projections of potential yield of poplar and willow crops across the United States. 相似文献
57.
Sequence dependent modulating effect of camptothecin on the DNA-cleaving activity of the calf thymus type I topoisomerase. 总被引:3,自引:2,他引:1 下载免费PDF全文
I I Gromova V L Buchman R A Abagyan A V Ulyanov I B Bronstein 《Nucleic acids research》1990,18(3):637-645
High-resolution mapping of topol cleavages in the regions of human DNA including the oncogene c-Ha-ras and p53, has revealed three kinds of topol cleavage sites: cleavage sites not affected by camptothecin; cleavage sites reinforced only in the presence of camptothecin, and cleavage sites which weaken in the presence of camptothecin. Statistical analysis of sequences revealed certain nucleotide or dinucleotide preferences for three groups studied. The preferences in camptothecin-reduced sites predominate upstream from the cleavage point, whereas in camptothecin-induced sites the situation is reversed. The influence of camptothecin on cleavage sites induced by two molecular forms of topol has been also studied. 相似文献
58.
59.
60.
The mechanism by which nitrite inhibits outgrowing spores of Bacillus cereus T was examined by using techniques developed earlier for nitrite analogs. The morphological stage of inhibition, cooperativity effects, effect of pH on inhibition, kinetics of protection against iodoacetate incorporation into membrane sulfhydryl groups, and protection against the bacteriocidal effect of carboxymethylation by iodoacetate indicate that nitrite acts as a membrane-directed sulfhydryl agent. The mechanism by which nitrite modifies the chemical reactivity of the sulfhydryl group could be either direct covalent modification or inactivation through communication with another modified membrane component. Profiles of pH effects suggest that the active agent is the protonated form of nitrite. The nitrite concentrations which modify membrane sulfhydryl activity coincide with those which have a bacteriostatic effect. These results are consistent with membrane sulfhydryl modification as a component of the mechanism of nitrite-induced bacteriostasis in this aerobic sporeformer. 相似文献