全文获取类型
收费全文 | 90篇 |
免费 | 2篇 |
专业分类
92篇 |
出版年
2021年 | 1篇 |
2016年 | 1篇 |
2014年 | 3篇 |
2013年 | 1篇 |
2012年 | 6篇 |
2011年 | 1篇 |
2010年 | 2篇 |
2009年 | 1篇 |
2007年 | 1篇 |
2006年 | 3篇 |
2005年 | 1篇 |
2004年 | 3篇 |
2001年 | 3篇 |
1999年 | 2篇 |
1998年 | 4篇 |
1997年 | 1篇 |
1996年 | 4篇 |
1995年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1959年 | 2篇 |
1958年 | 4篇 |
1957年 | 1篇 |
1956年 | 3篇 |
1955年 | 4篇 |
1954年 | 2篇 |
1953年 | 2篇 |
1952年 | 4篇 |
1951年 | 1篇 |
1950年 | 5篇 |
1949年 | 3篇 |
1948年 | 1篇 |
1946年 | 2篇 |
排序方式: 共有92条查询结果,搜索用时 15 毫秒
81.
82.
Population structure of the yellow-footed rock-wallaby Petrogale xanthopus (Gray, 1854) inferred from mtDNA sequences and microsatellite loci 总被引:1,自引:0,他引:1
The yellow-footed rock-wallaby Petrogale xanthopus is considered to be potentially vulnerable to extinction. This wallaby inhabits naturally disjunct rocky outcrops which could restrict dispersal between populations, but the extent to which that occurs is unknown. Genetic differences between populations were assessed using mitochondrial DNA (control region) sequencing and analysis of variation at four microsatellite loci among three geographically close sites in south-west Queensland (P. x. celeris) and, for mtDNA only, samples from South Australia (P. x. xanthopus) as well. Populations from South Australia and Queensland had phylogenetically distinct mtDNA, supporting the present classification of these two groups as evolutionarily distinct entities. Within Queensland, populations separated by 70 km of unsuitable habitat differed significantly for mtDNA and at microsatellite loci. Populations separated by 10 km of apparently suitable habitat had statistically homogeneous mtDNA, but a significant difference in allele frequency at one microsatellite locus. Tests for Hardy-Weinberg equilibrium and micro-geographical variation at microsatellite loci did not detect any substructuring between two wallaby aggregations within a colony encircling a single rock outcrop. Although the present study was limited by small sample sizes at two of the three Queensland locations examined, the genetic results suggest that dispersal between colonies is limited, consistent with an ecological study of dispersal at one of the sites. Considering both the genetic and ecological data, we suggest that management of yellow-footed rock-wallabies should treat each colony as an independent unit and that conservation of the Queensland and South Australian populations as separate entities is warranted. 相似文献
83.
GAD PERRY GORDON RODDA THOMAS FRITTS & THOMAS SHARP 《Global Ecology and Biogeography》1998,7(5):353-365
We sampled the lizard fauna of twenty-two small islets fringing the Pacific island of Guam and used these data to shed light on the processes responsible for present-day diversity. Habitat diversity, measured by islet area and vegetation complexity, was significantly correlated with the number of species found on an islet. However, islet distance and elevation were not significant predictors of diversity. Distribution patterns were slightly different for the two major families in our sample, Scincidae and Gekkonidae: skinks needed larger islets to maintain a population than did geckos. Presence/absence patterns were highly and significantly nested, and population density was correlated with the number of islets on which a species was found. An area cladogram was poorly supported and showed no faunal similarity between nearby islands. These patterns indicate that extinctions on most islets were due mostly to non-catastrophic, long-acting biological causes. The presence on the islets of species extirpated on Guam and the lack of significant nestedness on islands with greater maximum elevation highlight the impact that predators (primarily brown treesnakes) can have. Our findings also show that small reserves will not suffice to protect endangered lizard faunas, and that the islets may serve as a short-term repository of such species until snake-free areas can be established on Guam 相似文献
84.
The design of a simple instrument to monitor leaf expansionin grasses is described. The instrument was used to comparethe effects of water stress on leaf extension of two cultivarsof maize and sorghum. The effect of withholding water for 3days was an appreciable reduction in the rate of leaf expansionin both plants, particularly during the light period. In well-wateredplants of both species, leaf extension continued at a steadyrate even when leaf turgor fell to around 0.1 MPa. In water-stressedmaize plants, leaf turgor during the light period fell to zeroand leaf growth ceased. When turgor was restored, followingstomatal closure, leaf extension resumed at a slow rate. Inunwatered sorghum plants, leaf turgor remained at a value greaterthan 0.1 MPa but the rate of leaf extension was significantlyreduced. The reduction in leaf turgor in the unwanted plantsresulted partly from an increase in solute potential. Zea mays L, maize, Sorghum bicolor L, leaf expansion, leaf turgor, water stress 相似文献
85.
86.
87.
The first successful cryopreservation of Ochromonas danica and Ochromonas malhamensis is reported. The freezing method was consistently reproducible for the former, but not for the latter. Ochromonas danica cultures established from frozen material still could be used as test organisms for assay of thiamin. This is the first report of a protozoon retaining its assay property after being frozen to -196 C. 相似文献
88.
Dephosphorylation of adenosine triphosphate by concentrates of the virus of avian erythromyeloblastic leucosis 总被引:4,自引:0,他引:4
89.
Evolution of antibiotic resistance genes: the DNA sequence of a kanamycin resistance gene from Staphylococcus aureus 总被引:14,自引:0,他引:14
The kanamycin resistance gene from Staphylococcus aureus has been sequenced
and its structure compared with similar genes isolated from Streptomyces
fradiae and from two transposons, Tn5 and Tn903, originally isolated from
Klebsiella pneumoniae and Salmonella typhimurium, respectively. The genes
are all homologous but, since their common ancestor, have undergone
extensive divergence, with more than 43% divergence between the closest
pair. The phylogeny of the genes cannot be made congruent to the phylogeny
of the taxa from which they were isolated without requiring rather
improbable differences in rates. One is therefore led to conclude that
there have been multiple occurrences of gene transfer between these
species. Thus, although they are homologous, they are neither orthologous
nor paralogous. It is suggested that homologous genes of this type be
called xenologous.
相似文献
90.
The stability of plasmids and lysogenic bacteriophage in 16 strains of Escherichia coli was studied following storage in liquid nitrogen for periods up to 2 years. A comparison was made with cultures which had undergone lyophilization. Plasmid loss was detected by plating on selective media containing antibiotics to which resistance was conferred by plasmid DNA. Loss of plasmid DNA was not evident following storage in liquid nitrogen or lyophilization. 相似文献