全文获取类型
收费全文 | 727篇 |
免费 | 19篇 |
国内免费 | 1篇 |
专业分类
747篇 |
出版年
2014年 | 4篇 |
2013年 | 19篇 |
2012年 | 15篇 |
2011年 | 23篇 |
2010年 | 48篇 |
2009年 | 55篇 |
2008年 | 33篇 |
2007年 | 46篇 |
2006年 | 23篇 |
2005年 | 19篇 |
2004年 | 17篇 |
2003年 | 9篇 |
2001年 | 6篇 |
2000年 | 9篇 |
1999年 | 7篇 |
1998年 | 11篇 |
1997年 | 18篇 |
1996年 | 14篇 |
1995年 | 9篇 |
1994年 | 7篇 |
1993年 | 14篇 |
1992年 | 10篇 |
1991年 | 10篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 17篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1977年 | 5篇 |
1974年 | 5篇 |
1973年 | 7篇 |
1972年 | 6篇 |
1971年 | 7篇 |
1969年 | 5篇 |
1959年 | 8篇 |
1958年 | 16篇 |
1957年 | 17篇 |
1956年 | 16篇 |
1955年 | 9篇 |
1954年 | 16篇 |
1953年 | 24篇 |
1952年 | 12篇 |
1951年 | 10篇 |
1950年 | 14篇 |
1948年 | 8篇 |
排序方式: 共有747条查询结果,搜索用时 15 毫秒
1.
The distribution and percent frequency of the 13 species and one variety of Danthonia recorded on the New England Tablelands were examined in a survey of 110 predominantly pastoral sites. The relationships between site factors and the presence and percent frequency of the different Danthonia species were subjected to canonical correlation analysis. Individual species were found to have particular sets of environmental factors associated with their distribution and percent frequency; a fact not well recognized in the past. The most common factor associated with the abundance of Danthonia was either a long time since the last cultivation or no cultivation at all. Other factors such as altitude, drainage, soil parent material, soil texture and total soil phosphorus status were also of some importance. The six species most commonly recorded throughout the New England Tablelands were D. racemosa RBr, D. pilosa RBr, D. laevis J. Vickery, D. linkii Kunth., D. richardsonii Cashmore and D. tenuior (Steud.) Conert. Of these the first three are ecologically wide species. The first two can coexist with other naturalized species under grazing while D. laevis tends to disappear with disturbance. D. linkii and D. richardsonii require fairly narrow habitat conditions but these conditions and consequently these species are common. Both species respond positively to disturbance. Danthonia linkii increases with cultivation and D. richardsonii colonizes eroded soil surfaces. Danthonia tenuior has more restricted requirements and is not so frequent The other seven species and one variety did not occur often enough for detailed study and the occurrence of two of these species, D. carphoides and D. caespitosa must be considered as rare. 相似文献
2.
3.
4.
5.
Field techniques for assessing the reaction of winter wheat cultivars to Septoria nodorum 总被引:3,自引:0,他引:3
Methods of assessing the reaction of winter wheat cultivars to Septoria nodorum were compared in nine field trials. Plots were inoculated either by scattering naturally infected wheat debris or by spraying with a suspension of conidia at different growth stages, and were subsequently kept damp by several methods. Disease development was monitored by assessing the percentage cover of different organs by lesions, or by measuring the maximum height of lesions above ground. Major differences between cultivars were evident with all methods of inoculation and assessment, but time of assessment was critical for best differentiation. Inoculation with conidia soon after ear emergence, followed by assessment of lesions on flag leaves and ears, was a simple and effective method. Yield loss due to disease was measured by making comparisons either between inoculated and uninoculated plots, with various degrees of separation, or between inoculated plots with and without fungicidal treatment: the first of these methods was more effective in maintaining a disease differential. Yield loss was positively correlated with severity of disease and there was little evidence that cultivars differed in tolerance of disease. Disease was measured more easily and with less error than yield loss. 相似文献
6.
DAVID WEETMAN LORENZ HAUSER PAUL W. SHAW MICHELLE K. BAYES 《Molecular ecology resources》2005,5(2):361-362
Five microsatellite loci are described for the commercially exploited marine gastropod, Buccinum undatum. Levels of polymorphism were variable with three to 19 alleles per locus and expected heterozygosities of 0.26–0.94 in 60 individuals of the population from which the loci were isolated. Homozygote excess at two of the loci might be attributable to null alleles, and these loci should not be used in, for example, parentage analysis. Nevertheless, because null allele frequencies can be estimated and their effects partitioned, all are useful markers for studies of population differentiation. 相似文献
7.
SCOTT LYELL GARDNER 《Zoological Journal of the Linnean Society》1991,102(2):169-201
Coevolution was studied in six species of rodents of the genus Ctenomys and their parasitic nematodes of the genus Paraspidodera , collected in Bolivia. Representatives of the families Octodontidae and Caviidae were used as outgroups for the mammals, and nematodes from caviids were used as outgroups of the nematodes from ctenomyids. For the nematodes, quantitative and qualitative morphological characteristics of both males and females and electrophoretic characters of both sexes were used to generate phylogenetic hypotheses of evolutionary relationships of the OTUs occurring in hosts of different species. Concordance estimates of cladograms generated from biochemical—genetic and morphological data of the nematodes show a percentage incongruence (Mickevich-Farris Incongruence Statistic or I MF ) of 8.23% in the character sets. Parsimony mapping, testing concordance of topologies between the trees derived from both analysis of both morphological and biochemical—genetic data indicates an overall agreement of 82.3°. Comparisons of topologies of the host and parasite cladograms, as measured with parsimony mapping, showed 70.8% concordance, indicating substantially more cospeciation than host-switching in the Ctenomys-Paraspidodera host-parasite system. Nematodes of the genus Paraspidodera appear to have invaded the Ctenomys lineage from an origin in caviids sometime before the ctenomyids began to diversify in early Pleistocene time. 相似文献
8.
The Structure of Calf Liver Cytochrome <Emphasis Type="Italic">b</Emphasis><Subscript>5</Subscript> at 2.8 Å Resolution 总被引:5,自引:0,他引:5
CYTOCHROME b5 is a haem-containing protein in the microsomes of liver tissue. It interacts specifically with a flavo-protein, cytochrome b5 reductase, which catalyses the transfer of electrons from NADH to the haem iron of the cytochrome1. The microsomal cytochrome b5 system has been implicated in fatty acid desaturation reactions2 and a similar system in erythrocytes may catalyse the reduction of methaemoglobin3. Calf liver cytochrome b5, solubilized by pancreatic lipase, has a molecular weight of 11,000 and consists of ninety-three amino-acids in the sequence shown in Fig. 1 (refs. 4 and 5). The haem group is non-covalently bound to the protein and can be removed reversibly by acid acetone treatment6. 相似文献
9.
Agonistic Behavior of Mice and Rats: A Review 总被引:1,自引:0,他引:1
This paper reviews the results of research on the agonisticbehavior of wild and tame strains of house mice (Mus musculus)and the Norway rat (Rattus norvegicus) and covers papers thathave appeared since an earlier review (Scott and Fredericson,1951) Various methods for observing, measuring, and eliciting fightingare described and their appropriate uses discussed. Measuresof latency and frequency are more satisfactory than arbitraryrating scales. Situations involving competition over food presentcomplex motivational problems. The basic agonistic behavior patterns (ethograms) of rats andmice are compared. Differences appear in the lack of playfulfighting and weaning threats in mice, the dual "boxing" postureof rats, and the unique "tail rattling" in mice. Both speciesare alike in their inability to form complex dominance hierarchiesin which fighting is reduced to threat and avoidance. As a general theory, each species has evolved behavior patternsand physiological mechanisms of behavior which are adaptivelyrelated to its own social organization and population dynamics.Mice and rats have evolved along different lines both from eachother and other species of mammals. 相似文献
10.
SCOTT COOK 《American anthropologist》1966,68(2):323-345
"Analytic work begins with material provided by our vision of things, and this vision is ideological almost by definition. It embodies the picture of things as we see them, and wherever there is any possible motive for wishing to see them in a given rather than another light, the way in which we see things can hardly be distinguished from the nay in which we wish to see them. The more honest and naive our vision is, the more dangerous it is to the eventual emergence of anything for which general validity can be claimed. The inference for the social sciences is obvious, antl it is not even true that he who hates a social system will form an objectively more correct vision of it than he who loves it. For love distorts indeed, but hate distorts still more." 相似文献