全文获取类型
收费全文 | 198篇 |
免费 | 11篇 |
专业分类
209篇 |
出版年
2013年 | 5篇 |
2012年 | 4篇 |
2011年 | 3篇 |
2010年 | 7篇 |
2009年 | 8篇 |
2007年 | 7篇 |
2006年 | 4篇 |
2005年 | 4篇 |
2000年 | 2篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 6篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 8篇 |
1988年 | 3篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 4篇 |
1981年 | 3篇 |
1980年 | 7篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1972年 | 4篇 |
1971年 | 6篇 |
1970年 | 1篇 |
1969年 | 2篇 |
1967年 | 1篇 |
1964年 | 1篇 |
1958年 | 6篇 |
1957年 | 1篇 |
1956年 | 2篇 |
1955年 | 1篇 |
1954年 | 7篇 |
1953年 | 6篇 |
1952年 | 3篇 |
1951年 | 2篇 |
1950年 | 3篇 |
1949年 | 1篇 |
1948年 | 5篇 |
1940年 | 1篇 |
排序方式: 共有209条查询结果,搜索用时 15 毫秒
1.
2.
We assessed the extent to which plants can acquire amino acids when supplied as single N-sources or when plants have access to a mixture of amino- and inorganic N sources. Because the uptake of different N-sources is temperature-dependent, the effects of temperature on amino-N uptake were also tested. Lolium perenne (perennial rye-grass) was grown hydroponically at 11 °C or 21 °C. Uptake of N was determined using 15N tracers at the growth temperature from solutions containing either nitrate, ammonium or glycine as single N sources and from a mixture containing all three N-forms. Estimates of the relative importance of amino acids such as glycine to the total N budget of plants will have been underestimated in studies where uptake was determined in single source solutions compared with those from solutions containing a mixture of N-forms. The proportion of total N acquired from the mixed N source as ammonium increased as temperature was reduced. Regarding the uptake and initial metabolism of glycine, uptake was probably the rate limiting step at 11 °C whilst it was the metabolism of glycine to serine at 21 °C. Although 15N incorporation into the plant amino-N pool was generally in proportion to the abundance of individual amino acids, its incorporation into the glycine pool was sometimes significantly less than predicted. 相似文献
3.
The higher classification of the ant subfamily Leptanillinae (Hymenoptera: Formicidae) 总被引:3,自引:0,他引:3
BARRY BOLTON 《Systematic Entomology》1990,15(3):267-282
Abstract. Until now the ant subfamily Leptanillinae has been closely linked with the army ant subfamilies Dorylinae and Ecitoninae, but on relatively tenuous evidence. The current phylogenetic study strongly indicates that this view is incorrect and that the leptanillines really constitute the sister-group of subfamily Ponerinae, and are at a consider- ably greater taxonomic distance from the Army Ant subfamilies. Three tribes are now recognized within the Leptanillinae (Leptanillini; Anomalornyrmini, new tribe; and Apomyrmini, transferred here from Ponerinae: Arnblyoponini), containing a total of eight genera with fewer than fifty species in all. The subfamily and its component tribes are diagnosed and discussed here, and a key to genera provided. New taxa described include Anomalomyrma Taylor gen.n., type-species A. taylori Bolton sp.n. and Protanilla Taylor gen.n., type-species P. rafflesi Taylor sp.n. 相似文献
4.
Pyrazines as warning odour components in the Monarch butterfly, Danaus plexippus, and in moths of the genera Zygaena and Amata (Lepidoptera) 总被引:1,自引:0,他引:1
MIRIAM ROTHSCHILD BARRY P. MOORE W. VANCE BROWN 《Biological journal of the Linnean Society. Linnean Society of London》1984,23(4):375-380
2-Methoxy-3-alkylpyrazines have been identified as odour components in Danaus plexippus, Zygaena lonicerae and an Australian Amata species, by means of coupled gas chromatography/mass spectrometry, with selected ion monitoring. Wide variability in pyrazine content was observed with D. plexippus and this was correlated with similar variability in the larval food-plants, Asclepias spp., which are presumed to be the odour sources for this butterfly. The importance of pyrazines in the biology of aposematic insects and their role as ingredients of a generalized warning system are discussed. 相似文献
5.
6.
Abstract Habitat usage characteristics of two species of native murid rodents, Pseudomys gracilicaudatus and Rattus lutreolus were investigated on an area of coastal heathland at Myall Lakes National Park. A grid of 151 trap stations comprising 17 traplines was positioned across a mosaic of habitats. At each trap station 19 structural vegetation and physical variables known to affect the microdistribution of small mammals were measured. Multivariate statistical procedures identified those microhabitat variables that contribute to individual species' habitat use and habitat partitioning, and reduce potential competition for space. Cluster analysis classified trap stations into one of six habitat types that were mapped on the study area, identifying a heterogeneous assemblage of interlocking habitats. The pattern is a consequence of topographic variation on the site and, to a lesser extent, its fire history. Trapping results show P. gracilicaudatus and R. lutreolus exhibit similar macrohabitat selection, preferring topographically low habitats, with both species predominantly occupying short dense heath with dense sedge cover. The high overlap in macrohabitat use is greatly reduced when considered trap station by trap station, so that discriminant function and multiple regression analyses demonstrate marked microhabitat selection. Elevation was a highly significant variable, accounting for 41% and 27% of the variance in the habitat used by P. gracilicaudatus and R. lutreolus, respectively. This variable represents a soil moisture gradient that determines changes in the floristic and structural components of the biotic environment. Two other structural vegetation variables and vegetation height contributed 30% of the variance in P. gracilicaudatus distribution. Sedge cover was found to be significant and explained 13% of the variance in R. lutreolus distribution. Within-habitat separation was explained best with a linear combination of variables in a discriminant function, rather than by any single variable. Differential microhabitat selection, interference competition and diet separation appear to be the major factors facilitating coexistence of these two species. 相似文献
7.
Abstract A competitive interaction between two species of sympatric native rodents was demonstrated experimentally in Myall Lakes National Park (NSW, Australia). The previously documented replacement of Pseudomys gracilicaudatus by Rattus lutreolus in areas of wet heath in this region implied that competition may be the mechanism facilitating this succession. We present experimental evidence to support this suggestion. Removal of the larger R. lutreolus caused a significant increase in abundance of P. gracilicaudatus on five experimental sites in comparison to five unmanipulated control sites. The sites used in this experiment were chosen from three ages of wet heath regenerating after fire. These three ages represent key stages in the replacement of P. gracilicaudatus by R. lutreolus and the occurrence of competition in each of these ages indicates that competition plays an important role in this succession. In the initial stages of the experiment when R. lutreolus were removed they were replaced by new R. lutreolus individuals. This has been interpreted as evidence of strong intraspecific competition. After removal of R. lutreolus, P. gracilicaudatus expanded its habitat range into microhabitats that had formerly been occupied by R. lutreolus and reduced its range in microhabitats previously occupied by P. gracilicaudatus. This leads us to believe that P. gracilicaudatus was occupying inferior microhabitat before the removal of R. lutreolus, which had excluded it from more preferable microhabitat. 相似文献
8.
Results are presented for three controlled replicated experiments in which mice were transplanted onto the early successional stages of two regenerating mining paths. Naive house mice (Mus musculus) were successfully introduced on to sites regenerating for 8 years, with 56% remaining after 3 days and 2% still present after 69 days. Naive New Holland mice (Pseudomys novaehollandiae) had marginal establishment success on sites regenerating for 4.9 years, with 21% remaining after 3 days and 3% still present after 22 days. However, on sites regenerating for 3.9 years and 4.4 years, all 17 transplanted New Holland mice had dispersed from the treatment plots after 3 days. New Holland mice have been shown to be competitively dominant to house mice. Hence, we conclude that the reason New Holland mice do not colonize mining paths regenerating for less than 4.9 years is the unsuitability of the habitat rather than their exclusion by house mice. These results are consistent with the early stages of the mammalian succession following sand mining and fit the same habitat accommodation model proposed for post-fire mammalian succession. 相似文献
9.
10.
The internal phylogeny of ants (Hymenoptera: Formicidae) 总被引:5,自引:0,他引:5
Abstract. The higher phylogeny of the Formicidae was analysed using 68 characters and 19 taxa: the 14 currently recognized ant subfamilies plus 5 potentially critical infrasubfamilial taxa. The results justified the recognition of 3 additional subfamilies: Aenictogitoninae Ashmead (new status), Apomyrminae Dlussky & Fedoseeva (new status), and Leptanilloidinae Bolton (new subfamily). A second analysis on these better delimited 17 subfamilies resulted in 24 equally most parsimonious trees. All trees showed a basal division of extant Formicidae into two groups, the first containing (Myrmicinae, Pseudomyrmecinae, Nothomyrmeciinae, Myrmeciinae, Formicinae, Dolichoderinae, Aneuretinae) and the second the remaining subfamilies. Clades appearing within these groups included the Cerapachyinae plus 'army ants', the Nothomyrmeciinae plus Myrmeciinae, the 'formicoid' subfamilies (Aneuretinae + Dolichoderinae + Formicinae), and the Old World army ants (Aenictinae + Aenictogitoninae + Doryline), but relationships within the last two groups were not resolved, and the relative positions of the Apomyrminae, Leptanillinae and Ponerinae remained ambiguous. Moreover, a bootstrap analysis produced a consensus tree in which all branches were represented in proportions much lower than 95%. A reconstruction of the ground plan of the Formicidae indicated that the most specialized of all recent ants are the members of the subfamily Dorylinae and the least specialized ones are the monotypic Apomyrminae. 相似文献