全文获取类型
收费全文 | 489篇 |
免费 | 35篇 |
专业分类
524篇 |
出版年
2013年 | 5篇 |
2012年 | 5篇 |
2011年 | 7篇 |
2010年 | 12篇 |
2009年 | 8篇 |
2008年 | 10篇 |
2007年 | 9篇 |
2006年 | 12篇 |
2002年 | 5篇 |
2001年 | 6篇 |
2000年 | 7篇 |
1999年 | 10篇 |
1998年 | 6篇 |
1997年 | 8篇 |
1996年 | 4篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 9篇 |
1989年 | 13篇 |
1988年 | 6篇 |
1987年 | 12篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 13篇 |
1978年 | 12篇 |
1977年 | 5篇 |
1975年 | 6篇 |
1974年 | 6篇 |
1973年 | 6篇 |
1972年 | 8篇 |
1971年 | 9篇 |
1959年 | 12篇 |
1958年 | 18篇 |
1957年 | 22篇 |
1956年 | 24篇 |
1955年 | 31篇 |
1954年 | 13篇 |
1953年 | 18篇 |
1952年 | 14篇 |
1951年 | 11篇 |
1950年 | 9篇 |
1949年 | 10篇 |
1948年 | 8篇 |
排序方式: 共有524条查询结果,搜索用时 9 毫秒
181.
JOCELYN DELA-CRUZ TIM PRITCHARD GEOFFREY GORDON PENELOPE AJANI 《Freshwater Biology》2006,51(5):951-972
1. Periphytic diatoms are used as indicators of water quality because their ecological tolerances or preferences to environmental variables are thought to be predictable. However, much of the present autecological information for periphytic diatoms has been derived from studies conducted in the northern hemisphere. In this present study we used periphytic diatoms to determine the impacts of inorganic nutrient pollution in a tidal river system in the temperate latitudes of south‐east Australia. In so doing, we assess the suitability of the use of the ‘northern hemisphere’ ecological tolerance/preference data for periphytic diatoms. 2. Artificial substrates were used to collect periphytic diatoms at 35 sites, which were positioned along the riverbanks and the middle of the river at various distances upstream and downstream of the sewage outfall. The sampling design took into account tidal excursions and the observed sewage plume dynamics. Periphytic diatoms were collected during the austral winter month of August and the austral spring months of September and October. We deployed the artificial substrates for 4 weeks to allow the periphytic diatoms to recruit and colonise, before identifying and enumerating the assemblages. 3. Data analysis included two approaches: multivariate visualisations of combinations of environmental and biological data to investigate shifts in species structure of the periphytic diatom assemblage and multimetric indices based on ecological tolerance/preference data. 4. We found that the spatial patterns inferred from multivariate and multimetric analyses were consistent. Temporal variation in the composition of the periphytic diatom assemblage was greater than the spatial variation along horizontal sections of the river (in any one deployment) due mainly to shifts between winter and spring species. 5. Outfall effects were most apparent in winter, possibly because subsequent deployments were swamped by growth of spring periphytic diatoms. The outfall effects included a shift towards pollutant tolerant species and a reduction in the variability of the periphytic diatom assemblage across the river. 6. We conclude that the use of periphytic diatoms and associated ecological tolerance/preference data as a means of assessing impacts of point source inorganic nutrient pollution is effective. An understanding of river and sewage flow patterns is essential to the design of appropriate monitoring programmes and to the interpretation of results, especially as periphytic diatoms are sensitive to many environmental variables. 相似文献
182.
Distribution and conservation of mobile elements in the genus Drosophila 总被引:13,自引:1,他引:12
Essentially nothing is known of the origin, mode of transmission, and
evolution of mobile elements within the genus Drosophila. To better
understand the evolutionary history of these mobile elements, we examined
the distribution and conservation of homologues to the P, I, gypsy, copia,
and F elements in 34 Drosophila species from three subgenera. Probes
specific for each element were prepared from D. melanogaster and hybridized
to genomic DNA. Filters were washed under conditions of increasing
stringency to estimate the similarity between D. melanogaster sequences and
their homologues in other species. The I element homologues show the most
limited distribution of all elements tested, being restricted to the
melanogaster species group. The P elements are found in many members of the
subgenus Sophophora but, with the notable exception of D. nasuta, are not
found in the other two subgenera. Copia-, gypsy-, and F-element homologues
are widespread in the genus, but their similarity to the D. melanogaster
probe differs markedly between species. The distribution of copia and P
elements and the conservation of the gypsy and P elements is inconsistent
with a model that postulates a single ancient origin for each type of
element followed by mating-dependent transmission. The data can be
explained by horizontal transmission of mobile elements between
reproductively isolated species.
相似文献
183.
The structure and interrelationship of fungal populations in native and cultivated soils 总被引:4,自引:1,他引:3
Mitochondrial DNA haplotypes have been characterized for 120 isolates of the asexual fungus Fusarium oxysporum. Sixty of these isolates were obtained from soil in a native grassland in the San Joaquin Valley of California, including 20 isolates from each of six different sampling locations. The same sampling strategy was used to obtain 60 additional isolates from an agricultural field of the same soil type directly adjacent to the native soil. Twenty-three different mitochondrial DNA haplotypes were identified among the 120 isolates, including 11 haplotypes represented by two or more isolates and 12 that were unique. The five most common mitochondrial DNA haplotypes accounted for 93 (78%) of the 120 isolates. Isolates representing each of these five mitochondrial DNA haplotypes were found both in the cultivated and in the native soil. Seventy-two per cent of the isolates found in the cultivated soil were associated with the same mitochondrial DNA haplotype as one or more isolates in the native soil. The remaining isolates in the cultivated soil were associated with comparatively rare mitochondrial DNA haplotypes, most of which showed a close relationship to one of the haplotypes found in the native soil. Hierarchial gene diversity analysis indicated that a significant proportion of the mitochondrial DNA haplotype diversity was attributable to differences between sampling sites in the native soil but not in the cultivated soil. This may reflect significant spatial structuring of genetic diversity in populations of F. oxysporum in a native soil. The proportion of mtDNA haplotype diversity attributable to differences between populations in the native and cultivated soils was not significant. This suggests that our entire collection, encompassing strains from both native and cultivated soils, is representative of a single population of F. oxysporum. 相似文献
184.
JOHN ALCOCK EDWARD M. BARROWS GORDON GORDH L. JAN HUBBARD LARRY KIRKENDALL DONALD W. PYLE TERRY L. PONDER FRANK G. ZALOM 《Zoological Journal of the Linnean Society》1978,64(4):293-326
Males of aculeate Hymenoptera differ in the behavioural adaptations employed to locate and secure mates. The ecological and evolutionary bases of these differences are explored in this paper. Male bees and wasps search for females by patrolling widely within emergence-nesting areas or within patches of flowers attractive to conspecific females, or by waiting at landmarks, at specific emergence sites, or at nests. Nest dispersion, flower distribution, the type of female mating system and the nature of male-male competition appear to be key factors in determining the mate-locating behaviour of males. Of special interest in multiple-mating by females, which may be an evolutionary response to the costs of attempting to resist copulation in certain situations. When polyandry occurs, males are under selection pressure to be the last male to copulate with a female prior to oviposition if sperm precedence occurs. In species in which females mate just once, a selective premium is placed on being the first male to reach a virgin female. In either case, because receptive females are a limited resource, there is intense competition among males for access to the resource. The density of competitor males may play an important role in determining whether holding a relatively restricted territory is preferable to the strategy of patrolling widely at various sites which may have females. Territoriality is practiced by males of several species of aculeate Hymenoptera when the number of male competitors is relatively few in number and the distribution of emergence sites or foraging areas of females is clumped in space. 相似文献
185.
Long-term Partitioning, Storage and Re-mobilization of 14C Assimilated by Lolium perenne (cv. Melle)
The youngest fully expanded leaves of single tillers of vegetativeperennial ryegrass plants were exposed to 14CO2. Thereafter,quantitative and fractional analysis of the partitioning, storageand re-mobilization after defoliation of the 14C-labelled assimilatewas sequentially conducted over a 22 d period. In undefoliated plants, most 14C reached its final destinationwithin 56 of feeding. Forty per cent of assimilated 14Cwas subsequently lost through respiration, while 13.5, 8.5 and34 per cent remained in roots, stem bases and tops respectively.At least some 14C was distributed to tillers throughout theplant, but secondary tillers subtended by the fed tiller madethe greatest demand on 14C translocated from the fed tiller. A small, but significant portion of 14C was invested into longterm storage in undefoliated plants, four per cent of the totalassimilated still being present in a labile chemical form inroots and stem bases 22 d after feeding. In plants that wereseverely defoliated 4 d after feeding, depletion of reserve14C was observed relative to undefoliated plants. The depletiontook place from stem bases, not roots, and both low and highmolecular weight storage compounds were involved. A portionof the depleted 14C was incorporated into new growth after defoliation. Lolium perenne, perennial ryegrass, assimilate partitioning, storage, re-mobilization, defoliation 相似文献
186.
187.
188.
WE have used bilayer lipid membranes (BLM) as models for biological membranes to study the transport of metal halides as ion-pairs. In the presence of iodine (I2) we find1 that various monovalent and divalent halides can readily move across BLM along a concentration gradient. The rate of transport increases as the size of the cation increases. As the membrane interior is thought to be of a liquid hydrocarbon nature, these results can be related to those of solvent extraction studies2 which showed that various metal halides could be extracted efficiently from aqueous solutions into organic solvents; the extraction efficiency increases as the cation size increases. 相似文献
189.
Morphological variation of chewing lice (Insecta: Phthiraptera) from different skua taxa 总被引:4,自引:0,他引:4
ROSLI RAMLI MAGGIE CUSACK GORDON B. CURRY ROBERT W. FURNESS 《Biological journal of the Linnean Society. Linnean Society of London》2000,71(1):91-101
The lice, Haffneria grandis and Saemundssonia were extracted from their hosts, skuas (Aves: Stercorariidae). Lice were extracted from dead birds by combing feathers, while lice from live birds were extracted using a delousing chamber containing chloroform vapour. Lice were measured and the data analysed by canonical discriminant analysis. Lice show variation in morphology that is useful in identifying some of these hosts. This variation is presumably due to micro-environmental pressure provided by each host. Our interpretation is that these chewing lice have evolved to be adapted to each skua taxon; different sizes and morphologies of skua taxa result in different sizes and morphologies of Haffneria grandis and Saemundssonia. 相似文献
190.