全文获取类型
收费全文 | 316篇 |
免费 | 8篇 |
国内免费 | 1篇 |
出版年
2015年 | 4篇 |
2014年 | 3篇 |
2013年 | 6篇 |
2012年 | 9篇 |
2011年 | 14篇 |
2010年 | 22篇 |
2009年 | 16篇 |
2008年 | 18篇 |
2007年 | 13篇 |
2006年 | 13篇 |
2005年 | 12篇 |
2004年 | 14篇 |
2003年 | 7篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 13篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 8篇 |
1991年 | 3篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1980年 | 4篇 |
1977年 | 2篇 |
1975年 | 3篇 |
1973年 | 3篇 |
1971年 | 2篇 |
1959年 | 4篇 |
1958年 | 9篇 |
1957年 | 4篇 |
1956年 | 3篇 |
1955年 | 7篇 |
1954年 | 2篇 |
1953年 | 2篇 |
1952年 | 4篇 |
1951年 | 1篇 |
1950年 | 1篇 |
1949年 | 8篇 |
1948年 | 4篇 |
1947年 | 2篇 |
1935年 | 1篇 |
排序方式: 共有325条查询结果,搜索用时 15 毫秒
71.
HORACIO ZAGARESE CRAIG WILLIAMSON TIMOTHY VAIL OLAF OLSEN & CLAUDIA QUEIMALIÑOS 《Freshwater Biology》1997,37(1):99-106
1. The freshwater calanoid copepod Boeckella gibbosa is typical of high elevation lakes and ponds in Patagonia (Argentina). Previous studies have shown that this species is highly tolerant to short-term exposure to natural and artificial UVB radiation, and that its tolerance is due to photoreactivation by longer wavelength radiation. In this study, we investigate the potential sublethal effects of solar radiation after prolonged exposure.
2. We incubated B. gibbosa at 1 m depth in oligotrophic Lake Toncek for 24 days. The incubation chambers were 1.2 l acrylic cylinders covered with appropriate filters in order to obtain three radiation treatments: visible radiation only, visible radiation + UVA and visible radiation + UVA + UVB.
3. The three treatments did not differ significantly in variables considered as indicators of survival (number of individuals), reproduction (proportion of ovigerous females, clutch size) and development (instar composition). Although resistance to solar UVB radiation is certainly a requisite to live in transparent high elevation habitats, the fact of being effectively exposed to natural levels of UVB radiation does not seem to have measurable consequences on an already adapted species, such as B. gibbosa 相似文献
2. We incubated B. gibbosa at 1 m depth in oligotrophic Lake Toncek for 24 days. The incubation chambers were 1.2 l acrylic cylinders covered with appropriate filters in order to obtain three radiation treatments: visible radiation only, visible radiation + UVA and visible radiation + UVA + UVB.
3. The three treatments did not differ significantly in variables considered as indicators of survival (number of individuals), reproduction (proportion of ovigerous females, clutch size) and development (instar composition). Although resistance to solar UVB radiation is certainly a requisite to live in transparent high elevation habitats, the fact of being effectively exposed to natural levels of UVB radiation does not seem to have measurable consequences on an already adapted species, such as B. gibbosa 相似文献
72.
MARK A. ADAMS REAS RICHTER ALAN K. HILL & TIMOTHY D. COLMER 《Plant, cell & environment》2005,28(6):772-787
In four species of salt-tolerant eucalypts (Eucalyptus raveretiana, E. spathulata, E. sargentii and E. loxophleba), we found substantial concentrations of quercitol – a cyclitol known for its accumulation in seeds of Quercus. Quercitol was absent in old foliage of E. globulus, a species noted for greater susceptibility to salinity, and also absent in the moderately tolerant E. camaldulensis, but, relative to other species, both had higher foliar concentrations of inositol. Simple sugars and cyclitols accumulated to osmotically significant concentrations in all species. The osmotic potential of expressed sap was always less than that of the external ‘soil’ solution and increasing salinity produced predictable reductions in growth and increases in ion concentrations in foliage of saplings of four eucalypt species. The more salt-tolerant species, E. spathulata, E. loxophleba and E. sargentii, were able to maintain well-regulated leaf Na+ concentrations even at 300 mol m−3 NaCl. These more salt-tolerant species also showed an apparent increase in net selectivity for K+ over Na+ as salinity increased, irrespective of the Na+ : Ca2+ ratio of the external medium (range 25 : 1 to 75 : 1; Ca2+ always ≥ 4.0 mol m−3). By contrast, E. globulus was unable to exclude Na+ when exposed to higher NaCl concentrations (e.g. 200 and 300 mol m−3). Carbon isotope signatures of foliage reflected imposed salinity but were not strongly enough correlated with growth to support previous suggestions that isotope discrimination be a means of evaluating salt tolerance. On the other hand, patterns of sugar and cyclitol accumulation should be further explored in eucalypts as traits contributing to salt tolerance, and with potential use as markers in breeding programmes. 相似文献
73.
74.
TIMOTHY J. KURTTI K. ROGER TSANG MARION A. BROOKS 《The Journal of eukaryotic microbiology》1983,30(4):652-657
Nosema disstriae, a parasite of the forest tent caterpillar, Malacosoma disstria, was cultured with cell lines UMN-MDH-1 (Malacosoma disstria), IPLB-1075 (Heliothis zea), and BTC-32 (Triatoma infestans). Infected cultured cells were used to infect the healthy cell lines. Electron micrographs of thin sections of 6-day-old cultures revealed infected cells that exocytosed vesicles containing vegetative and immature sporulating forms of the parasite. Some of these forms were believed to be responsible for intercellular transmission of the parasite. The spread of infection was augmented by culturing the cells at high densities; if the density was too low, there was little or no cross infection. Cross infection was inhibited, but not blocked completely, by high osmolality of the culture medium. The yield of spores from a confluent cell monolayer at the end of growth was generally 1–4 × 107 per ml of culture medium. 相似文献
75.
TIMOTHY F. MURPHY 《Bioethics》2013,27(4):224-232
Sex selection of children by pre‐conception and post‐conception techniques remains morally controversial and even illegal in some jurisdictions. Among other things, some critics fear that sex selection will distort the sex ratio, making opposite‐sex relationships more difficult to secure, while other critics worry that sex selection will tilt some nations toward military aggression. The human sex ratio varies depending on how one estimates it; there is certainly no one‐to‐one correspondence between males and females either at birth or across the human lifespan. Complications about who qualifies as ‘male’ and ‘female’ complicate judgments about the ratio even further. Even a judiciously estimated sex ratio does not have, however, the kind of normative status that requires society to refrain from antenatal sex selection. Some societies exhibit lopsided sex ratios as a consequence of social policies and practices, and pragmatic estimates of social needs are a better guide to what the sex ratio should be, as against looking to ‘nature’. The natural sex ratio cannot be a sound moral basis for prohibiting parents from selecting the sex of their children, since it ultimately lacks any normative meaning for social choices. 相似文献
76.
CAROLYNE BARDELEBEN RACHAEL L. MOORE JAMES A. NICHOLLS JEREMY J. AUSTIN TIMOTHY E. ROBSON 《Molecular ecology resources》2005,5(2):305-307
We isolated 13 polymorphic microsatellite markers from the satin bowerbird, Ptilonorhynchus violaceus from a genomic library enriched in (AAGG)n repetitive elements and characterized them in 20 individuals. The number of alleles ranged from two to 18 per locus with the observed heterozygosity ranging from 0.15 to 1.00. These markers will be useful for analysing questions concerning parentage, population genetic structure and models of speciation. 相似文献
77.
Sabellid regeneration and the expression of polarity are knownto be under direct neural control. During regeneration nervesexhibit both neurotrophic and neurosecretory capacities. Indeed,it is proposed that the basic neurotrophic control mechanismis neurosecretory in nature, involving the synthesis, packagingand transport of neurosecretory granules to the wound site.The expression of polarity is envisioned as a cellular differentiatingresponse to a threshold increase in neurohormone. Under maximalstimulus, anterior regeneration involves the sequential synthesisand assembly of a rich hyaluronate matrix followed by the depositionof chondroitin sulfate and collagen to form the normal adultcrown. Posterior regeneration, according to the model, is underminimal stimulation and does not progress beyond the hyaluronatestage. The relationship between neural control and the assemblyof extracellular macromolecules during regeneration is discussedin relation to the proposed model. 相似文献
78.
79.
MICHAEL TURELLI JERRY A. COYNE TIMOTHY PROUT 《Biological journal of the Linnean Society. Linnean Society of London》1984,22(2):95-106
Three capture-recapture experiments were performed on natural populations of Drosophila from orchards in California. Two of the experiments investigated choice between known breeding substrates: oranges v. grapefruits and figs v. peaches. The third involved choice between citrus baits in a citrus orchard and both banana and citrus baits in an adjacent almond orchard. Drosophila pseudoobscura showed no fidelity to the bait or area of initial capture. However, a statistically significant but low level of resource fidelity was displayed by D. melanogaster/D. simulans. An index of differential choice is used to compare our results to those of previous studies. Possible biological bases of choice behaviour are discussed. 相似文献
80.