全文获取类型
收费全文 | 124篇 |
免费 | 12篇 |
出版年
2023年 | 1篇 |
2021年 | 4篇 |
2019年 | 3篇 |
2018年 | 2篇 |
2016年 | 3篇 |
2015年 | 4篇 |
2014年 | 1篇 |
2013年 | 8篇 |
2012年 | 4篇 |
2011年 | 10篇 |
2010年 | 3篇 |
2009年 | 7篇 |
2008年 | 11篇 |
2007年 | 2篇 |
2006年 | 4篇 |
2005年 | 6篇 |
2004年 | 4篇 |
2003年 | 2篇 |
2002年 | 5篇 |
2001年 | 4篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 2篇 |
1997年 | 4篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1992年 | 4篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1939年 | 1篇 |
1911年 | 1篇 |
排序方式: 共有136条查询结果,搜索用时 15 毫秒
21.
Aim Most studies of avian insular adaptations have focused on oceanic islands, which may not allow characters that are insular adaptations to be teased apart from those that benefit dispersal and colonization. Using birds on continental islands, we investigated characters that evolved in situ in response to insular environments created by late Pleistocene sea level rise. Location Trinidad and Tobago and continental South America. Methods We weighed fresh flight muscles and measured museum skeletal specimens of seven species of birds common to the continental islands of Trinidad and Tobago. Results When corrected for body size, study species exhibited significantly smaller flight muscles, sterna and sternal keels on Tobago than on larger Trinidad and continental South America. Tobago populations were more ‘insular’ in their morphologies than conspecifics on Trinidad or the continent in other ways as well, including having longer bills, longer wings, longer tails and longer legs. Main conclusions We hypothesize that the longer bills enhance foraging diversity, the longer wings and tails compensate for the smaller pectoral assemblage (allowing for retention of volancy, but with a probable reduction in flight power and speed), and the longer legs expand perching ability. Each of these differences is likely to be related to the lower diversity and fewer potential predators and competitors on Tobago compared with Trinidad. These patterns of smaller flight muscles and larger bills, legs, wings and tails in island birds are not the results of selection for island dispersal and colonization, but probably arose from selection pressures acting on populations already inhabiting these islands. 相似文献
22.
23.
The influence of net surface charge on the interaction of uropathogenic Escherichia coli with human neutrophils 总被引:2,自引:0,他引:2
R Steadman J Knowlden M Lichodziejewska J Williams 《Biochimica et biophysica acta》1990,1053(1):37-42
Escherichia coli strains, grown to suppress fimbrial expression, synthesised enhanced quantities of polysaccharide capsule, which significantly lessened their binding to heparin sepharose columns. In the presence of poly-L-lysine, these strains were strongly retained on the columns confirming their highly anionic nature. Uropathogenic strains of E. coli expressing type 1 fimbrial adhesins activated the respiratory burst, the degranulation response and the release of leukotrienes from human neutrophils (PMN) to a significantly greater extent than the same strains grown in a medium to suppress this fimbrial expression. The addition of the poly-cation poly-L-lysine, however, selectively increased neutrophil activation in response to these non-fimbriate strains. This dose-dependent effect was reversed by the addition of heparin suggesting a mechanism dependent on surface charge. The results of this study suggest that non-specific mechanisms involving the neutralisation of surface charge, in addition to specific receptor and adhesin mediated events could affect neutrophil activation at sites of infection. 相似文献
24.
L-Lactate dehydrogenase (L-LDH, E.C. 1.1.1.27) is encoded by two or three
loci in all vertebrates examined, with the exception of lampreys, which
have a single LDH locus. Biochemical characterizations of LDH proteins have
suggested that a gene duplication early in vertebrate evolution gave rise
to Ldh-A and Ldh-B and that an additional locus, Ldh-C arose in a number of
lineages more recently. Although some phylogenetic studies of LDH protein
sequences have supported this pattern of gene duplication, others have
contradicted it. In particular, a number of studies have suggested that
Ldh-C represents the earliest divergence among vertebrate LDHs and that it
may have diverged from the other loci well before the origin of
vertebrates. Such hypotheses make explicit statements about the
relationship of vertebrate and invertebrate LDHs, but to date, no closely
related invertebrate LDH sequences have been available for comparison. We
have attempted to provide further data on the timing of gene duplications
leading to multiple vertebrate LDHs by determining the cDNA sequence of the
LDH of the tunicate Styela plicata. Phylogenetic analyses of this and other
LDH sequences provide strong support for the duplications giving rise to
multiple vertebrate LDHs having occurred after vertebrates diverged from
tunicates. The timing of these LDH duplications is consistent with data
from a number of other gene families suggesting widespread gene duplication
near the origin of vertebrates. With respect to the relationships among
vertebrate LDHs, our data are not consistent with previous claims that
Ldh-C represented the earliest divergence. However, the precise
relationships among some of the main lineages of vertebrate LDHs were not
resolved in our analyses.
相似文献
25.
Witherington J Abberley L Briggs MA Collis K Dean DK Gaiba A King NP Kraus H Shuker N Steadman JG Takle AK Sanger G Wadsworth G Butler S McKay F Muir A Winborn K Heightman TD 《Bioorganic & medicinal chemistry letters》2008,18(6):2203-2205
A series of small molecule orally bioavailable ghrelin receptor agonists have been identified through systematic optimisation of a high throughput screening hit. 相似文献
26.
Rowena L. Long Kathryn J. Steadman F. Dane Panetta Stephen W. Adkins 《Plant and Soil》2009,320(1-2):131-140
To investigate the effects of soil type on seed persistence in a manner that controlled for location and climate variables, three weed species—Gomphocarpus physocarpus (swan plant), Avena sterilis ssp. ludoviciana (wild oat) and Ligustrum lucidum (broadleaf privet)—were buried for 21 months in three contrasting soils at a single location. Soil type had a significant effect on seed persistence and seedling vigour, but soil water content and temperature varied between soils due to differences in physical and chemical properties. Warmer, wetter conditions favoured shorter persistence. A laboratory-based test was developed to accelerate the rate of seed ageing within soils, using controlled superoptimal temperature and moisture conditions (the soil-specific accelerated ageing test, SSAAT). The SSAAT demonstrated that soil type per se did not influence seed longevity. Moreover, the order in which seeds aged was the same whether aged in the field or SSAAT, with L. lucidum being shortest-lived and A. sterilis being longest-lived of the three species. 相似文献
27.
28.
Aim By establishing a prehistoric fossil record for the vertebrate fauna of Guam, we can document the composition of the island’s fauna prior to Western contact. It will also complement the extensive prehistoric fossil data already known from the nearby islands of Rota, Tinian and Aguiguan, and improve our understanding of natural distributional patterns through a chronologically deeper view of the biogeography of the Mariana Islands. Location Northern Guam, Mariana Islands, Micronesia. Methods Fossil‐bearing sediment was removed from karst features with vertically and horizontally controlled excavation methods. Age determinations of the fossils were estimated by radiocarbon dating associated wood charcoal samples. Results The 3314 bones (fossils) of terrestrial vertebrates date to the past two millennia and represent 34 species. Among 12 species of squamate reptiles, one is extinct (a gekkonid lizard) and another, the monitor lizard, Varanus indicus, was introduced to Guam no later than about 1600 years ago. The 17 species of birds feature five that are extinct. Two others not recorded previously on Guam are extirpated. Eight others were lost on Guam in historic times. We also recovered bones of two species of indigenous bats, (Pteropus sp., extirpated; Emballonura semicaudata, probably extirpated), and non‐native rodents, (Rattus spp.). Main conclusions Guam’s contemporary faunal losses have been well documented since invasion of the Brown Treesnake (Boiga irregularis) during or shortly after WWII. Other exotics, as well as habitat deterioration, have abetted this snake’s deleterious presence. Our data further show that human impact led to declines or losses of vertebrate populations by late prehistoric times. Numerous anthropogenic extirpations would not be evident without studying fossils. Our chrono‐stratigraphic evidence argues for the arrival of non‐native rats (Rattus sp.) only about 1000 years ago, which helps to explain why a flightless rail survived on Guam into modern times, unlike on most other islands in Oceania. 相似文献
29.
Rowena L. Long Ilse Kranner F. Dane Panetta Simona Birtic Steve W. Adkins Kathryn J. Steadman 《Plant and Soil》2011,338(1-2):511-519
Seeds in the field experience wet-dry cycling that is akin to the well-studied commercial process of seed priming in which seeds are hydrated and then re-dried to standardise their germination characteristics. To investigate whether the persistence (defined as in situ longevity) and antioxidant capacity of seeds are influenced by wet-dry cycling, seeds of the global agronomic weed Avena sterilis ssp. ludoviciana were subjected to (1) controlled ageing at 60% relative humidity and 53.5°C for 31 days, (2) controlled ageing then priming, or (3) ageing in the field in three soils for 21 months. Changes in seed viability (total germination), mean germination time, seedling vigour (mean seedling length), and the concentrations of the glutathione (GSH) / glutathione disulphide (GSSG) redox couple were recorded over time. As controlled-aged seeds lost viability, GSH levels declined and the relative proportion of GSSG contributing to total glutathione increased, indicative of a failing antioxidant capacity. Subjecting seeds that were aged under controlled conditions to a wet-dry cycle (to ?1 MPa) prevented viability loss and increased GSH levels. Field-aged seeds that underwent numerous wet-dry cycles due to natural rainfall maintained high viability and high GSH levels. Thus wet-dry cycles in the field may enhance seed longevity and persistence coincident with re-synthesis of protective compounds such as GSH. 相似文献
30.
Danica E. Goggin Stephen B. PowlesPeter E. Toorop Kathryn J. Steadman 《Journal of plant physiology》2011,168(6):527-533
Dormancy release in freshly matured, imbibed annual ryegrass (Lolium rigidum) seeds is inhibited by light and involves a decrease in seed sensitivity to abscisic acid. Other processes involved in dormancy release in the dark were investigated by measuring seed storage compound mobilisation and the activity of cell wall-degrading enzymes. Activities of endo-β-mannanase and total peroxidase were higher in dark-stratified compared to light-stratified seeds, indicating that weakening of the structures constraining the embryo was accelerated in the dark. A dramatic degradation of storage proteins in light-stratified seeds, accompanied by induction of a high molecular mass protease, suggests that maintenance of storage(-like) proteins is also important in dark-mediated dormancy release. α-Amylase activity was induced in dark-stratified seeds at least 48 h prior to radicle emergence upon transfer to conditions permitting germination, or in light-stratified seeds supplied with exogenous gibberellin A4. This suggests that (a) α-amylase is involved in stimulation of germination of non-dormant L. rigidum seeds, and (b) dark-stratified seeds have an increased sensitivity to gibberellins which permits the rapid induction of α-amylase activity upon exposure to germination conditions. Overall, it appears that a number of processes, although possibly minor in themselves, occur in concert during dark-stratification to contribute to dormancy release. 相似文献