全文获取类型
收费全文 | 288篇 |
免费 | 34篇 |
国内免费 | 32篇 |
出版年
2022年 | 3篇 |
2021年 | 3篇 |
2020年 | 6篇 |
2019年 | 4篇 |
2018年 | 9篇 |
2017年 | 6篇 |
2016年 | 8篇 |
2015年 | 6篇 |
2014年 | 9篇 |
2013年 | 12篇 |
2012年 | 16篇 |
2011年 | 21篇 |
2010年 | 17篇 |
2009年 | 17篇 |
2008年 | 14篇 |
2007年 | 8篇 |
2006年 | 21篇 |
2005年 | 11篇 |
2004年 | 8篇 |
2003年 | 7篇 |
2002年 | 9篇 |
2001年 | 8篇 |
2000年 | 4篇 |
1999年 | 9篇 |
1998年 | 13篇 |
1997年 | 9篇 |
1996年 | 6篇 |
1995年 | 4篇 |
1994年 | 7篇 |
1992年 | 7篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1978年 | 2篇 |
1977年 | 5篇 |
1971年 | 2篇 |
1963年 | 2篇 |
1959年 | 2篇 |
1958年 | 3篇 |
1957年 | 2篇 |
1955年 | 3篇 |
1953年 | 1篇 |
1952年 | 1篇 |
1949年 | 5篇 |
1948年 | 2篇 |
排序方式: 共有354条查询结果,搜索用时 31 毫秒
51.
A Emami-Khoyi DA Hartley RH Cruickshank LJ Boren JG Ross 《New Zealand journal of zoology.》2016,43(4):322-335
New Zealand fur seals are one of many pinniped species that survived the commercial sealing of the eighteenth and nineteenth centuries in dangerously low numbers. After the enforcement of a series of protection measures in the early twentieth century, New Zealand fur seals began to recover from the brink of extinction. We examined the New Zealand fur seal populations of Banks Peninsula, South Island, New Zealand using the mitochondrial DNA control region. We identified a panmictic population structure around Banks Peninsula. The most abundant haplotype in the area showed a slight significant aggregated structure. The Horseshoe Bay colony showed the least number of shared haplotypes with other colonies, suggesting a different origin of re-colonisation of this specific colony. The effective population size of the New Zealand fur seal population at Banks Peninsula was estimated at approximately 2500 individuals. The exponential population growth rate parameter for the area was 35, which corresponds to an expanding population. In general, samples from adjacent colonies shared 4.4 haplotypes while samples collected from colonies separated by between five and eight bays shared 1.9 haplotypes. The genetic data support the spill-over dynamics of colony expansion already suggested for this species. Approximate Bayesian computations analysis suggests re-colonisation of the area from two main clades identified across New Zealand with a most likely admixture coefficient of 0.41 to form the Banks Peninsula population. Approximate Bayesian computations analysis estimated a founder population size of approximately 372 breeding individuals for the area, which then rapidly increased in size with successive waves of external recruitment. The population of fur seals in the area is probably in the late phase of maturity in the colony expansion dynamic. 相似文献
52.
ALOYSIO DA S. FERRÃO FILHO WILLIAM R. DEMOTT & ALAN J. TESSIER 《Freshwater Biology》2005,50(6):954-964
1. The response of three tropical cladocerans to a gradient of resource quality was compared in a series of growth bioassays using seston collected from five lakes of different depth and trophic structure in Michigan, U.S.A. To assess the food quality in terms of digestibility, assimilation experiments were performed with 32P‐labelled seston from the same lakes. Animals were also analysed for P‐content in their tissues at the end of these assays. 2. In general, assimilation efficiency was higher when animals fed on seston from shallow compared to deep lakes, and was significantly correlated with growth rates, suggesting that shallow lakes have the best food resources in terms of digestibility and P availability. 3. Results also showed that all cladoceran species responded similarly to the resource gradient, with lower growth rates in deep lakes and higher growth rates in shallow lakes, although the strength of response (sensitivity) was different among the species tested. 4. The cladoceran Moina micrura was the most sensitive species, and also displayed the highest P‐content and maximal growth rate, a pattern consistent with the growth rate hypothesis. 5. However, seston C : P ratio and growth rates in the different resources did not correlate with the animals’ P‐contents, showing an uncoupling between RNA‐phosphorus demands for growth and seston food quality. 6. In conclusion, our results support the idea that digestion resistance in algae is a major constraint to cladocerans in natural plankton communities. 相似文献
53.
54.
In China, croplands account for a relatively large form of vegetation cover. Quantifying carbon dioxide exchange and understanding the environmental controls on carbon fluxes over croplands are critical in understanding regional carbon budgets and ecosystem behaviors. In this study, the net ecosystem exchange (NEE) at a winter wheat/summer maize rotation cropping site, representative of the main cropping system in the North China Plain, was continuously measured using the eddy covariance technique from 2005 to 2009. In order to interpret the abiotic factors regulating NEE, NEE was partitioned into gross primary production (GPP) and ecosystem respiration (Reco). Daytime Reco was extrapolated from the relationship between nighttime NEE and soil temperature under high turbulent conditions. GPP was then estimated by subtracting daytime NEE from the daytime estimates of Reco. Results show that the seasonal patterns of the temperature responses of Reco and light‐response parameters are closely related to the crop phenology. Daily Reco was highly dependent on both daily GPP and air temperature. Interannual variability showed that GPP and Reco were mainly controlled by temperature. Water availability also exerted a limit on Reco. The annual NEE was ?585 and ?533 g C m?2 for two seasons of 2006–2007 and 2007–2008, respectively, and the wheat field absorbed more carbon than the maize field. Thus, we concluded that this cropland was a strong carbon sink. However, when the grain harvest was taken into account, the wheat field was diminished into a weak carbon sink, whereas the maize field was converted into a weak carbon source. The observations showed that severe drought occurring during winter did not reduce wheat yield (or integrated NEE) when sufficient irrigation was carried out during spring. 相似文献
55.
RACE技术是一项扩增基因末端序列的新技术。该研究从牛BMP4基因出发,以牛软骨的RNA为模板,按照不同物种BMP4基因的相似性设计特异引物,运用PCR和RACE技术扩增并获得了特异片段,该片段经PCR、酶切和测序验证,证实所克隆序列为牛BMP4的3′端序列,包含有1170bp组成的开放读码框(ORF),编码389个氨基酸,3′非编码区121bp个核苷酸和poly(A)15。同源性分析结果表明,牛BMP4 cDNA最大开放读码框所推测的氨基酸序列与已知人、小鼠、大鼠、狗、羊和鸡等真核生物BMP4氨基酸序列进行比较,分别有94.5%、93.1%、91.9%、87.4%、94.2%、79%的同源性。这为克隆其他物种的BMP4基因提供了依据,同时牛骨形态发生蛋白的测序为我们更好的理解牛的生骨机理提供帮助。 相似文献
56.
57.
MÁRCIA FERNANDES DE AQUINO SANTOS JOSÉ RICARDO M. MERMUDES VERA MARIA MEDINA DA FONSECA 《Palaeontology》2011,54(4):807-814
Abstract: A specimen of Curculionidae (Curculioninae) is described as Arariperhinus monnei gen. et sp. nov. The specimen is preserved on a laminated limestone sample of the Crato Formation (Santana Group), Lower Cretaceous (Aptian–Albian), and was collected from a quarry near Nova Olinda, Chapada do Araripe, State of Ceará, Brazil. The genus is placed in the subfamily Curculioninae because of its strongly convex body and relatively slender rostrum, but mainly by its rounded eyes and lack of a prosternal sulcus and tibial spurs. The very prominent eyes in lateral view, a cylindrical rostrum and a straight posterior margin of ventrite II are strong indications that this fossil belongs to the tribe Anthonomini. However, the claws, which would resolve the exact placement of this fossil, are poorly preserved. Arariperhinus monnei gen. et sp. nov. is distinguishable by the combination of several characters and the first record of the family Curculionidae in the Santana Group; it is the oldest record of a member of the subfamily Curculioninae. 相似文献
58.
RICARDO CRUZ DE CARVALHO ANABELA BERNARDES DA SILVA RENATA SOARES ANDRÉ M. ALMEIDA ANA VARELA COELHO JORGE MARQUES DA SILVA CRISTINA BRANQUINHO 《Plant, cell & environment》2014,37(7):1499-1515
All bryophytes evolved desiccation tolerance (DT) mechanisms during the invasion of terrestrial habitats by early land plants. Are these DT mechanisms still present in bryophytes that colonize aquatic habitats? The aquatic bryophyte Fontinalis antipyretica Hedw. was subjected to two drying regimes and alterations in protein profiles and sucrose accumulation during dehydration and rehydration were investigated. Results show that during fast dehydration, there is very little variation in protein profiles, and upon rehydration proteins are leaked. On the other hand, slow dehydration induces changes in both dehydration and rehydration protein profiles, being similar to the protein profiles displayed by the terrestrial bryophytes Physcomitrella patens (Hedw.) Bruch and Schimp. and, to what is comparable with Syntrichia ruralis (Hedw.) F. Weber and D. Mohr. During dehydration there was a reduction in proteins associated with photosynthesis and the cytoskeleton, and an associated accumulation of proteins involved in sugar metabolism and plant defence mechanisms. Upon rehydration, protein accumulation patterns return to control values for both photosynthesis and cytoskeleton whereas proteins associated with sugar metabolism and defence proteins remain high. The current results suggest that bryophytes from different ecological adaptations may share common DT mechanisms. 相似文献
59.
Graham RD Jones Sabrina DA Koetsier 《The Clinical biochemist. Reviews / Australian Association of Clinical Biochemists》2014,35(4):243-250
Reference intervals are commonly considered to allow for between-laboratory bias. The RCPAQAP Liquid Serum Chemistry Program has collected data on laboratory measurements as well as reference intervals. This allows assessment of the between-laboratory variation in results, reference intervals and the information transmitted by the combination of these factors. For the majority of common chemistry analytes, the between-laboratory variation in reference intervals is greater than the variation in results. Additionally the reference interval variation is generally not related to bias between the results. Use of common reference intervals, either as an average of the current intervals in use, or the intervals proposed by the AACB Harmonisation Group, improved the variation seen in the information produced by different laboratories. 相似文献
60.