全文获取类型
收费全文 | 287篇 |
免费 | 8篇 |
出版年
2015年 | 2篇 |
2013年 | 6篇 |
2012年 | 7篇 |
2011年 | 9篇 |
2010年 | 19篇 |
2009年 | 29篇 |
2008年 | 21篇 |
2007年 | 23篇 |
2006年 | 10篇 |
2005年 | 11篇 |
2004年 | 4篇 |
2003年 | 3篇 |
2002年 | 2篇 |
1999年 | 5篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 6篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 5篇 |
1973年 | 9篇 |
1972年 | 8篇 |
1971年 | 7篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1958年 | 1篇 |
1957年 | 1篇 |
1955年 | 1篇 |
1954年 | 1篇 |
1952年 | 1篇 |
排序方式: 共有295条查询结果,搜索用时 21 毫秒
31.
Although conchologically an etheriid mussel, M. siamensis from northeastern Thailand is, according to anatomical and larval features, an unionid. Its unique character states among the Unionidae are: relatively distant spacing of septa in all four demibranchs in both, males and females, absence of a foot in adults, and generation of subelliptical glochidial larvae. This species is seemingly a phylogenetic relic. 相似文献
32.
33.
34.
35.
In arid Australia, changes to historic fire regimes may now produce more large‐scale wildfire events. The impacts of these fires on fauna communities are poorly known. We sought to test the impacts of fire on the occurrence of two arid‐zone snake species, the desert death adder (Acanthophis pyrrhus) and monk snake (Parasuta monachus), specialist inhabitants of hummock grassland and mulga shrubland, respectively. We also examined the influence of fire on the occurrence of a habitat generalist, the sympatric Stimson's python (Antaresia stimsoni). Under an Information‐Theoretic framework we modelled the occurrence of each species with a range of habitat variables, including fire history, using logistic regression. As predicted, the two habitat specialists were more likely to be encountered at locations that had a lower percentage of surrounding area burnt in the most recent wildfires (2002), while fire variables failed to predict the occurrence of the habitat generalist. Acanthophis pyrrhus, already predisposed to endangerment through a suite of life‐history characteristics, may be at increased risk through accidental and deliberate burning and fragmentation of old‐growth hummock grasslands. We stress the importance of prescribed burning and natural fire breaks in maintaining areas of old‐growth hummock grassland across the landscape. 相似文献
36.
CARLOS DANIEL CADENA BERNABÉ LÓPEZ-LANÚS JOHN M. BATES NIELS KRABBE NATHAN H. RICE F. GARY STILES JUAN DIEGO PALACIO & PAUL SALAMAN 《Ibis》2007,149(4):814-825
Based on analyses of variation in plumage, morphometrics, vocalizations, and mitochondrial and nuclear DNA sequences, we document the occurrence of interspecific hybridization between a Chestnut-naped Antpitta Grallaria nuchalis and a Chestnut-crowned Antpitta G. ruficapilla in a high-elevation forest fragment in the Cordillera Central of the Andes of Colombia. One hybrid individual was collected and at least two were recorded singing. The hybrid specimen exhibits a combination of phenotypic traits that exclude other species of antpittas as potential parents, and its vocalizations combine elements of songs of both of its parental species. Genetic analyses demonstrate that the hybrid has G. nuchalis mitochondrial DNA and mixed nuclear DNA with copies corresponding to G. nuchalis and G. ruficapilla alleles, demonstrating that the female parent was G. nuchalis. Hybridization is considered extremely rare in most groups of suboscine passerines, but it may be facilitated when populations are decimated as a consequence of deforestation and habitat fragmentation. 相似文献
37.
IN 1958 Cremer et al.1 observed that the concentration of bilirubin (la) in the plasma could be reduced by exposing newborn infants to fluorescent light. Since that time phototherapy has come into wide use to lower elevated bilirubin levels associated with neonatal jaundice (hyperbilirubin-aemia)2, 3. This condition in the newborn has been associated with retarded motor development, irreversible brain damage or even death. Phototherapy lowers bilirubin levels (by conversion of this lipid-soluble pigment to water-soluble products)4 and therefore presumably helps to prevent brain damage. But there are two possible dangers in this treatment: light may have other deleterious effects on the newborn and the photo-products of bilirubin may themselves be toxic. At present neither the structures of the bilirubin photo-products nor their toxicities have been established. Although the photo-destruction of bilirubin has been studied in vivo and in vitro by Ostrow5–7, Schmid4, 7 and others8, 9, these authors investigated principally the visible-ultraviolet spectral changes during the course of bilirubin photo-oxidation and recorded paper chromatographic separations of the photo-products for comparison with the bile or urine of the congenitally jaundiced Gunn rat. More recently McDonagh has shown that singlet oxygen is involved in the self-sensitized photo-oxidation of bilirubin10. In view of the paucity of structural.information on the bilirubin photo-products, we wish to report preliminary findings on the in vitro photo-oxidation products from bilirubin IXa (1a), mesobilirubin IXa (1b) and 5′-oxo-3′,4,4′-triethyl-3,5-dimethyl-l′,5′-dihydro-(2.2′)-dipyrrylmethene (2). We synthesized and carried out our initial studies on 2, which serves as a simplified model for rings I and II of 1a and 1b because it lacks the vinyl group of 1a and the propionic acid β-substituent of 1a and 1b. The visible-ultraviolet spectrum of 2 is quite similar to that of either 1a or 1b11–13. 相似文献
38.
KAUSHIK GHOSE SANJUKTA DEY HANNAH BARTON GARY J. LOAKE DEBABRATA BASU 《Molecular Plant Pathology》2008,9(6):763-775
The lack of availability of sources of resistance against Alternaria brassicicola within the family Brassicaceae has made oilseed mustard plants a target for one of the most damaging and widespread fungal diseases, Alternaria black spot. Of the other non‐host‐resistant/tolerant plants, Sinapis alba, white mustard, is considered to be the most important apart from Arabidopsis. To understand the defence response of S. alba upon incompatible interaction with this pathogen, a functional genomic approach using cDNA amplified fragment length polymorphism was performed. The highly reproducible bands, found to be either more amplified or uniquely present in infected S. alba plants compared with non‐infected plants, were further subjected to comparative reverse Northern analysis in the incompatible white mustard (S. alba) and compatible India mustard (Brassica juncea L.) plants. The suppression of 46% of the genes in the compatible background indicates the possibility of effective and specific recognition of Alternaria in S. alba. Analysis of the 118 genes up‐regulated specifically in infected S. alba compared with B. juncea showed that 98 genes have similarity to proteins such as receptor‐like protein kinase genes, genes involved with calcium‐mediated signalling and salicylic acid‐dependent genes as well as other genes of known function in Arabidopsis. The apparent expression profile data were further confirmed for selected genes by quantitative real‐time polymerase chain reaction analysis. Classification of these genes on the basis of their induction pattern in Arabidopsis indicates that the expression profile of several of these genes was distinct in S. alba compared with B. juncea. 相似文献
39.
CINZIA FISSORE CHRISTIAN P. GIARDINA† CHRISTOPHER W. SWANSTON‡ GARY M. KING§ RANDALL K. KOLKA¶ 《Global Change Biology》2009,15(9):2295-2310
We investigated mean residence time (MRT) for soil organic carbon (SOC) sampled from paired hardwood and pine forests located along a 22 °C mean annual temperature (MAT) gradient in North America. We used acid hydrolysis fractionation, radiocarbon analyses, long-term laboratory incubations (525-d), and a three-pool model to describe the size and kinetics of the acid insoluble C (AIC), active and slow SOC fractions in soil. We found that active SOC was 2 ± 0.2% (mean ± SE) of total SOC, with an MRT of 33 ± 6 days that decreased strongly with increasing MAT. In contrast, MRT for slow SOC and AIC (70 ± 6% and 27 ± 6% of total SOC, respectively) ranged from decades to thousands of years, and neither was significantly related to MAT. The accumulation of AIC (as a percent of total SOC) was greater in hardwood than pine stands (36% and 21%, respectively) although the MRT for AIC was longer in pine stands. Based on these results, we suggest that the responsiveness of most SOC decomposition in upland forests to global warming will be less than currently modeled, but any shifts in vegetation from hardwood to pine may alter the size and MRT of SOC fractions. 相似文献
40.
ABSTRACT Species reintroductions are used commonly as a tool for conservation, but rigorous, quantitative assessments of their outcome rarely occur. Such assessments are critical for determining success of the reintroduction and for identifying management actions needed to ensure persistence of reintroduced populations. We collected 9 years of demographic data on populations of brown-headed nuthatches (Sitta pusilla) and Eastern bluebirds (Sialia sialis) reintroduced via translocation into Long Pine Key, Everglades National Park, Florida, USA. Realized population growth of brown-headed nuthatches was positive in the first 3 years after cessation of translocations (λ2002 = 1.15, SE = 0.13; λ2003 = 1.28, SE = 0.12; λ2005 = 1.32, SE = 0.20) but became negative thereafter (λ2006 = 0.67, SE = 0.10; λ2007 = 0.77, SE = 0.13). Realized growth rate for the Eastern bluebird population did not vary among years and indicated either a stable or a slowly declining population (λ = 0.92, SE = 0.04). Reintroductions were a qualified success; they resulted in the re-establishment of populations of both species, but neither population grew to the extent expected and both remained at risk of extinction. 相似文献