全文获取类型
收费全文 | 1275篇 |
免费 | 150篇 |
出版年
2023年 | 8篇 |
2022年 | 8篇 |
2021年 | 33篇 |
2020年 | 16篇 |
2019年 | 24篇 |
2018年 | 24篇 |
2017年 | 18篇 |
2016年 | 33篇 |
2015年 | 47篇 |
2014年 | 65篇 |
2013年 | 90篇 |
2012年 | 84篇 |
2011年 | 104篇 |
2010年 | 44篇 |
2009年 | 67篇 |
2008年 | 70篇 |
2007年 | 68篇 |
2006年 | 51篇 |
2005年 | 40篇 |
2004年 | 49篇 |
2003年 | 42篇 |
2002年 | 49篇 |
2001年 | 20篇 |
2000年 | 23篇 |
1999年 | 11篇 |
1998年 | 13篇 |
1997年 | 9篇 |
1996年 | 12篇 |
1994年 | 13篇 |
1993年 | 12篇 |
1992年 | 15篇 |
1991年 | 19篇 |
1990年 | 9篇 |
1989年 | 16篇 |
1988年 | 22篇 |
1987年 | 7篇 |
1986年 | 18篇 |
1985年 | 15篇 |
1984年 | 13篇 |
1983年 | 10篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1975年 | 7篇 |
1973年 | 9篇 |
1972年 | 6篇 |
1970年 | 5篇 |
1964年 | 5篇 |
排序方式: 共有1425条查询结果,搜索用时 46 毫秒
41.
42.
Effect of norleucine on mycelial fragmentation in Cephalosporium acremonium. 总被引:2,自引:0,他引:2 下载免费PDF全文
DL-Norleucine, which is known to replace methionine for stimulation of cephalosporin C formation, also mimics methionine's effect on arthrospore formation. Thus, hyphal fragmentation, like antibiotic biosynthesis, is divorced from a sulfur donation role. 相似文献
43.
44.
45.
46.
Aaron Brewer Zoe Harrold Elliot Chang Drew Gorman‐Lewis Fang‐Zhen Teng 《Geobiology》2020,18(2):225-236
Bacillus subtilis endospore‐mediated forsterite dissolution experiments were performed to assess the effects of cell surface reactivity on Mg isotope fractionation during chemical weathering. Endospores present a unique opportunity to study the isolated impact of cell surface reactivity because they exhibit extremely low metabolic activity. In abiotic control assays, 24Mg was preferentially released into solution during forsterite dissolution, producing an isotopically light liquid phase (δ26Mg = ?0.39 ± 0.06 to ?0.26 ± 0.09‰) relative to the initial mineral composition (δ26Mg = ?0.24 ± 0.03‰). The presence of endospores did not have an apparent effect on Mg isotope fractionation associated with the release of Mg from the solid into the aqueous phase. However, the endospore surfaces preferentially adsorbed 24Mg from the dissolution products, which resulted in relatively heavy aqueous Mg isotope compositions. These aqueous Mg isotope compositions increased proportional to the fraction of dissolved Mg that was adsorbed, with the highest measured δ26Mg (?0.08 ± 0.07‰) corresponding to the highest degree of adsorption (~76%). The Mg isotope composition of the adsorbed fraction was correspondingly light, at an average δ26Mg of ?0.49‰. Secondary mineral precipitation and Mg adsorption onto secondary minerals had a minimal effect on Mg isotopes at these experimental conditions. Results demonstrate the isolated effects of cell surface reactivity on Mg isotope fractionation separate from other common biological processes, such as metabolism and organic acid production. With further study, Mg isotopes could be used to elucidate the role of the biosphere on Mg cycling in the environment. 相似文献
47.
Spencer J. Livengood Richard H. Drew John R. Perfect 《Current fungal infection reports》2020,14(1):40-49
The purpose of this review is to summarize and evaluate relevant literature on combination antifungal therapy for invasive fungal infections (IFIs). Cryptococcal meningitis has the largest body and highest quality in support of combination therapy with amphotericin B and flucytosine. More recent data in treatment of invasive aspergillosis suggest combination therapy with voriconazole and echinocandins may be effective in select patients. Quality studies are needed to define combination therapy in rare mold infections. Multiple strategies have been employed to optimize treatment of the growing incidence of IFIs. With exceptions as noted above, justification for the use of combination antifungal therapy is most often based on uncontrolled and/or underpowered studies, in vitro data, and case reports. 相似文献
48.
49.
The role of tree mortality in the global carbon balance is complicated by strong spatial and temporal heterogeneity that arises from the stochastic nature of carbon loss through disturbance. Characterizing spatio‐temporal variation in mortality (including disturbance) and its effects on forest and carbon dynamics is thus essential to understanding the current global forest carbon sink, and to predicting how it will change in future. We analyzed forest inventory data from the eastern United States to estimate plot‐level variation in mortality (relative to a long‐term background rate for individual trees) for nine distinct forest regions. Disturbances that produced at least a fourfold increase in tree mortality over an approximately 5 year interval were observed in 1–5% of plots in each forest region. The frequency of disturbance was lowest in the northeast, and increased southwards along the Atlantic and Gulf coasts as fire and hurricane disturbances became progressively more common. Across the central and northern parts of the region, natural disturbances appeared to reflect a diffuse combination of wind, insects, disease, and ice storms. By linking estimated covariation in tree growth and mortality over time with a data‐constrained forest dynamics model, we simulated the implications of stochastic variation in mortality for long‐term aboveground biomass changes across the eastern United States. A geographic gradient in disturbance frequency induced notable differences in biomass dynamics between the least‐ and most‐disturbed regions, with variation in mortality causing the latter to undergo considerably stronger fluctuations in aboveground stand biomass over time. Moreover, regional simulations showed that a given long‐term increase in mean mortality rates would support greater aboveground biomass when expressed through disturbance effects compared with background mortality, particularly for early‐successional species. The effects of increased tree mortality on carbon stocks and forest composition may thus depend partly on whether future mortality increases are chronic or episodic in nature. 相似文献
50.
Rachel J. Derscheid Jack M. Gallup Cory J. Knudson Steven M. Varga Drew D. Grosz Albert van Geelen Shannon J. Hostetter Mark R. Ackermann 《PloS one》2013,8(12)
Respiratory syncytial virus (RSV) is the most frequent cause of bronchiolitis in infants and children worldwide. There are currently no licensed vaccines or effective antivirals. The lack of a vaccine is partly due to increased caution following the aftermath of a failed clinical trial of a formalin-inactivated RSV vaccine (FI-RSV) conducted in the 1960’s that led to enhanced disease, necessitating hospitalization of 80% of vaccine recipients and resulting in two fatalities. Perinatal lamb lungs are similar in size, structure and physiology to those of human infants and are susceptible to human strains of RSV that induce similar lesions as those observed in infected human infants. We sought to determine if perinatal lambs immunized with FI-RSV would develop key features of vaccine-enhanced disease. This was tested in colostrum-deprived lambs immunized at 3–5 days of age with FI-RSV followed two weeks later by RSV infection. The FI-RSV-vaccinated lambs exhibited several key features of RSV vaccine-enhanced disease, including reduced RSV titers in bronchoalveolar lavage fluid and lung, and increased infiltration of peribronchiolar and perivascular lymphocytes compared to lambs either undergoing an acute RSV infection or naïve controls; all features of RSV vaccine-enhanced disease. These results represent a first step proof-of-principle demonstration that the lamb can develop altered responses to RSV following FI-RSV vaccination. The lamb model may be useful for future mechanistic studies as well as the assessment of RSV vaccines designed for infants. 相似文献