收费全文 | 2113篇 |
免费 | 189篇 |
国内免费 | 1篇 |
2024年 | 7篇 |
2023年 | 14篇 |
2022年 | 24篇 |
2021年 | 44篇 |
2020年 | 19篇 |
2019年 | 26篇 |
2018年 | 31篇 |
2017年 | 42篇 |
2016年 | 54篇 |
2015年 | 94篇 |
2014年 | 112篇 |
2013年 | 118篇 |
2012年 | 165篇 |
2011年 | 156篇 |
2010年 | 101篇 |
2009年 | 84篇 |
2008年 | 144篇 |
2007年 | 156篇 |
2006年 | 125篇 |
2005年 | 138篇 |
2004年 | 116篇 |
2003年 | 99篇 |
2002年 | 111篇 |
2001年 | 22篇 |
2000年 | 18篇 |
1999年 | 28篇 |
1998年 | 29篇 |
1997年 | 18篇 |
1996年 | 15篇 |
1995年 | 15篇 |
1994年 | 16篇 |
1993年 | 9篇 |
1992年 | 15篇 |
1991年 | 12篇 |
1990年 | 11篇 |
1989年 | 8篇 |
1988年 | 11篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 13篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 9篇 |
1975年 | 4篇 |
1972年 | 3篇 |
Pedicularis dudleyi (Dudley’s Lousewort, Orobanchaceae) is an extremely rare plant endemic to the redwood forests of Central California. Until recently, the species was known only from three extant natural populations. However, in 2019, one of those populations was described as a novel species (P. rigginsiae D.J. Keil) based on morphological and ecological data leaving only two populations described as P. dudleyi. While little is known about the past distribution of the species, historical records have led to speculation that the species was once more widespread and may have suffered from habitat destruction as a result of widespread logging during the early twentieth century. We utilized a combination of ddRAD SNP and Sanger sequencing data to: (1) Test the morphological hypothesis that P. rigginsiae is distinct from P. dudleyi; (2) Describe the genetic diversity and population structure of P. dudleyi and; (3) Test the hypothesis that the species underwent a bottleneck corresponding with increased logging of redwood forests in the early twentieth century. Our results support the recognition of P. rigginsiae as distinct from P. dudleyi, increasing the conservation priority of both species. Genetic diversity statistics and analyses of genetic structure suggest that both populations of P. dudleyi are highly differentiated from each other with one population exhibiting unexpected substructure. Finally, demographic modeling supports a scenario where the contemporary rarity of the species is explained by a recent bottleneck.
相似文献We assessed the potential for microbial interactions influencing a well-documented host–pathogen system. Mycoplasma agassizii is the known etiological agent of upper respiratory tract disease in Mojave desert tortoises (Gopherus agassizii), but disease in wild animals is extremely heterogeneous. For example, a much larger proportion of animals harbor M. agassizii than those that develop disease. With the availability of a new quantitative PCR assay for a microbe that had previously been implicated in disease, Pasteurella testudinis, we tested 389 previously collected samples of nasal microbes from tortoise populations across the Mojave desert. We showed that P. testudinis is a common commensal microbe. However, we did find that its presence was associated with higher levels of M. agassizii among the tortoises positive for this pathogen. The best predictor of P. testudinis prevalence in tortoise populations was average size of tortoises, suggesting that older populations have higher levels of P. testudinis. The prevalence of co-infection in populations was associated with the prevalence of URTD, providing additional evidence for an indirect interaction between the two microbes and inflammatory disease. We showed that URTD, like many chronic, polymicrobial diseases involving mucosal surfaces, shows patterns of a polymicrobial etiology.
相似文献