全文获取类型
收费全文 | 20721篇 |
免费 | 2856篇 |
国内免费 | 12099篇 |
专业分类
35676篇 |
出版年
2024年 | 300篇 |
2023年 | 764篇 |
2022年 | 1178篇 |
2021年 | 1258篇 |
2020年 | 1164篇 |
2019年 | 1317篇 |
2018年 | 945篇 |
2017年 | 856篇 |
2016年 | 889篇 |
2015年 | 1190篇 |
2014年 | 1713篇 |
2013年 | 1518篇 |
2012年 | 2057篇 |
2011年 | 2023篇 |
2010年 | 1733篇 |
2009年 | 1790篇 |
2008年 | 1974篇 |
2007年 | 1876篇 |
2006年 | 1854篇 |
2005年 | 1477篇 |
2004年 | 1245篇 |
2003年 | 1127篇 |
2002年 | 1058篇 |
2001年 | 959篇 |
2000年 | 863篇 |
1999年 | 625篇 |
1998年 | 314篇 |
1997年 | 193篇 |
1996年 | 166篇 |
1995年 | 103篇 |
1994年 | 107篇 |
1993年 | 86篇 |
1992年 | 75篇 |
1991年 | 108篇 |
1990年 | 70篇 |
1989年 | 69篇 |
1988年 | 69篇 |
1987年 | 44篇 |
1986年 | 43篇 |
1985年 | 53篇 |
1984年 | 47篇 |
1983年 | 28篇 |
1982年 | 61篇 |
1981年 | 26篇 |
1980年 | 15篇 |
1964年 | 13篇 |
1957年 | 18篇 |
1955年 | 12篇 |
1954年 | 16篇 |
1950年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
Wei CHEN Liqing PENG Lichun JIANG David A.PIKE Christopher R.FRIESEN Gregory BROWN 《亚洲两栖爬行动物研究(英文版)》2018,(1)
Environmental unpredictability can influence strategies of maternal investment among eggs within a clutch. Models predict that breeding females should adopt a diversified bet-hedging strategy in unpredictable environments, but empirical field evidence from Asia is scarce. Here we tested this hypothesis by exploring spatial patterns in egg size along an altitudinal gradient in a frog species(Rana kukunoris) inhabiting the Tibetan Plateau. Within-clutch variability in egg size increased as the environment became variable(e.g., lower mean monthly temperature and mean monthly rainfall at higher altitudes), and populations in environments with more unpredictable rainfall produced eggs that were smaller and more variable in size. We provide support for a diversified bet-hedging strategy in high-altitude environments, which experience dynamic weather patterns and therefore are of unpredictable environmental quality. This strategy may be an adaptive response to lower environmental quality and higher unpredictable environmental variance. Such a strategy should increase the likelihood of breeding success and maximize maternal lifetime fitness by producing offspring that are adapted to current environmental conditions. We speculate that in high-altitude environments prone to physical disturbance, breeding females are unable to consistently produce the optimal egg size due to physiological constraints imposed by environmental conditions(e.g., duration of the active season, food availability). Species and populations whose breeding strategies are adapted to cope with uncertain environmental conditions by adjusting offspring size and therefore quality show a remarkable degree of ability to cope with future climatic changes. 相似文献
2.
用β-内啡肽放射免疫分析法测定10例无疼痛主诉的患者电针前后侧脑室脑脊液中B-内啡肽样免疫活性物质(B-EPIS)的含量,其中6例用弹簧棒测定了痛阈和耐痛阈。结果表明,每 ml 脑脊液中β-EPIS 的含量比针前增加126.7 fmol/ml(P<0.02),病人的痛阈比针前升高29.5%,耐痛阈升高28.1%(P<0.05)。β-EPIS 含量的增加量与痛阈、耐痛阈的升高值呈直线相关,r_1=0.776,r_2=0.741(P<0.05)。表明电针能促使脑内释放β-内啡肽,释放增加的β-内啡肽参与镇痛作用,这可能是电针镇痛机制的重要环节之一。 相似文献
3.
The formation of hyphae that grow solely by apical extension is a defining feature of filamentous fungi. Hyphal morphogenesis involves several key steps, including the establishment and maintenance of a stable polarity axis, as well as cell division via the deposition of septa. Several filamentous fungi have been employed in attempts to decipher the mechanisms underlying these steps. Amongst these fungi, Aspergillus nidulans has proven to be a particularly valuable model. The genetic tractability of this fungus coupled with the availability of sophisticated post-genomics resources has enabled the identification and characterization of numerous genes involved in hyphal morphogenesis. Here, we summarize current progress towards understanding the function of these genes and the mechanisms involved in polarized hyphal growth and septation in A. nidulans. We also highlight important areas for future investigation. 相似文献
4.
5.
6.
Integrated species delimitation and conservation implications of an endangered weevil Pachyrhynchus sonani (Coleoptera: Curculionidae) in Green and Orchid Islands of Taiwan 下载免费PDF全文
YEN‐TING CHEN HUI‐YUN TSENG MING‐LUEN JENG YONG‐CHAO SU WEN‐SAN HUANG CHUNG‐PING LIN 《Systematic Entomology》2017,42(4):796-813
Oceanic islands are productive habitats for generating new species and high endemism, which is primarily due to their geographical isolation, smaller population sizes and local adaptation. However, the short divergence times and subtle morphological or ecological divergence of insular organisms may obscure species identity, so the cryptic endemism on islands may be underestimated. The endangered weevil Pachyrhynchus sonani Kôno (Coleoptera: Curculionidae: Entiminae: Pachyrhynchini) is endemic to Green Island and Orchid Island of the Taiwan‐Luzon Archipelago and displays widespread variation in coloration and host range, thus raising questions regarding its species boundaries and degree of cryptic diversity. We tested the species boundaries of P. sonani using an integrated approach that combined morphological (body size and shape, genital shape, coloration and cuticular scale), genetic (four genes and restriction site‐associated DNA sequencing, RAD‐seq) and ecological (host range and distribution) diversity. The results indicated that all the morphological datasets for male P. sonani, except for the colour spectrum, reveal overlapping but statistically significant differences between islands. In contrast, the morphology of the female P. sonani showed minimum divergence between island populations. The populations of P. sonani on the two islands were significantly different in their host ranges, and the genetic clustering and phylogenies of P. sonani established two valid evolutionary species. Integrated species delimitation combining morphological, molecular and ecological characters supported two distinct species of P. sonani from Green Island and Orchid Island. The Green Island population was described as P. jitanasaius sp.n. Chen & Lin, and it is recommended that its threatened conservation status be recognized. Our findings suggest that the inter‐island speciation of endemic organisms inhabiting both islands may be more common than previously thought, and they highlight the possibility that the cryptic diversity of small oceanic islands may still be largely underestimated. 相似文献
7.
The purpose of this review is to provide an overview of the effects of adenovirus and influenza virus infections on obesity in various experimental models. We reviewed studies that were conducted within the past 10 years and were related to virus infection and obesity prevalence. Here, we discuss a different causal relationship between adenovirus and influenza infections with obesity. Adenovirus infection can cause obesity, whereas obesity can be a risk factor for increasing influenza virus infection and increases the risk of morbidity and mortality. The prevalence of obesity due to adenovirus infections may be due to an increase in glucose uptake and reduction in lipolysis caused by an increase in corticosterone secretion. Adenovirus infections may lead to increases in appetite by decreasing norepinephrine and leptin levels and also cause immune dysfunction. The relationship between obesity and influenza virus infection could be summarized by the following features: decreases in memory T-cell functionality and interferon (IFN)-α, IFN-β, and IFN-γ mRNA expression, increases in viral titer and infiltration, and impaired dendritic cell function in obese individuals. Moreover, leptin resistance may play an important role in increasing influenza virus infections in obese individuals. In conclusion, prevention of adenovirus infections could be a good approach for reducing obesity prevalence, and prevention of obesity could reduce influenza virus infections from the point of view of viral infections and obesity. 相似文献
8.
Summary Lactic acid produced by continuous culture of L.casei in an upflow packed bed reactor, was recovered with Amberlite IRA 400 in a fluidized bed column. Bed expansions of 1.25 and 2.25 were applied. Reutilization did not alter the capability of net recovery of 0.048 ± 0.01 g lactic acid/g resin. When 2200 cm/h of ascensional velocity was used, (bed expansion of 2.25), the resin adsorbed 39.3% of the initial lactic acid and 63.5% was eluted. This resin supported the highest exchange capacity of 0.126 g lactic acid/g resin. Applying high flow rates, the process has potential industrial applications due to the short time employed. 相似文献
9.
10.