全文获取类型
收费全文 | 593篇 |
免费 | 56篇 |
专业分类
649篇 |
出版年
2023年 | 6篇 |
2022年 | 5篇 |
2021年 | 12篇 |
2020年 | 7篇 |
2018年 | 8篇 |
2017年 | 8篇 |
2016年 | 8篇 |
2015年 | 21篇 |
2014年 | 15篇 |
2013年 | 30篇 |
2012年 | 30篇 |
2011年 | 31篇 |
2010年 | 13篇 |
2009年 | 22篇 |
2008年 | 22篇 |
2007年 | 19篇 |
2006年 | 14篇 |
2005年 | 25篇 |
2004年 | 14篇 |
2003年 | 19篇 |
2002年 | 14篇 |
2001年 | 19篇 |
2000年 | 24篇 |
1999年 | 18篇 |
1998年 | 12篇 |
1997年 | 11篇 |
1996年 | 7篇 |
1995年 | 5篇 |
1994年 | 8篇 |
1993年 | 7篇 |
1992年 | 8篇 |
1991年 | 7篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1978年 | 12篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1973年 | 12篇 |
1969年 | 8篇 |
1967年 | 5篇 |
1955年 | 3篇 |
1951年 | 3篇 |
1940年 | 3篇 |
1938年 | 4篇 |
排序方式: 共有649条查询结果,搜索用时 0 毫秒
641.
642.
M Burrows 《BMJ (Clinical research ed.)》1981,282(6259):243-248
643.
Vegetation covers a substantial proportion of the terrestrial Earth surface and where removed from human influence has evolved influenced mainly by climatic and edaphic constraints. Over the past 200 years, substantial tracts of the wheat-sheep belt of southern Australia has been cleared of native vegetation and replaced by annual crop and pasture species. As a consequence, there has been a change in the water balance of many catchments as now more water leaks beneath the roots of introduced plants, contributing to groundwater rise and expansion in the amount of land affected by salinization. In an attempt to arrest root zone leakage, scientists and managers acknowledge the need for new agricultural practices which mimic the ecohydrological behaviour patterns of remnant vegetation.
In this paper, we examine the water use characteristics of four woodland Eucalyptus species growing in different topographic and edaphic environments in south eastern Australia. Eucalyptus sideroxylon and E. rossii were the sub- and dominant species inhabiting the stony ridges, while in the adjacent valleys E. macrorhyncha and E. albens were the sub- and dominant species, respectively. At the two locations, sub-dominant species were highly responsive to episodic rainfall events both during and following drought, and exhibited distinct seasonality in daily transpiration rate; while the dominant species at each location was less responsive to rainfall and water use appeared to be less seasonally dependent. Analysis of the water use response indicated that the two sub-dominant species had shallower roots while the major of roots of the dominant trees were likely to be located deeper in the substratum. This suggested that the stony ridges may store water deeper in the substratum than previously thought, to sustain some remnant vegetation over the dry summers, and ultimately, contribute less recharge to groundwater. 相似文献
In this paper, we examine the water use characteristics of four woodland Eucalyptus species growing in different topographic and edaphic environments in south eastern Australia. Eucalyptus sideroxylon and E. rossii were the sub- and dominant species inhabiting the stony ridges, while in the adjacent valleys E. macrorhyncha and E. albens were the sub- and dominant species, respectively. At the two locations, sub-dominant species were highly responsive to episodic rainfall events both during and following drought, and exhibited distinct seasonality in daily transpiration rate; while the dominant species at each location was less responsive to rainfall and water use appeared to be less seasonally dependent. Analysis of the water use response indicated that the two sub-dominant species had shallower roots while the major of roots of the dominant trees were likely to be located deeper in the substratum. This suggested that the stony ridges may store water deeper in the substratum than previously thought, to sustain some remnant vegetation over the dry summers, and ultimately, contribute less recharge to groundwater. 相似文献
644.
645.
646.
Shun Lu Maximiliano Giuliani Hanjeong Harvey Lori?L. Burrows Robert?A. Wickham John?R. Dutcher 《Biophysical journal》2015,108(12):2865-2875
Type IV pili (T4P) are very thin protein filaments that extend from and retract into bacterial cells, allowing them to interact with and colonize a broad array of chemically diverse surfaces. The physical aspects that allow T4P to mediate adherence to many different surfaces remain unclear. Atomic force microscopy (AFM) nanoscale pulling experiments were used to measure the mechanical properties of T4P of a mutant strain of Pseudomonas aeruginosa PAO1 unable to retract its T4P. After adhering bacteria to the end of an AFM cantilever and approaching surfaces of mica, gold, or polystyrene, we observed adhesion of the T4P to all of the surfaces. Pulling of single and multiple T4P on retraction of the cantilever from the surfaces could be described using the worm-like chain (WLC) model. Distinct peaks in the measured distributions of the best-fit values of the persistence length Lp on two different surfaces provide strong evidence for close-packed bundling of very flexible T4P. In addition, we observed force plateaus indicating that adhesion of the T4P to both hydrophilic and hydrophobic surfaces occurs along extended lengths of the T4P. These data shed new light, to our knowledge, on T4P flexibility and support a low-affinity, high-avidity adhesion mechanism that mediates bacteria-surface interactions. 相似文献
647.
648.
The melatonin supersensitivity to light has been suggested as a biological marker of bipolar disorder. However previous studies have been inconsistent with regard to light induced suppression of melatonin and raising questions regarding its reproducibility and reliability. The present study examined the test re-test reliability of the melatonin suppression by light in healthy subjects. Study was divided into two parts. The first examined the melatonin suppression by 200 lux of light while the second examined effect 500 lux of light. Subjects were tested twice, separated by one week for each part of the study. On each night subjects reported to the study at 1800 h. The first sample was collected at 2100 h (in the light). Subjects were then placed in a dark room, with a background light intensity of 10–20 lux. Further blood samples were collected at regular intervals. After each collection, blood samples were centrifuged and plasma separated and stored frozen at –20ºC. Plasma melatonin concentrations were determined by a specific radioimmunoassay. Results showed poor test re-test reliability for nights 1 and 2 for both light intensities suggesting that the melatonin suppression by light is not reproducible and has poor reliability. The poor test re-test reliability may provide an explanation for the inconsistencies in previous studies. 相似文献
649.
Malcolm Burrows 《BMC biology》2009,7(1):27-17