全文获取类型
收费全文 | 251篇 |
免费 | 16篇 |
出版年
2019年 | 2篇 |
2017年 | 5篇 |
2016年 | 7篇 |
2015年 | 11篇 |
2014年 | 10篇 |
2013年 | 8篇 |
2012年 | 8篇 |
2011年 | 6篇 |
2010年 | 6篇 |
2009年 | 4篇 |
2007年 | 5篇 |
2006年 | 3篇 |
2005年 | 8篇 |
2001年 | 4篇 |
2000年 | 7篇 |
1999年 | 6篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1992年 | 3篇 |
1990年 | 4篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 6篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1975年 | 2篇 |
1973年 | 5篇 |
1972年 | 5篇 |
1961年 | 1篇 |
1960年 | 1篇 |
1959年 | 4篇 |
1958年 | 5篇 |
1957年 | 4篇 |
1956年 | 7篇 |
1955年 | 3篇 |
1954年 | 8篇 |
1953年 | 2篇 |
1952年 | 5篇 |
1951年 | 10篇 |
1950年 | 7篇 |
1949年 | 4篇 |
1948年 | 5篇 |
1946年 | 3篇 |
1934年 | 1篇 |
排序方式: 共有267条查询结果,搜索用时 46 毫秒
31.
32.
33.
J. PEARSON D. M. WELLS K. J. SELLER A. BENNETT A. SOARES J. WOODALL & M. J. INGROUILLE 《The New phytologist》2000,147(2):317-326
Mosses have been used as biomonitors of atmospheric pollution for some years, but few studies have been carried out on the effect of NOx emissions from traffic on moss tissue N. Eight species of moss (102 samples) growing on walls or roofs next to roads exposed to different traffic densities were collected from urban and rural sites in the UK. The shoots were sampled for total N, their stable isotope 15 N/14 N content (δ15 N) and heavy metal content (Pb, Zn). There was a lack of correlation between tissue total N and traffic exposure, but a very good correlation between traffic exposure and tissue δ15 N. Plants collected near motorways or busy urban roads had δ15 N values ranging between +6 and −1‰, while in rural areas with hardly any traffic these ranged from −2 to −12‰. In a separate survey of mosses, the average δ15 N of shoots from busy roadsides in London was +3.66‰, whereas from samples collected from farm buildings near poultry or cattle pens it was −7.8‰. This indicates that the two main atmospheric N sources, NOx and NHx , have different δ15 N signatures, the former tending to be positive and the latter negative. Tissue concentrations of both Pb and Zn show a strong positive correlation with traffic exposure, with Zn in particular being greater than Pb. The results are discussed with regard to the use of moss tissue Zn as a means for monitoring or mapping pollution from vehicles, and of δ15 N as an aid to distinguish between urban (NOx ) and rural (NHx ) forms of N pollution. 相似文献
34.
35.
Jung JA Jhang E. John Staba Jung Yun Kim 《In vitro cellular & developmental biology. Plant》1974,9(4):253-259
Summary Roots, stems, or leaves of American (Panax quinquefolium) and Korean (Panax ginsing) ginseng were grown as callus or supension tissue cultures. Tissue cultures ofP. ginseng would occasionally form plantlets. The fundamental chemical composition, inorganic analysis, and saponin (panaquilin) content
of American and Korean ginseng plants and tissue cultures were determined. The crude saponin content is very similar to, but
approximately one-half (1.3%, fresh weight) of that present in ginseng roots. Two-dimensional thin layer chromatographic analysis
revealed minor differences in the panaquilins present in American and Korean ginseng tissue cultures. The sapogenin, panaxadiol,
was isolated from Korean ginseng callus. 相似文献
36.
37.
Synthesis of Cell Coat in Normal and Transformed Cells 总被引:6,自引:0,他引:6
THE surface of transformed cells has been a focus of considerable attention recently because some of the properties which distinguish these cells from their precursors, such as decreased cell adhesiveness, altered cell orientation and loss of contact and density dependent inhibition1–3, may relate to changes on their surface. A common feature of vertebrate cells is the cell coat, a glycoprotein structure surrounding the plasma membrane4. Electron microscopy has revealed that transformed cells have a thicker coat than normal cells5 and we have now found that coat synthesis in cells transformed by an oncogenic DNA virus and in cells transformed by a chemical carcinogen occurs faster than in normal controls whereas only in the virus-transformed cells is the coat significantly thicker. 相似文献
38.
JA Nboyine S Boyer D Saville MJ Smith SD Wratten 《New Zealand journal of zoology.》2016,43(4):336-350
The endemic New Zealand ground wētā (Hemiandrus sp. ‘promontorius’) has a Naturally Uncommon conservation status. This is because of the paucity of information on its density and distribution. Here, the biology, density and distribution of a population of this wētā found in and around vineyards in the Awatere Valley, Marlborough was studied. Wētā density was assessed in vineyards, paddocks and shrublands in this valley. Soil moisture, penetration resistance, pH and organic matter were recorded at locations with and without wētā. Wētā density in vineyards was significantly higher than in either paddocks or shrub habitats. In vineyards, the density of this insect was significantly higher under-vines than in the inter-rows. Higher numbers of this wētā were found in moist soils that required lower force to burrow. Females laid an average of 55 eggs between March and April, which hatched in September. These findings highlight the intersection between agriculture and conservation. 相似文献
39.
Distinguishing morphologically cryptic taxa, by definition, requires genetic data such as DNA sequences. However, DNA sequences may not be obtained easily for taxa from remote sites. Here we provide the details of a high-resolution melt-curve-based method using taxon-specific primers that can distinguish two taxa of Adélie penguins, and that will be usable in Antarctica when combined with some of the newly developed field-deployable thermal cyclers. We suggest that the wider adoption of field-deployable polymerase-chain-reaction-based techniques will enable faster assignation of haplotype to individuals in situ, and so allow the targeting of observations and sample collection to specimens relevant to the research question. Targeting individuals will also reduce the need to repeatedly handle animals and reduce the time and travel required to complete field work. 相似文献
40.