全文获取类型
收费全文 | 1023篇 |
免费 | 99篇 |
出版年
2021年 | 10篇 |
2019年 | 7篇 |
2018年 | 9篇 |
2017年 | 12篇 |
2016年 | 22篇 |
2015年 | 32篇 |
2014年 | 35篇 |
2013年 | 48篇 |
2012年 | 56篇 |
2011年 | 44篇 |
2010年 | 34篇 |
2009年 | 25篇 |
2008年 | 53篇 |
2007年 | 43篇 |
2006年 | 51篇 |
2005年 | 43篇 |
2004年 | 41篇 |
2003年 | 44篇 |
2002年 | 40篇 |
2001年 | 36篇 |
2000年 | 29篇 |
1999年 | 23篇 |
1998年 | 22篇 |
1997年 | 14篇 |
1996年 | 17篇 |
1995年 | 16篇 |
1994年 | 16篇 |
1993年 | 10篇 |
1992年 | 18篇 |
1991年 | 11篇 |
1990年 | 15篇 |
1989年 | 20篇 |
1988年 | 16篇 |
1987年 | 18篇 |
1986年 | 13篇 |
1985年 | 12篇 |
1984年 | 9篇 |
1983年 | 11篇 |
1982年 | 7篇 |
1980年 | 9篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 6篇 |
1975年 | 9篇 |
1974年 | 10篇 |
1973年 | 7篇 |
1972年 | 6篇 |
1970年 | 6篇 |
1966年 | 7篇 |
1965年 | 7篇 |
排序方式: 共有1122条查询结果,搜索用时 125 毫秒
171.
172.
Cell Division and Prophage Induction in Escherichia coli: Studies of Nucleotide Levels 总被引:9,自引:6,他引:3 下载免费PDF全文
Cell division and prophage repression in the Escherichia coli mutant, T-44, are very sensitive to the levels of certain purine and pyrimidine derivatives in the media. The hypothesis that a change in the level of an adenine derivative in the small molecule pool of this strain was responsible for prophage induction and filament formation was tested. The nucleoside triphosphate pools in T-44 and C-600 nonlysogenic and lysogenic strains were labeled in experiments with (32)P and (33)P. Cultures were mixed, and the nucleotides were isolated. When adenine was present, the level of adenosine triphosphate (ATP) in T-44 compared to C-600 (as indicated by the isotope ratio) was increased up to twofold. Most of the other nucleotides increased but not to the same degree. In the lysogenic strain guanosine triphosphate and deoxycytidine triphosphate showed increases comparable to ATP, whereas increases noted in the deoxynucleotides in T-44 +/- lambda with adenine present were less. In experiments where T-44 and C-600 were incubated with (3)H- and (14)C-adenine, the levels of several compounds, including ATP, were slightly elevated in T-44. The combined data suggest that cultures of T-44 +/- lambda, grown in the presence of adenine, show a preferential increase in the level of ATP when compared to C-600 +/- lambda, but the increase in relation to the other nucleotides is less than twofold. In the experiment with (3)H- and (14)C-adenine, the level of inosine was found to be increased in T-44 relative to C-600. Cyclic AMP, when added to cultures of T-44 under various conditions, had no effect on prophage induction. Intracellular and extracellular levels of cyclic AMP in T-44 compared to C-600, incubated with had-acidin, guanosine, and cytidine (HGC) or with HGC plus adenine, were not significantly different. No compelling evidence for altered nucleotide metabolism in T-44 +/- lambda as a cause of prophage induction or filament formation was obtained. 相似文献
173.
174.
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
175.
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
176.
177.
178.
179.
The invasive freshwater snail Tarebia granifera (Lamarck, 1822) was first reported in South Africa in 1999 and it has become widespread across the country, with some evidence to suggest that it reduces benthic macroinvertebrate biodiversity. The current study aimed to identify the primary abiotic drivers behind abundance patterns of T. granifera, by comparing the current abundance of the snail in three different regions, and at three depths, of the highly modified Nseleni River in KwaZulu-Natal, South Africa. Tarebia granifera was well established throughout the Nseleni River system, with an overall preference for shallow waters and seasonal temporal patterns of abundance. Although it is uncertain what the ecological impacts of the snail in this system are, its high abundances suggest that it should be controlled where possible and prevented from invading other systems in the region. 相似文献
180.