全文获取类型
收费全文 | 961篇 |
免费 | 48篇 |
专业分类
1009篇 |
出版年
2023年 | 7篇 |
2022年 | 5篇 |
2021年 | 19篇 |
2020年 | 14篇 |
2019年 | 14篇 |
2018年 | 26篇 |
2017年 | 36篇 |
2016年 | 26篇 |
2015年 | 23篇 |
2014年 | 30篇 |
2013年 | 66篇 |
2012年 | 51篇 |
2011年 | 63篇 |
2010年 | 55篇 |
2009年 | 26篇 |
2008年 | 41篇 |
2007年 | 27篇 |
2006年 | 29篇 |
2005年 | 17篇 |
2004年 | 21篇 |
2003年 | 16篇 |
2002年 | 17篇 |
2001年 | 16篇 |
2000年 | 8篇 |
1999年 | 3篇 |
1998年 | 13篇 |
1997年 | 13篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1992年 | 2篇 |
1991年 | 6篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1971年 | 2篇 |
1955年 | 29篇 |
1954年 | 77篇 |
1953年 | 42篇 |
1952年 | 27篇 |
1951年 | 20篇 |
1950年 | 56篇 |
1949年 | 12篇 |
1948年 | 5篇 |
1947年 | 3篇 |
1936年 | 1篇 |
排序方式: 共有1009条查询结果,搜索用时 0 毫秒
101.
102.
103.
104.
105.
106.
107.
Genotypic variation in major components of the efficiency of nitrogen utilization and photosynthetic activity of flag leaves among old (released 1881-1963) and modern (released 1969-2003) cultivars of winter wheat was studied in field conditions under varied N fertilization levels (110, 90 and 80 kg N ha-1). Significant genotypic differences were observed for all characters. Their heritabilities ranged from 0.37 to 0.93 and were the lowest for the leaf efficiency of gas exchange, photosynthetic rate, straw N content and the economic index of N utilization efficiency (NUE). Some modern cultivars exhibited an enhanced tolerance to N shortage and several attributes of efficient N utilization (e.g. later senescing and more photosynthetically active flag leaves, increased ability to redistribute N into grains). The genotypes may serve as donors of appropriate characteristics for breeding. The observed cultivar-by-fertilization interactions suggest, however, that evaluations under diverse fertilization regimes may be necessary when searching for improved wheat efficiency and adaptation to less favourable environments. 相似文献
108.
The oxidative effect of tannic acid and its two derivatives (ellagic and gallic acid), naturally occurring plant polyphenols, has been studied on digestive gland cells of the fresh-water mussel Unio tumidus. A spectrophotometric method was used to determine the protein thiol groups after incubation of the cells with the polyphenols at concentrations of 1, 15 and 60 microM. The results showed that the oxidative modification of proteins increased in a concentration-dependent manner but no changes were observed at the concentration of 1 microM. The comet assay (single-cell gel electrophoresis assay) with the formamido-pyrimidine glycosylase (FPG) protein was used to assess oxidative DNA base damage. The cells were treated with polyphenols at the concentrations of 30 and 60 microM and post-incubated with FPG. FPG strongly enhanced DNA damage induced by the polyphenols, indicating that N-7 guanine oxidation is responsible for the observed effect. Using the comet assay in combination with proteinase K we were able to demonstrate the presence of DNA-protein cross-links as the probable cause of the decrease in DNA migration. After treatment of the cells with tannic acid and its metabolites at concentrations of 120, 180 and 240 microM, they were post-incubated with proteinase K. After this treatment an increased DNA migration was observed, indicating the presence of DNA-protein cross-links. We have also used a fluorescence method with Hoechst 33258/propidium iodide DNA-binding dyes to study the extent of DNA fragmentation after exposure of the cells to polyphenols at concentrations of 1, 5 and 60 microM. The results demonstrate that the polyphenols can induce apoptosis and necrosis at higher concentrations (5 and 60 microM). All experimental data suggest that tannic, ellagic and gallic acids at concentrations above 1 microM are able to interact with proteins and DNA, which leads to their degradation or changes in their function. 相似文献
109.
Unusual growth pattern in the Frasnian alveolitids (Tabulata) from the Holy Cross Mountains (Poland)
Abstract: Growth periodicity is a phenomenon occurring in fossil and modern corals. The most apparent feature is growth banding, and environmental changes are broadly accepted as controls on this phenomenon. If environment controls the growth, then all corallites within a colony should repeat the same growth pattern, as individuals are clones and must have shared the same environment. A study on several species of Alveolitidae (Anthozoa, Tabulata) from the Late Devonian (Early Frasnian) of the Holy Cross Mountains (Poland) shows that the growth pattern varies between neighbouring individuals within the same corallum. This contradicts observations of closely related Favositida as demonstrated on Pachyfavosites sp. from the Givetian of Avesnois, France, where neighbouring individuals repeat the same pattern. Therefore, environmental control on growth rhythm in Alveolitidae can be excluded; the causes of differences between individuals remain unknown. 相似文献
110.