全文获取类型
收费全文 | 121556篇 |
免费 | 6222篇 |
国内免费 | 57篇 |
专业分类
127835篇 |
出版年
2018年 | 1418篇 |
2017年 | 1559篇 |
2016年 | 3609篇 |
2015年 | 7545篇 |
2014年 | 7135篇 |
2013年 | 7372篇 |
2012年 | 6861篇 |
2011年 | 3890篇 |
2010年 | 3377篇 |
2009年 | 3217篇 |
2008年 | 2262篇 |
2007年 | 2188篇 |
2006年 | 2047篇 |
2005年 | 8028篇 |
2004年 | 6647篇 |
2003年 | 4689篇 |
2002年 | 2303篇 |
2001年 | 2293篇 |
2000年 | 1539篇 |
1999年 | 2540篇 |
1998年 | 925篇 |
1997年 | 725篇 |
1996年 | 658篇 |
1995年 | 689篇 |
1992年 | 2899篇 |
1991年 | 2746篇 |
1990年 | 2830篇 |
1989年 | 2744篇 |
1988年 | 2600篇 |
1987年 | 2399篇 |
1986年 | 2229篇 |
1985年 | 2354篇 |
1984年 | 1620篇 |
1983年 | 1251篇 |
1982年 | 800篇 |
1981年 | 738篇 |
1979年 | 1453篇 |
1978年 | 1101篇 |
1977年 | 853篇 |
1976年 | 856篇 |
1975年 | 1070篇 |
1974年 | 1285篇 |
1973年 | 1253篇 |
1972年 | 1141篇 |
1971年 | 1069篇 |
1970年 | 927篇 |
1969年 | 948篇 |
1968年 | 838篇 |
1967年 | 829篇 |
1966年 | 656篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
131.
Jacob van Etten 《Journal of ethnobiology and ethnomedicine》2006,2(1):1-17
Small-scale studies on long-term change in agricultural knowledge might uncover insights with broader, regional implications. This article evaluates change in farmer knowledge about crop genetic resources in highland Guatemala between 1927/37 and 2004. It concentrates on maize (Zea mays ssp. mays L.) in one Guatemalan township, Jacaltenango, an area with much ecological and maize diversity. It relies on a particular type of baseline information: lists of farmer-defined cultivars drawn up by ethnographers in the first half of the twentieth century. A questionnaire format based on two independent lists of local farmer cultivars dating from 1927 and 1937 was used to assess changes in maize diversity. Comparisons between attributes given to each cultivar in the past and in 2004 were used as a partial test of the stability of cultivar identity. In farmers' perceptions, cultivar loss was low and limited to certain cultivars adapted to the warmer environments. Crop production problems were mentioned as the main motives for change. No evidence for a loss of cultivars due to the political violence of the 1980s was found. In the lower areas many newly introduced cultivars were found, which reportedly provide solutions for the production problems the older cultivars have. The article contrasts these findings with those of an earlier study which suggested much cultivar loss due to political violence, and draws conclusions about the methodological implications. 相似文献
132.
133.
134.
A comparison of the respiratory chain in particles from Paracoccus denitrificans and bovine heart mitochondria by EPR spectroscopy 总被引:6,自引:0,他引:6
S P Albracht H W van Verseveld W R Hagen M L Kalkman 《Biochimica et biophysica acta》1980,593(2):173-186
A study is presented on the EPR characteristics of the paramagnetic groups in the respiratory chain present in membrane particles of Paracoccus denitrificans, the respiratory system of which is very similar to that in submitochondrial particles from beef heart. All paramagnetic prosthetic groups of the mitochondrial system are also found in the bacterial plasma membrane. Their properties suggest that the respiratory groups are embedded in very similar protein environments in the two systems. 相似文献
135.
136.
137.
138.
139.
The use of the artificial antigen abequosylmannoside copolymer with acrylamide in the enzyme immunoassay for the determination of antibodies in the sera of salmonellosis patients has enhanced the specificity of the serological diagnosis of group B salmonellosis in comparison with the use of the natural antiren, S. typhimurium lipopolysaccharide. 相似文献
140.