全文获取类型
收费全文 | 134456篇 |
免费 | 3958篇 |
国内免费 | 1017篇 |
专业分类
139431篇 |
出版年
2018年 | 12276篇 |
2017年 | 11101篇 |
2016年 | 8041篇 |
2015年 | 1277篇 |
2014年 | 1175篇 |
2013年 | 1356篇 |
2012年 | 5399篇 |
2011年 | 13986篇 |
2010年 | 12795篇 |
2009年 | 9043篇 |
2008年 | 10913篇 |
2007年 | 12466篇 |
2006年 | 1426篇 |
2005年 | 1497篇 |
2004年 | 1915篇 |
2003年 | 1920篇 |
2002年 | 1657篇 |
2001年 | 3023篇 |
2000年 | 2609篇 |
1999年 | 1822篇 |
1998年 | 619篇 |
1997年 | 546篇 |
1996年 | 479篇 |
1995年 | 400篇 |
1994年 | 390篇 |
1993年 | 372篇 |
1992年 | 1299篇 |
1991年 | 1195篇 |
1990年 | 1102篇 |
1989年 | 1050篇 |
1988年 | 945篇 |
1987年 | 935篇 |
1986年 | 802篇 |
1985年 | 793篇 |
1984年 | 570篇 |
1983年 | 530篇 |
1979年 | 630篇 |
1978年 | 451篇 |
1977年 | 414篇 |
1976年 | 368篇 |
1975年 | 499篇 |
1974年 | 575篇 |
1973年 | 553篇 |
1972年 | 817篇 |
1971年 | 821篇 |
1970年 | 525篇 |
1969年 | 526篇 |
1968年 | 426篇 |
1967年 | 374篇 |
1966年 | 409篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
12.
Rice fields are an alternative habitat for waterbirds and provide food and shelter for many avian species, but there is a lack of information about how the use of rice fields translates into population level effects. The aim of this study was to test the relationship between the use of rice fields by European waterbirds and trends in their populations. We tested this relationship during the autumn migration season and during the breeding season. Based on counts conducted over the last 23 years in natural marshes and areas of rice fields in Doñana (SW Spain), an index of rice field use was constructed for 76 bird species, which was then compared to these species’ European population trends obtained from the literature. A positive relationship was found between waterbird population trends and the use of rice fields during autumn migration season. Our study suggests that changes in the Common Agriculture Policy in Europe leading to reductions in areas of rice cultivation may have important effects on waterbirds. The restoration of former marsh areas and the maintenance of rice cultivation would seem to be more environmentally friendly approaches than the use of these areas to grow alternative crops or solar farms. 相似文献
13.
V A Frolov G Mall P Rieger Kh Derks Z Antoni 《Biulleten' eksperimental'no? biologii i meditsiny》1987,104(12):739-741
Intensive synthesis of collagen-like substance was revealed in the rabbit myocardium during experimental diphtheria intoxication. It was more marked in the right ventricle 24 hours after the injection of diphtheria toxin. Since similar changes (the substance was mainly formed around blood vessels) have been observed in other cases of toxic myocardial alterations (i.e. ethanol intoxication, injection of pharmacological agents, etc.), it can be assumed that it is a standard protective reaction of the altered heart to the penetration of toxic agents from the blood into the myocardial tissue. 相似文献
14.
15.
16.
Background
Recent studies point to a great diversity of non-ribosomal peptide synthesis systems with major roles in amino acid and co-factor biosynthesis, secondary metabolism, and post-translational modifications of proteins by peptide tags. The least studied of these systems are those utilizing tRNAs or aminoacyl-tRNA synthetases (AAtRS) in non-ribosomal peptide ligation. 相似文献17.
18.
19.
20.