全文获取类型
收费全文 | 204307篇 |
免费 | 7343篇 |
国内免费 | 897篇 |
专业分类
212547篇 |
出版年
2023年 | 720篇 |
2022年 | 624篇 |
2021年 | 1405篇 |
2020年 | 1327篇 |
2019年 | 1278篇 |
2018年 | 14194篇 |
2017年 | 12771篇 |
2016年 | 10902篇 |
2015年 | 5757篇 |
2014年 | 5588篇 |
2013年 | 7460篇 |
2012年 | 12240篇 |
2011年 | 19387篇 |
2010年 | 15940篇 |
2009年 | 11166篇 |
2008年 | 14984篇 |
2007年 | 16226篇 |
2006年 | 5605篇 |
2005年 | 5048篇 |
2004年 | 5442篇 |
2003年 | 5068篇 |
2002年 | 4475篇 |
2001年 | 3120篇 |
2000年 | 2933篇 |
1999年 | 2147篇 |
1998年 | 922篇 |
1997年 | 689篇 |
1996年 | 714篇 |
1995年 | 639篇 |
1992年 | 1236篇 |
1991年 | 1138篇 |
1990年 | 1059篇 |
1989年 | 1146篇 |
1988年 | 981篇 |
1987年 | 960篇 |
1986年 | 887篇 |
1985年 | 941篇 |
1984年 | 840篇 |
1983年 | 710篇 |
1982年 | 583篇 |
1979年 | 775篇 |
1978年 | 603篇 |
1975年 | 695篇 |
1974年 | 747篇 |
1973年 | 739篇 |
1972年 | 913篇 |
1971年 | 860篇 |
1970年 | 637篇 |
1969年 | 692篇 |
1968年 | 655篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
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
12.
13.
A. Španová 《Folia microbiologica》1980,25(4):281-288
Temperature-sensitive (ts) mutations of the G101 phage were isolated after mutagenesis with hydroxylamine. A complementation analysis of 61ts mutants showed that these mutants may be divided into at least 12 complementation groups. Twots mutants probably originated in genes which control lytic functions of the G101 phage. It was shown by three factor crosses
that all of the 12ts mutations tested are localized on that side of the “c” region where the probablecI repressor gene is positioned. Sevents mutations is closely linked to thecI
26 clear marker, three exhibit a closer linkage and two do not exhibit any linkage withcI. All mutations isolated until now can be arrange linearly. According to the present knowledge the preliminary genetic map
of the G101 phage is linear. 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.
The sensorimotor area of rat cerebral cortex was subjected to repeated electrical stimulation at 10-min intervals, with resultant formation and progressive lengthening of self-sustained after-discharges (SSAD). One and 60 min after the third SSAD ended, we carried out an electron microscopy morphometric analysis of the agranular synaptic vesicles in type I synapses (after Gray) in the second cortical layer of the homotopic area of the unstimulated hemisphere. One minute after the seizure ended, 5.8% enlargement of the synaptic vesicles compared with the control was demonstrated in zone II of the synapse (0.1-0.2 micron from the active zone of the synapse). Neither the size nor the shape of the synaptic vesicles in the other parts of the synaptic apparatus altered. Sixty min after the seizure ended, a 5.5% enlargement of the synaptic vesicles in zone I (0.0-0.1 micron) and a 5.4% enlargement of those in zone II was found. The synaptic vesicles in zone I in the experimental animals were more oval than in the controls. Our findings support the vesicular theory and testify that hyperfunction, up to temporary exhaustion of the synaptic apparatuses, produces a change in the transmitter content of the synaptic vesicles. A raised amount of transmitter in the synaptic vesicles near the active zone could be one of the factors responsible for continued hyperexcitability of the tissue one hour after the seizure had ended. The results likewise support the concept of two mechanisms of synaptic vesicle formation, and hence of the existence of two different vesicle populations. 相似文献
18.
19.
Background
Substituted catechols are important precursors for large-scale synthesis of pharmaceuticals and other industrial products. Most of the reported chemical synthesis methods are expensive and insufficient at industrial level. However, biological processes for production of substituted catechols could be highly selective and suitable for industrial purposes. 相似文献20.
"Pertussis toxin induces tachycardia and impairs the increase in blood pressure produced by alpha 2-adrenergic agonists" 总被引:4,自引:0,他引:4
Administration of purified pertussis toxin to rats induced persistent tachycardia, (observed in conscious rats but not after pithing); as little as 0.05 microgram/100 g produced a significant effect. Pertussis toxin-treatment did not affected the pressor response produced in the pithed rats by the alpha 2-adrenergic agonist methoxamine but markedly diminished the pressor effect of the alpha 2-adrenergic agonists clonidine and azepexole. A role of adenylate cyclase inhibition in the action of postsynaptic vascular alpha 2-adrenergic receptors is suggested. 相似文献