全文获取类型
收费全文 | 483108篇 |
免费 | 49272篇 |
国内免费 | 226篇 |
出版年
2018年 | 4357篇 |
2016年 | 5910篇 |
2015年 | 8348篇 |
2014年 | 9547篇 |
2013年 | 13992篇 |
2012年 | 15657篇 |
2011年 | 16146篇 |
2010年 | 10882篇 |
2009年 | 9943篇 |
2008年 | 14135篇 |
2007年 | 14841篇 |
2006年 | 14051篇 |
2005年 | 13561篇 |
2004年 | 13573篇 |
2003年 | 13154篇 |
2002年 | 12917篇 |
2001年 | 18273篇 |
2000年 | 18143篇 |
1999年 | 14939篇 |
1998年 | 5892篇 |
1997年 | 5958篇 |
1996年 | 5703篇 |
1995年 | 5367篇 |
1994年 | 5265篇 |
1993年 | 5278篇 |
1992年 | 12954篇 |
1991年 | 12654篇 |
1990年 | 12618篇 |
1989年 | 12404篇 |
1988年 | 11702篇 |
1987年 | 10982篇 |
1986年 | 10277篇 |
1985年 | 10765篇 |
1984年 | 8995篇 |
1983年 | 7725篇 |
1982年 | 6156篇 |
1981年 | 5500篇 |
1980年 | 5116篇 |
1979年 | 8426篇 |
1978年 | 6680篇 |
1977年 | 6303篇 |
1976年 | 5974篇 |
1975年 | 6411篇 |
1974年 | 7021篇 |
1973年 | 6878篇 |
1972年 | 6327篇 |
1971年 | 5740篇 |
1970年 | 4966篇 |
1969年 | 4972篇 |
1968年 | 4607篇 |
排序方式: 共有10000条查询结果,搜索用时 654 毫秒
301.
The influence of age, weaning, season of the year and body weight on the peripheral levels of progesterone, oestradiol-17β and luteinizing hormone (LH) were studied during neonatal, perinatal and peripubertal periods in buffalo heifers. The buffalo heifers exhibited oestrus only after 30 months of age and had higher levels of LH and oestradiol-17β and a lower level of progesterone on the day of oestrus. The progesterone concentration was affected significantly (P < 0.01) by different seasons, by weaning (P < 0.05) and varied between pubertal and neonatal periods (P < 0.01), whereas the oestradiol-17β level was affected significantly (P < 0.01) by weaning and varied at different seasons and with body weight. However, the LH concentration was greater during the neonatal period than the pre- and peripubertal periods and changed significantly (P < 0.01) between groups of ages and body weights. The results suggest that increases in the levels of oestradiol-17β and progesterone after 30 months of age are probably indicative of the onset of puberty in buffalo heifers. However, a further increase in oestradiol-17β, LH, and a decrease in progesterone are essential for oestrus and cyclicity to be exhibited in buffalo heifers. 相似文献
302.
303.
304.
G Amiconi E Antonini M Brunori E Chiancone P Vecchini 《The Journal of biological chemistry》1971,246(6):1883-1888
305.
Virginia Edgcomb William Orsi John Bunge Sunok Jeon Richard Christen Chesley Leslin Mark Holder Gordon T Taylor Paula Suarez Ramon Varela Slava Epstein 《The ISME journal》2011,5(8):1344-1356
Microbial diversity and distribution are topics of intensive research. In two companion papers in this issue, we describe the results of the Cariaco Microbial Observatory (Caribbean Sea, Venezuela). The Basin contains the largest body of marine anoxic water, and presents an opportunity to study protistan communities across biogeochemical gradients. In the first paper, we survey 18S ribosomal RNA (rRNA) gene sequence diversity using both Sanger- and pyrosequencing-based approaches, employing multiple PCR primers, and state-of-the-art statistical analyses to estimate microbial richness missed by the survey. Sampling the Basin at three stations, in two seasons, and at four depths with distinct biogeochemical regimes, we obtained the largest, and arguably the least biased collection of over 6000 nearly full-length protistan rRNA gene sequences from a given oceanographic regime to date, and over 80 000 pyrosequencing tags. These represent all major and many minor protistan taxa, at frequencies globally similar between the two sequence collections. This large data set provided, via the recently developed parametric modeling, the first statistically sound prediction of the total size of protistan richness in a large and varied environment, such as the Cariaco Basin: over 36 000 species, defined as almost full-length 18S rRNA gene sequence clusters sharing over 99% sequence homology. This richness is a small fraction of the grand total of known protists (over 100 000–500 000 species), suggesting a degree of protistan endemism. 相似文献
306.
307.
308.
309.
Extracts from the stems and leaves of Pitavia punctata Mol. were examined. The neutral fraction yielded β-sitosterol, daucosterin, quercetin, avicularin, and the previously undescribed quercetin 3-rhamnosylarabinoside. Braylin was co-extracted with the basic constituents, dictamnine, skimmianine and γ-fagarine. Acid hydrolysis of the leaves yielded cyanidin and delphinidin. 相似文献
310.
High resolution nuclear magnetic resonance study of base pairing in four purified transfer RNA molecules 总被引:3,自引:0,他引:3
The high resolution nuclear magnetic resonance spectra of hydrogen bonded protons in four purified tRNA molecules are reported. From the temperature and concentration dependence it is shown that these resonances arise from intramolecular hydrogen bonds. 相似文献