收费全文 | 41786篇 |
免费 | 3339篇 |
国内免费 | 2406篇 |
2024年 | 60篇 |
2023年 | 484篇 |
2022年 | 1132篇 |
2021年 | 1965篇 |
2020年 | 1223篇 |
2019年 | 1506篇 |
2018年 | 1410篇 |
2017年 | 1084篇 |
2016年 | 1623篇 |
2015年 | 2427篇 |
2014年 | 2848篇 |
2013年 | 3090篇 |
2012年 | 3586篇 |
2011年 | 3398篇 |
2010年 | 1943篇 |
2009年 | 1794篇 |
2008年 | 2086篇 |
2007年 | 1867篇 |
2006年 | 1604篇 |
2005年 | 1420篇 |
2004年 | 1199篇 |
2003年 | 1053篇 |
2002年 | 905篇 |
2001年 | 851篇 |
2000年 | 740篇 |
1999年 | 716篇 |
1998年 | 435篇 |
1997年 | 464篇 |
1996年 | 441篇 |
1995年 | 390篇 |
1994年 | 378篇 |
1993年 | 308篇 |
1992年 | 441篇 |
1991年 | 383篇 |
1990年 | 331篇 |
1989年 | 246篇 |
1988年 | 236篇 |
1987年 | 198篇 |
1986年 | 141篇 |
1985年 | 185篇 |
1984年 | 121篇 |
1983年 | 104篇 |
1982年 | 80篇 |
1981年 | 60篇 |
1980年 | 55篇 |
1979年 | 72篇 |
1978年 | 66篇 |
1977年 | 46篇 |
1976年 | 48篇 |
1973年 | 48篇 |
Non-native invasive plant species influence plant community composition and competitively eradicate native species. However, there is doubt regarding how global invasive species increase and explosively interfere with native plants. Invasive plants always have strong allelopathic potential. In this study, allelopathic effects of switchgrass on redroot pigweed and crabgrass growth were investigated by field and laboratory experiments. Within a 0.4-m distance of switchgrass, density and shoot biomass of native species were significantly suppressed in the field, with 95.1% and 93.0% inhibition on density of redroot pigweed and crabgrass and with 99.0% and 97.7% inhibition on shoot biomass, respectively, during the third growing season. Significant inhibitory effects on shoot and root biomass were observed at the 5:5 (switchgrass–native species) proportion in glass bottles, by 41.57% and 51.21% for shoot and root biomass of redroot pigweed and by 33.42% and 56.95% for shoot and root biomass of crabgrass, respectively. Results of a glass bottle experiment showed that shoot and root biomass of redroot pigweed and crabgrass could be significantly inhibited by contact with switchgrass root. Results of a Petri dish experiment showed that aqueous extracts of switchgrass significantly inhibited germination process of both species at high concentrations, with 90.74% and 18.62% inhibition on germination rate and plumule length of redroot pigweed and with 63.59%, 16.38%, and 19.92% inhibition on germination rate, plumule, and radicle lengths of crabgrass, respectively, at the concentration of 0.1 g·mL?1. This report demonstrated that switchgrass had allelopathic effects on redroot pigweed and crabgrass growth.
相似文献The skin secretions of amphibians are a rich source of bioactive peptides. We isolated chensirin-1 and chensirin-2 from the skin secretion of the Chinese frog Rana chensinensis. Sephadex-G-50 and RP-HPLC were employed to purify these peptides. The amino acid sequences of these peptides were VLPLVGNLLNDLLGE and IIPLPLGYFAKKT, respectively, as determined by Edman degradation. The molecular weights were 1578.7 and 1460.8 Da, respectively, as analyzed by HPLC-ESI-MS. The chensirin cDNA was cloned by 5′ and 3′ amplification of cDNA ends, synthesized and purified. The antibacterial activities of the chensirins were tested using minimum inhibitory concentration, the results indicated that chensirins inhibit the growth of gram-negative and gram-positive bacteria. Among them, chensirin-1 is a novel peptide with a higher antibacterial activity compared to other similar antimicrobial peptides. These low molecular weight peptides with good antimicrobial efficacy are considered potential sources for developing new antimicrobial agents to improve traditional drug resistance.
相似文献