全文获取类型
收费全文 | 19380篇 |
免费 | 1297篇 |
国内免费 | 712篇 |
专业分类
林业 | 1255篇 |
农学 | 1693篇 |
基础科学 | 611篇 |
2848篇 | |
综合类 | 3307篇 |
农作物 | 852篇 |
水产渔业 | 1231篇 |
畜牧兽医 | 7669篇 |
园艺 | 389篇 |
植物保护 | 1534篇 |
出版年
2023年 | 159篇 |
2022年 | 220篇 |
2021年 | 329篇 |
2020年 | 325篇 |
2019年 | 421篇 |
2018年 | 512篇 |
2017年 | 548篇 |
2016年 | 505篇 |
2015年 | 567篇 |
2014年 | 585篇 |
2013年 | 866篇 |
2012年 | 820篇 |
2011年 | 1006篇 |
2010年 | 817篇 |
2009年 | 740篇 |
2008年 | 851篇 |
2007年 | 846篇 |
2006年 | 750篇 |
2005年 | 733篇 |
2004年 | 537篇 |
2003年 | 558篇 |
2002年 | 466篇 |
2001年 | 870篇 |
2000年 | 838篇 |
1999年 | 704篇 |
1998年 | 285篇 |
1997年 | 272篇 |
1996年 | 226篇 |
1995年 | 235篇 |
1994年 | 232篇 |
1993年 | 192篇 |
1992年 | 381篇 |
1991年 | 383篇 |
1990年 | 320篇 |
1989年 | 287篇 |
1988年 | 292篇 |
1987年 | 333篇 |
1986年 | 254篇 |
1985年 | 198篇 |
1984年 | 173篇 |
1983年 | 149篇 |
1982年 | 96篇 |
1981年 | 86篇 |
1980年 | 80篇 |
1979年 | 141篇 |
1978年 | 94篇 |
1977年 | 73篇 |
1975年 | 77篇 |
1974年 | 79篇 |
1969年 | 70篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
DNA直接导入法获得烟草新品系 总被引:2,自引:0,他引:2
为解决烟草栽培品种香气品质低下的问题,选用香料烟品种Ti245、高密度腺毛品种Ti1068、浓香型品系6251为供体,将三者总DNA混合后经花粉管通道导入烟草栽培品种K326;后代产生了广泛变异,经过连续3年的选育,获得了优良品系K809。与受体K326相比,K809田间农艺性状优良、叶片腺毛密度增加、香气物质含量大幅度提高,评吸结果表明香气品质得以改善;RAPD分子检测发现,K809含有来源于受体和供体的所有片段,一些片段仅与某供体特异同源,表明供体Ti245、Ti1068、6251的某些DNA片段在K809基因组中进行有效了整合,并且对烟叶的香气品质具有积极作用。 相似文献
82.
83.
84.
85.
Kermit Cromack Phillip Sollins William C. Graustein Karen Speidel Allen W. Todd Gody Spycher Ching Y. Li Robert L. Todd 《Soil biology & biochemistry》1979,11(5):463-468
Fungal mats of Hysterangium crassum (Tul. and Tul.) Fischer occupied a mean of 9.6% of the upper 10 cm of soil developed under a 40–65 yr old stand of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in Oregon. This hypogeous basidiomycete exudes large amounts of oxalic acid, some of which precipitates with Ca in microscopic crystals of calcium oxalate, resulting in a mean CaC2O4 content of 82g m?2 for the entire soil. Soil oxalate concentration was significantly greater within fungal mats (P < 0.01) and soil pH was significantly lower (P < 0.01) than in soil adjacent to mats. The quantity of Ca present as CaC2O4 is 0.5 the amount of exchangeable Ca in the soil and exceeds the mass of Ca lost annually in runoff. Scanning electron micrographs show intense chemical weathering, attributable to oxalate attack, in the immediate vicinity of hyphae. X-ray diffraction patterns of clay indicate bulk weathering is more intense within the fungal mats than in adjacent uncolonized soil. 相似文献
86.
H. KATAE K. KOUNO Y. TAKASE H. MIYAZAKI M. HASHIMOTO M. SHIMIZU 《Journal of fish diseases》1979,2(4):321-335
Abstract. The activity of piromidic acid (PA, 8-ethyl-5, 8-dihydro-5-oxo-2-prior lidinopyrido [2,3–d] pyrimidine-6-carboxylic acid) against bacteria in fish was evaluated in vitro and in vivo .
In vitro , PA was effective against a wide range of pathogenic bacteria stocked in this laboratory and freshly isolated from fish. PA was active against organisms showing multiple drug-resistance and showed no cross-resistance to chloramphenicol, tetracycline and sulfamonomethoxine.
In vivo , orally administered PA was effective against experimental infections with Aeromonas hydrophila and Vibrio spp. in goldfish and eels. The efficacy of PA against these infections was equal to or higher than that of chloramphenicol, and higher than that of tetracycline. Distribution of PA in the Ash was investigated by bioassay and autoradiography. In the bioassay, orally administered PA was easily absorbed and distributed in the blood and tissues of major organs within 1 h after the administration. Peak levels were attained 2 to 4 h post dosing, and no residue was detected in the blood and tissues 48 h post dosing. The results obtained from the autoradiographic investigation in goldfish were similar to those from the bioassay. Therapeutic levels of PA were well tolerated by fish when administered with feed. 相似文献
In vitro , PA was effective against a wide range of pathogenic bacteria stocked in this laboratory and freshly isolated from fish. PA was active against organisms showing multiple drug-resistance and showed no cross-resistance to chloramphenicol, tetracycline and sulfamonomethoxine.
In vivo , orally administered PA was effective against experimental infections with Aeromonas hydrophila and Vibrio spp. in goldfish and eels. The efficacy of PA against these infections was equal to or higher than that of chloramphenicol, and higher than that of tetracycline. Distribution of PA in the Ash was investigated by bioassay and autoradiography. In the bioassay, orally administered PA was easily absorbed and distributed in the blood and tissues of major organs within 1 h after the administration. Peak levels were attained 2 to 4 h post dosing, and no residue was detected in the blood and tissues 48 h post dosing. The results obtained from the autoradiographic investigation in goldfish were similar to those from the bioassay. Therapeutic levels of PA were well tolerated by fish when administered with feed. 相似文献
87.
88.
The metabolism of fluorodifen (p-nitrophenyl α,α,α,-trifluoro-2-nitro-p-tolyl ether) by soil microorganisms in the presence or absence of other carbon and nitrogen sources was studied. The degradation of this herbicide continued for 5 days, when benzoate or acetate and ammonium sulphate were included in the cultures, and for more than 5 weeks when fluorodifen was used as a sole source of carbon and nitrogen.Under all conditions nitrite ions were produced at concentrations ranging between 5 and 80 per cent of the nitro-nitrogen of the fluorodifen present. The highest concentration of nitrite was obtained when added carbon sources were used with fluorodifen. The lowest nitrite concentration accumulated when the fluorodifen was used as sole source of carbon and nitrogen. The nitrite reached a maximum value after a few days of incubation, followed by rapid disappearance.p-Nitrophenol and quinol were identified in the acid-ether extract of cultures. It is suggested that the first step in the degradation of fluorodifen is the hydrolysis of the ether linkage followed by the direct elimination of the nitro-groups as nitrite ions. 相似文献
89.
90.