全文获取类型
收费全文 | 696441篇 |
免费 | 82672篇 |
国内免费 | 2759篇 |
专业分类
781872篇 |
出版年
2018年 | 6819篇 |
2017年 | 6312篇 |
2016年 | 8698篇 |
2015年 | 12166篇 |
2014年 | 14333篇 |
2013年 | 19482篇 |
2012年 | 21925篇 |
2011年 | 22191篇 |
2010年 | 14562篇 |
2009年 | 13722篇 |
2008年 | 18870篇 |
2007年 | 19202篇 |
2006年 | 18062篇 |
2005年 | 17312篇 |
2004年 | 16652篇 |
2003年 | 16320篇 |
2002年 | 15741篇 |
2001年 | 33744篇 |
2000年 | 34086篇 |
1999年 | 26734篇 |
1998年 | 8945篇 |
1997年 | 9704篇 |
1996年 | 9129篇 |
1995年 | 8486篇 |
1994年 | 8310篇 |
1993年 | 8416篇 |
1992年 | 21963篇 |
1991年 | 21682篇 |
1990年 | 20708篇 |
1989年 | 20368篇 |
1988年 | 18780篇 |
1987年 | 17811篇 |
1986年 | 16507篇 |
1985年 | 16446篇 |
1984年 | 13339篇 |
1983年 | 11613篇 |
1982年 | 8692篇 |
1981年 | 7709篇 |
1980年 | 7421篇 |
1979年 | 12572篇 |
1978年 | 9944篇 |
1977年 | 8989篇 |
1976年 | 8259篇 |
1975年 | 9274篇 |
1974年 | 9614篇 |
1973年 | 9525篇 |
1972年 | 8549篇 |
1971年 | 7883篇 |
1970年 | 6735篇 |
1969年 | 6564篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
841.
842.
843.
Among the enterobacterial strains under study, more organisms in the stationary phase of growth have been found to have nicks in their DNA than those in the exponential phase. Bacteria less sensitive to ultraviolet irradiation have the least number of nicks in each phase of growth. The number of nicks in different strains belonging to the serovar is sufficiently stable. Virulent and avirulent forms show no difference in this characteristic. 相似文献
844.
845.
846.
847.
848.
849.
Mutagenesis of hydroxylamine oxidoreductase in Nitrosomonas europaea by transformation and recombination. 总被引:2,自引:0,他引:2 下载免费PDF全文
Mutagenesis of Nitrosomonas europaea was achieved by electroporation and recombination. To demonstrate this, an aminoglycoside 3'-phosphotransferase (kan) gene was specifically inserted into each of the three gene copies of hao individually. Southern hybridizations and PCR analysis showed the incorporation of the kan gene at the chosen genetic loci. The isolation of mutant strains was achieved in 7 to 14 days when the strains were grown on solid medium. The induced mutations were stable even in the absence of kanamycin-selective pressure for periods of up to 45 days in culture. The mutant strains did not show an observable phenotype different from that of the wild type when grown under the same conditions. 相似文献
850.
Interactions between water availability and elevated atmosphericCO2 concentrations have the potential to be important factorsin determining future forage supply from temperate pastures.Using large turves from an established pasture, the responseof these communities at 350 or 700 l l1 CO2 to a soilmoisture deficit and to recovery from the deficit in comparisonto turves that were well-watered throughout was measured. Priorto this experiment the turves had been exposed to the CO2 treatmentsfor 324 d. Net CO2 exchange continued at elevated CO2 even when the volumetricsoil moisture content was less than 0.10 m3 m3 soil;at the same moisture deficit gas exchange at ambient CO2 waszero. The additional carbon fixed by the elevated CO2 turveswas primarily allocated below-ground as shown by the maintenanceof root length density at the same level as in well-wateredturves. When the dry turves were rewatered there was compensatorygrowth at ambient CO2 so that the above-ground growth rate exceededthat of turves that had not experienced a moisture deficit.At the start of this experiment, the turves that were growingat 700 l I1 CO2 had a greater proportion of legume (principallywhite clover, Trifolium repens L.) in the harvested herbage.There was a trend for the legume content at elevated CO2 tobe reduced under a soil moisture deficit. The results indicate different strategies in response to soilmoisture deficits depending on the CO2 concentration. At ambientCO2, growth stopped, but plants were able to respond stronglyon rewatering; while at elevated CO2 growth continued (particularlybelow-ground), but no additional growth was evident on rewatering.Ecosystem gas exchange measurements taken at the end of theexperiment (after 429 d of exposure to CO2) showed 33% moreCO2 was fixed at elevated CO2 with only a small (12%) and nonsignificantdownward regulation. Key words: Carbon dioxide, climate change, grassland, gas exchange, soil moisture deficit 相似文献