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961.
本文借用自动控制理论中主导极点的概念,指出在静态时,谐振变量决定于代表谐振特性的主导极点,而滤波变量则由于滤波极点的作用,近似为直流(忽略纹波);在动态时,控制频率的变化相对开关频率的变化为慢变化,慢变化仅仅引起样点的变化。在这一理论指导下,推导出混合串并联型谐振变换器小信号电路模型。 相似文献
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贵州植物区系新纪录 总被引:2,自引:0,他引:2
张宏达 《中山大学学报(自然科学版)》1987,(2)
乔木,嫩枝被黄色茸毛,顶芽近于秃净.叶长圆形,膜质,长8—13厘米,宽3.6—4.8厘米;先端锐尖或渐尖;基部圆形,平截或不等侧心形;上面干后黄绿色,除中肋有毛外秃净无毛;下面被黄色星状茸毛;侧脉8—11对,与网脉在上下两面均显著;全 相似文献
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记述了采自四川省昭觉地区及云南省昆明市草螽科的钩顶螽属二新种,即凉山钩顶螽 Ruspolia liangshanensis sp.n.和云南指顶螽R.yunnana sp.n.;并报道了该属一新组合种,即印度钩顶螽R.indica(Redtenbacher,1891)Conmb.n. 相似文献
966.
A new mechanism of invader success: Exotic plant inhibits natural vegetation restoration by changing soil microbe community 总被引:12,自引:0,他引:12
YUXingjun YUDan LUZhijun MAKeping 《科学通报(英文版)》2005,50(11):1105-1112
Since the 1950s of the last century, the exotic plant, Eupatorium adenophorum, has spread rapidly across southwest China, damaging native ecosystems and causing great economic losses. We examined the pH, N, P, K, and organic matter concentrations, and the bacterial community character (by Biolog EcoPlateTM) in soils from sites heavily and lightly invaded by this exotic species. Also, soil from the lightly invaded site was treated with a water extract of E.adenophorum roots to examine the effect of the plant on soil properties. We grew three plant species, one native and two exotic, in pot experiment using soil from heavily invaded site to examine the effects of the soil on these plants growth. The soil analysis demonstrated that the pH, organic matter, total N, total P and total K in soils from the heavily invaded site were only slightly different from those of the lightly invaded site, but concentrations of NH4^ , NO3^- and available P and K in the heavily invaded site were greater than those in the lightly invaded site. The catabolic activity of soil bacterial community in the heavily invaded site was different from that in the lightly invaded site. The catabolic activity of bacterial community in soils treated by the water extract of E.adenophorum roots changed and became similar to that in soils from the heavily invaded site. The pot experiment showed that the exotic plants growth in heavily invaded soil were not different from in lightly invaded soil; however, the native plant biomass decreased dramatically when grown in soil from the heavily invaded site as compared to soil from the lightly invaded site; and the same phenomenon was found when any potential allelopathic effects by E. adenophorum were eliminated by added activated carbon to those soils. Difference in soil nutrient availability and allelopathy could not explain this phenomenon of the native plant in the soils from the heavily and lightly invaded sites. Changes observed in the soil bacterial community were obviously related to native plant growth in those tow soils. Those results suggest that changing soil microbial community may be an important part of E. adenophorum invasion process. Since the soil microbial community serves as bridge in connection of exotic and natural plants, the exotic plant could inhibit the natural plant growth and reproduction by changing the soil microbial community in invaded site. 相似文献
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