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在晚秋或冬季,人们可以看见一品红(又名圣诞花)开花,在株顶的几片叶子显得十分鲜红,甚是娇艳。在自然状态下,不曾有人在夏天见过一品红开花,这是因为夏天日照长,不适于一品红开花,到了秋末冬初,由于日照长度缩短,一品红就能顺利地开花。日照长度决定植物开花的学说,早在1920年便为两位美国学者加纳尔和阿拉德所提出。他们在长期实践中观察到一种烟草
In the late autumn or winter, one can see the flowering of poinsettia (aka Christmas flower) and the few leaves on the top of the plant appear very bright red, very delicate and tender. In nature, no one had ever seen the flowering of poinsettia in the summer. This is because the summer is long and not suitable for the flowering of poinsettia. By the end of autumn and early winter, the poinsettia can flow smoothly because of the shortening of the sunshine. The theory that the length of the sunshine determines the flowering of plants was proposed by two American scholar Gardner and Allard back in 1920. They observed a tobacco in long-term practice