The flower of angiosperm consists of four different types of floral organs; sepals, petals, stamens, and carpels (pistils), which are produced in concentric circles called whorls. The ABC model has explained that combinations of only three classes of homeotic genes define each identity of the four floral organs. This simple model is addressed in all the high school “Advanced Biology” (Seibutsu) textbooks under the Course of Study 2009. The hypothesis of floral organ development was constructed by observations of altered flowers and genetic approach of Arabidopsis mutants. However, the flower structure diagrams of the mutants are not consistent with the photograph of flowers in many authorized textbooks. There would be two reasons for insufficient diagrams; the organ of the fourth whorl is regarded as one pistil in spite of two fused carpels in Arabidopsis flowers, and there is no explanation that Class C gene controls the flower determinate. Therefore, we propose that, for understanding the theory of ABC model, it should be described that angiosperm floral organs, sepals, petals, stamens, and carpels, are initiated as separate whorls and a pistil consists of fused some carpels or a single carpel. And then, the ABC model should be explained as a scientific approach to establish hypothesis based on experimental results.
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