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Why do butterfly bush flowers typically have four stamens?
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Lamiales
The lilac-pink flowers of the water mint attract butterflies and beetles.
In the order Lamiales, the number of stamens is often determined by the evolutionary suppression of parts from an ancestral five-part (pentamerous) plan. While some members of the order retain five stamens, most have reduced this number to two or four.1
In species with four stamens, such as the butterfly bush (Buddleja), the reduction occurs because the uppermost stamen is absent. This four-stamen arrangement typically consists of an upper pair and a lower pair. In many members of the Scrophulariaceae family, to which the butterfly bush belongs, these four stamens are further characterized by two being shorter than the other two.12