The morphological variation and systematic value of stamen pairs in the Magnoliatae

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Abstract
The distribution of paired stamens has been investigated in the Magnoliatae. Stamens can be inserted in pairs as the result of two developmental processes: (a) spatial shifts in the flower caused by the transition of a spiral into a cyclic arrangement of floral parts or (b) dédoublement of complex primordia. The first case is linked with a progressive loss of stamen whorls, ultimately leading to the replacement of all pairs by single stamens. Outer paired stamens are characteristic for polycyclic androecia and are often retained when the upper stamen whorls vanish. The outer pairs may be replaced by petals in some cases or by single stamens in other. The second situation arises by the splitting of complex primordia in two secondary primordia or by the independent initiation of the stamens of a pair. Replacement of single stamens by pairs reflects an increase in the pollination potential and has arisen several times, mainly in relation with oligomerous androecia.The distribution of these two types of stamen pairs on a Dahlgrenogram confirms the existence of two groups. These groups correspond more or less with the presence of the characters polymery and oligomery, proposed by RONSE DECRAENE & SMETS (1987, 1993b, 1995b).

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