A Study of Floral Morphological Diversity in Phytolacca (Phytolaccaceae) Based on Early Floral Ontogeny

Abstract

The floral ontogeny of four species of Phytolacca (P. acinosa, P. clavigera, P. dodecandra, and P. americana) shows major differences in the number and position of stamens and carpels. Five sepals are always formed in a 2/5 sequence. The number of stamens is mostly eight, with a sequential initiation: six outer stamens are arranged in three pairs opposite sepals 1-3; single stamens stand opposite sepals 4-5, except for P. americana, where all antesepalous stamens are positioned in pairs (10 stamens rather than eight). The stamens of the first, second, and third pair arise simultaneously as independent entities. The stamens of the third pair occasionally arise sequentially and are displaced in relation to the third sepal. Stamens opposite sepals 4 and 5 follow sequentially. The delayed plastochron of the stamens opposite sepal 3 indicates that the pentamerous flower has been derived from a trimerous precursor. Phytolacca dodecandra has 10 stamens arising in a different sequence: the five inner stamens are alternisepalous and the five outer, antesepalous, thus resembling diplostemony. The alternisepalous stamens correspond positionally to the two pairs opposite sepals 1-2 in other species, plus one stamen between sepals 3 and 5. The outer antesepalous stamens appear later and correspond to single stamens opposite sepals 3-4-5 in the other species, with two extra stamens opposite sepals 1-2. In other species a variable number of outer stamens may arise centrifugally in alternate whorls. The number of carpels corresponds with the number of stamens in the inner androecial whorl. Each carpel consists of a free abaxial part and an adaxial, continuous ovule-bearing part. Our data of the androecial ontogeny refute the existence of diplostemony in the Phytolaccaceae, with P. dodecandra occupying a pivotal position. It is suggested that the paired arrangement of stamens in Phytolacca is a basic condition in the family and is not the result of dédoublement. The centrifugal initiation of outer stamens is believed to reflect a reversal in the direction of stamen initiation and is not linked with a secondary stamen multiplication.