The floral development of Pleuropetalum darwinii, an anomalous member of the Amaranthaceae

Abstract

Summary The genus Pleuropetalum differs from the Amaranthaceae in its higher stamen and carpel number, coupled with numerous ovules. The floral development of Pleuropetalum darwinii is investigated with the SEM and is compared with the Amaranthaceae and other taxa of the Caryophyllales. Inflorescences are cymose with reduced partial inflorescences. Bracteoles are closely associated with the flower only separated by a short pedicel. Sepal initiation follows a 2/5 sequence but there is an important lag between the first two and the remaining sepals. The androecium arises on a circular primordium; the first four stamens are initiated as pairs opposite the two outer sepals, and are followed by paired or single stamens opposite the inner sepals. However, the number of stamens can fluctuate, or their sequence may be variable. The gynoecium starts its growth as a ring primordium with a number of apices enclosing a flattened apical dome. In early stages septal tissue can sometimes be observed. The central dome becomes completely subdivided in different sectors on which a large number of coiled ovules arise centrifugally. The development of flower and androecium shows similarities with other octandrous Caryophyllales, especially Phytolacca. The shared development is interpreted as a primitive feature. A comparison with other Amaranthaceae on the one hand, and with the Caryophyllales on the other shows evidence that a pluristaminate androecium with outer stamen pairs is plesiomorphic in the Amaranthaceae.