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

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 t he Amaranthaceae and other taxa of the Caryophyllales. Inflorescences are c ymose with reduced partial inflorescences. Bracteoles are closely associate d with the newer only separated by a short pedicel. Sepal initiation follow s a 2/5 sequence but there is an important lag between the first two and th e remaining sepals. The androecium arises on a circular primordium; the fir st four stamens are initiated as pairs opposite the two outer sepals, and a re followed by paired or single stamens opposite the inner sepals. However, the number of stamens can fluctuate, or their sequence may be variable. Th e gynoecium starts its growth as a ring primordium with a number of apices enclosing a flattened apical dome. In early stages septal tissue can someti mes be observed. The central dome becomes completely subdivided in differen t sectors on which a large number of coiled ovules arise centrifugally. The development of flower and androecium shows similarities with other octa ndrous Caryophyllales, especially Phytolacca. The shared development is int erpreted as a primitive feature. A comparison with other Amaranthaceae on t he one hand, and with the Caryophyllales on the other shows evidence that a pluristaminate androecium with outer stamen pairs is plesiomorphic in the Amaranthaceae.