This book is an exploration into the field of Plant Morphology. It deals with the placentation of the ovules in ten families of Centrospermae — including the Cactaceae — and in the Primulaceae. The core is formed by a very close observation and a complete documentation of the histogenesis of the ovary wall, the septs, and the placentae in four Caryophyllaceous species. Furthermore, the result is compared with similar known and newly discovered features in other species and in the other families. It appears that the ovary is composed of a cup of sterile phyllomes which surrounds a central body. This central part is built up by two alternating sets of five axial placentae bearing the ovules. The septs grow from the cup inwards and fuse with the placentae and their ovules. The pattern of the vascular bundles is in full accordance with the histogenetic results. Variations on this theme occur in the other species and families, the ultimate stage in reduction being an ovary with a solitary terminal ovule. However, the Primulaceae do not fit in this scheme; they cannot be considered as Centrospermae.