Floral Development of Galopina tomentosa with a Discussion of Sympetaly and Placentation in the Rubiaceae

Abstract

The floral ontogeny of Galopina tomentosa is investigated. Inflorescences consist of pleiothyrses whose smallest elements are three-flowered cymes. The upper flowers of each cyme are subtended by a bract with two prophylls, functioning as bract of the smaller flowers, and are surrounded by a variable number of colleters. Sepals fail to develop, are present as two pairs of different size, or as a single lateral pair. The four petals arise as broad diagonally inserted lobes around a central depression. The stamen primordia emerge on the inner slopes of the cup-like flower primordium in the interstices of the large petal lobes. The gynoecium develops externally as two shallow stylar primordia whereas most of the ovary is embedded in the receptacular tissue. The two erect styles are lined on their inner side with stigmatic papillae. Placentation is strictly spoken central- basal; an apical septum is formed after ovule initiation by downward growth of tissue subjacent to the styles and connects postgenitally with the placenta. Sterilisation of one of the sexes occurs at the onset of ovule initiation in staminate flowers, and after anther development in pistillate flowers. Differences of morphs become manifest just before anthesis with reflexed petals in staminate flowers and erect petals in the pistillate forms. There is no trace of a nectary. The use of the terms 'early' and 'late' sympetaly is discussed for the Rubiaceae. It is shown that the nature of a 'corolla tube' is not necessarily homologous in the family and relates to fluctuations of three processes: the extent of growth of a meristematic zone common to stamens and petals (stamen-corolla tube), the growth of a petal tube sensu stricto, and postgenital fusion of filaments with the corolla. The development of a concave floral primordium determines the existence of a peripheral collar or 'early sympetaly.' Gynoecial development is compared with other Rubiaceae. It is suggested that ovaries arise along a common pattern in the family. Differences in position of placentation are related with the heterogenous nature of the septum that consists of a basal and apical section. The flower of Galopina tomentosa is characteristic for the wind-pollinated Anthospermeae with a strong tendency for reduction. A comparison is made with other Anthospermeae and with Theligonum in particular.