Untersuchungen über die Biotechnik der Apocynaceenblüte ) : II. Bau und Funktion des Bestäubungsapparates

Abstract

Based on the functional morphology of the stylar head of the Apocynaceae , described in part I [Flora (1980) 170 : 394 — 432], the architecture and mechanical principle of the pollination apparatus as a whole, and its principal evolutionary trends, have been investigated. According to the degree of integration and differentiation of the style-anther complex, five steps of advancement are distinguishable, represented in the series Plumeria, Catharanthus, Vinca, Thevetia, Allamanda and Apocynoideae . In primitive Plumerioid genera with club-shaped style apex, pollen uptake by, and receipt from insects are still unprecise due to the absence of coadaptive arrangements. In all other Apocynaceae , a complicated, strictly herkogamous apparatus is formed by anthers, stylar head, and auxiliary channelling structures of the corolla. It is longitudinally divided into three superposed compartments: stigmatic chamber (lower end), glue zone (middle), and pollen depository (upper end). In radial dimension, the single anthers tend to join and even to fuse postgenitally with the stylar head in the more advanced groups. Five separate pollination units are thus formed, each containing a separate stigmatic zone, adhesive zone, and pollen mass. The sterile basal tails of thecae serve to force the insect's proboscis to pass one of the units, where eventually adherent cross pollen is scraped off by sharp edges (pollen scrapers) of the stylar head and conducted to the stigma beneath. The pollination process has been directly observed in Vinca minor . Bees first insert their mouthparts smoothly to extract nectar from the base of the corolla tube. Only during withdrawal is the tongue directed centripetally; thereby it passes first the pollen scraper and stigma, then the glue zone (here becoming sticky), and finally the pollen depository. The five functional units of the Asclepiadaceous gynostegium, in which the stylar adhesive is replaced by the secreted translators, show a principal relationship to the mechanism of the advanced Apocynaceae . The Periplocoideae exhibit a transitional position.