The pollen-collecting hairs of Campanula (Campanulaceae). II. Function and adaptive significance in relation to pollination

A peculiar and often discussed feature in the genus Campanula is the presence and function of the pollen-collecting hairs (PCHs), covering the style and the dorsal surface of the stigmatic lobes on the pistil. The hairs sweep up the pollen as the anthers dehisce, and this "stylar brush" serves as a secondary pollen presentation mechanism, facilitating the transfer of pollen from the flower to visiting insects. The adaptive significance of the hairs in relation to pollination has been variously interpreted by several authors during the past two centuries, but a satisfactory explanation for the floral mechanism is still lacking. The aim of the present investigation is to elucidate the function of the PCHs by experimental manipulations of the male phase. Comparisons are made 1) among species with various degrees of protandry and different mating systems, and 2) between annuals and perennials. The results clearly show that the hairs serve not only as pollen-collectors, but also as an important and efficient mechanism controlling stigma spreading and promoting cross-pollination. A tactile stimulation of the hairs causes a shortening of the length of the male phase and accelerates stigma maturation. The more frequently the hairs are treated, the shorter the male phase. Similar trends were found in all species regardless of differences in the degree of protandry, life span, or mating system. Geographically well-separated populations in two species showed differences in length of the male phase, depending on mating system and/or weather conditions. The relevance of the method used in this study compared to pollinator activity and its effect on the PCHs is discussed. The consequences of the retraction of the hairs with respect to self-pollination are briefly discussed