Meetings of the Royal Botanical Society of The Netherlands

A comparison has been made between the differences of the U.V.-light reflectance-absorbance pattern of the corollas of fresh, open flowers and of flowers in senescence. This has been done by using a Leitz-Wetzlar Ortholux fluorescence microscope supplied with a cut-off filter (A) for U.V.- light within the range of 340-380 pm. The process of senescence has two aspects: 1) a morphological and 2) a biological one. 1). The following changes have been ascertained: the polygonal front view of the flower changes into a circular one, and the corolla changes from purple into blue and ultimately starts shrinking. 2), Owing to biochemical changes the venation turns from yellow-green into blue; the initially invisible tiny vascular supply to the papillate epidermal cells later becomes blue coloured and conspicuous. The flavonoid cell pigments and maybe also other substances absorb U.V.-light, so that these cells become dark brown. During senescence a break-down of substances occurs after which they and the necrotic cell walls reflect a blue to wiry white fluorescence. These changes of the corolla act as a visual signal for bees and bumble bees to prevent them from visiting the non-rewarding and withered flowers.