POLLINATION BIOLOGY AND SEXUAL-DIFFERENTIATION OF OSYRIS-ALBA (SANTALACEAE) IN THE MEDITERRANEAN REGION

Osyris alba L. is a widespread dioecious hemiparasitic shrub of S Euro pe, N Africa, and SW Asia. Male inflorescences are multiflowered where as each female inflorescence is reduced to a single flower with persis tent enlarged bracts. Pollination is a prerequisite for fruit and seed development and wind is unlikely to be an effective means of pollen s pread. In southern Italy pollen is transported by small unspecialized flies and beetles. Both male and female flowers produce an indistingui shable sweet odour. Male flowers are produced in large numbers and ove r a larger period than the females and provide pollen, nectar, and sta minal hairs as rewards for pollinators. The presence and function of s taminal hairs with tip cells in Osyris alba has been reported for the first time. Female flowers are rewardless, producing neither mature po llen, nectar nor staminal hairs, but possess three modified yellow ind ehiscent anthers containing no viable pollen which may provide a stron g visual feeding stimulus for pollinators. It is suggested that pollin ators are attracted by deceit to female flowers by mimicry of the male s and the floral mimicry is, therefore, intraspecific and intersexual. The floral characteristics and flowering phenology of male and female plants are consistent with this kind of mimicry. The female flower po ssesses a tricarpellary ovary with three ovules of which only one deve lops. The single seed, containing a small embryo and a large, rich end osperm, is borne in a red fleshy bird-dispersed fruit. The reduction i n seed number per flower to one highly nutrient-invested seed, togethe r with a reduction of the multiflowered inflorescence to a solitary fl ower and the sequential production of ripe fruits over an extended fru iting season, suggest that the female function is markedly resource-li mited. It is suggested that, although all the reproductive characteris tics present in Osyris alba, as well as hemiparasitism, had probably e volved before the end of the tropical Tertiary, they are of adaptive a dvantage in the nutrient and water-limited environment of the Mediterr anean maquis.