Floral Biology of Chaenorrhinum minus (Scrophulariaceae) a Self-compatible Annual

The floral biology of Chaenorrhinum minus (L.) Lange (Scrophulariaceae) was studied at two sites in central New York State. Individual flowers are short-lived (approximately 24 hr). Small plants generally produce flowers intermittently. More vigorous individuals produce many flower-bearing branches, which increases the probability that they will have at least one flower open when a pollinator visits the patch. Chaenorrhinum minus is self-compatible, though it has adaptations for insect pollination. The only insect pollinators observed during 2 seasons were halictid bees. These visited infrequently. Accordingly, most of the flowers in the population of C. minus were selffertilized. INTRODUCTION The genus Chaenorrhinum consists of 10 herbaceous species, five perennials and five annuals (Tutin et al., 1972). Most of these species have a localized distribution in southern Europe. Only one of them, C. minus, has spread widely, throughout most of Europe and much of eastern and central North America. This paper describes the floral characteristics and phenology of Chaenorrhinum minus in central New York State. Observations on the relative frequency of autogamy and insect-mediated allogamy are also reported. The plant is an introduced annual, which is locally abundant as a pioneer on well-drained, gravelly soils, especially on the cinder ballast of railroad tracks. It was first recorded in the United States at Camden, New Jersey, in 1874 (Deam, 1940). The population dynamics and seed dispersal mechanisms of this species have been previously described (Arnold, 1981). MATERIALS AND METHODS Field studies were carried out on populations of Chaenorrhinum minus growing on railroad cinders near the village of Poolville, Madison Co., New York, in 1979, and at a patch of waste ground ("weedlot") near the village of Hamilton, Madison Co., N.Y. , in 1979 and 1980. These sites have been previously described (Arnold, 1981). Floral characteristics. -The lifespan of flowers was estimated by observing buds, uniquely marked with enamel paint, from opening to wilting. Observations were made at 3-hr intervals throughout the lifespan of each of the 15 flowers observed. The temporal relationship between anthesis and stigma receptivity was investigated by testing for the presence of peroxidase enzymes at the stigmatic surface in flowers at different stages of anthesis. Zeisler (1938) showed that the appearance of peroxidases in stigmatic tissue is correlated, in many plants, with the onset of stigma receptivity. To perform the test, excised stigma-style complexes were immersed in a 3 % solution of hydrogen peroxide. Formation of oxygen bubbles was used as an indicator of stigma receptivity. Pollen tube germination at stigmatic surfaces was studied in excised stigma-style complexes that were cut open longitudinally and stained with acetocarmine and basic fuchsin (Chandler, 1931). Phenology. Blooming times of Chaenorrhinum minus at the weedlot during 1980 were determined by weekly counts of the number of open flowers in five 1-M2 quadrats. A daily expected number of flowers per plant as a function of the number of flowerbearing branches was estimated by randomly sampling 60 plants each day for 14 days at the peak of the blooming season (July 1-July 13). Also spelled Chaenorhinum by some European authorities.