VISCIN THREADS, POLLINATION EFFICIENCY AND LOW POLLEN-OVULE RATIOS

Some Onagraceae are characterized by quite low pollen-ovule ratios. We suggest they are an evolutionary response to highly efficient pollination, mediated through viscin threads, which hold large numbers of pollen grains together, and by stigmas, which are large relative to the area of the pollinator over which the pollen is spread. The pollen grains of some Leguminosae are joined by "exinal connections" which are chemically and morphologically similar to viscin threads. In the legumes the area of the stigma is small relative to the area of pollen on the pollinator and pollination is facilitated by short connections which hold small numbers of pollen grains together. THE REPRODUCTIVE STRUCTURES of Onagraceae are conspicuous in two respects. First, the pollen grains are united by viscin threads (Skvarla, Raven, and Praglowski, 1975, 1976; Skvarla et al., 1978) and second, the stigmas are quite large. Because viscin threads hold numbers of pollen grains together it has been assumed that they play an important role in pollination. In some instances, correlations between viscin thread morphology and pollinator class (Skvarla et al., 1978) suggest coevolution. Similar structures, herein referred to as exinal connections, were recently reported in Leguminosae (Cruden and Hermann-Parker, 1979; Graham, Barker, and Freitas Da Silva, in press). Since the pterotribic transport (carried on the wings) of legume pollen (Cruden and Hermann-Parker, 1979; Heithaus, Opler, and Baker, 1974; Owen, 1971) contrasts strongly with the movement of Onagraceous pollen on the proboscis or body of the pollinator, exinal connections and viscin threads may play quite different roles in pollination. As we show (Table 1), Onagraceae are also conspicuous in having quite low pollen-ovule ratios (P/O's). We develop the hypothesis that viscin threads, which hold numbers of pollen grains together in large clumps, and stigmas, which are large relative to the area of pollen on the pollinator, contribute to quite efficient pollination, and the low P/O's reflect this efficiency. In ad1 Received for publication 20 November 1978; revision accepted 8 May 1979. We thank Kathryn Grove, Stephen D. P'-ndrix, Sharon Hermann-Parker, Peter H. Raven, John J. Skvarla and Sharon Miller Ward for critically reviewing the manuscript. Thomas Melchert provided the collections of Oenothera and Gaura. Pollen samples were obtained from specimens in the Field Museum, Chicago, Illinois, the University of California, Berkeley, California, and the University of Iowa, Iowa City, Iowa. dition, we compare pollination efficiency of Epilobium angustifolium L. (Onagraceae) with that of Caesalpinia pulcherrima (L.) Sw. (Leguminosae), and discuss viscin threads and exinal connections with respect to their morphology. MATERIALS AND METHODs-Pollen-ovule ratios were determined by counting the number of pollen grains in an anther, multiplying that number by the number of anthers and dividing by the number of ovules in the ovary. The number of pollen grains per clump, in the Onagraceae, was estimated by carefully removing the pollen from an anther into aniline bluelactophenol. The mass of pollen was then carefully teased apart. Absolute measurements of viscin thread length were difficult to obtain because of the elasticity of the threads, hence the measurements tend to be underestimates. The area of pollen on a butterfly was obtained by tracing an outline of the wing on graph paper, cutting out that area and weighing it. The weight of the wing divided by that of 1,000 mm2 provided an estimate of wing area. The area of pollen on the bumblebees that pollinate Epilobium angustifolium was measured directly on the bees. Stigmatic areas were obtained by measuring the diameters, etc., of the stigmas. Pollen grains of a number of species in Caesalpinia, Delonix, and Bauhinia (Leguminosae) were obtained from herbarium specimens, coated with gold and examined with a Cambridge Model S4 scanning electron microscope. Some pollen grains of C. pulcherrima and D. regia (Boj.) Rafin. were acetolyzed in a 9:1 /v:v mixture of acetic anhydride:sulfuric acid for 2 to 3 min prior to coating and SEM examination. Means plus or minus the standard error are given in the text.