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Abstract:The breeding systems, reproductive effica(des and population densities of" 75 species of trees, shrubs, perennial herbs and vines in a mont~me tropical cloud forest in Venezuela are investiga~ed. 56.96% and 44.32~ of the trees, versus the other life forms considered, respectively possess obligate outbreeding mechanisms. Two shrubs are non-pseudogamous apomiets. The percentage of dioecy among tree species (31~) is among the highest recorded in tropical forests, lgeproductive efficacy is similar under ~ll breeding systems i)) the forest interior. Ohligately outbred taxa are slower recolonizers of a disturbed border as compared with non-obligate outbreeders. Ext)lanati(ms are advanced for the high ineidence of dioecy c)mbincd with a low level of sell'-incompatibility among hermaphroditic species. It is concluded tkat the breeding system spectrum reflects an unpredictable pollination pattern, rather than insufficient pollinator servicing.

The effect of flowering time and of distance between pollen donor and pollen recipient on the proportion of filled achenes, aborted seeds, and seedling survivorship was studied in populations of Espeletia schultzii in the Venezuelan Andes. Hand-pollinations were performed in two different years and at different times during the flowering season. Pollinations within-population included crosses between plants within a few meters to a maximum of 500 m apart and between-population pollinations included crosses between plants more than 10 km and up to 78 km away. Late in the flowering season, plants suffered a reduction in the proportion of filled achenes and an increase in aborted seeds. The proportion of filled achenes and aborted seeds did not vary significantly between the distances between pollen and ovules in crosses done early in the flowering season. However, the proportion of filled achenes in crosses between individuals within 30 m distance were very variable, either very successful or not successful. Beyond 30 m, for some of the populations, the frequency of unsuccessful crosses was almost 0. This pattern was observed at three elevation sites in two consecutive years. When crosses were done late in the flowering season, the proportion of filled achenes was lower in crosses between close individuals and between very distant ones. Seedling survivorship varied significantly between the crossing distances at the higher elevation only. The results show that a clear effect of the incompatibility alleles on the crossing success of the plants is present in the different populations examined, while they suggest that the effect of inbreeding or outcrossing depression is less clear but might still be present only under certain conditions. Late in the season, when plants might have fewer resources available for reproduction and at the high and intermediate site where cold and dry environmental conditions are less favorable for the plants, the discrimination among distances was stronger.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Botanical Society of America is collaborating with JSTOR to digitize, preserve and extend access to American Journal of Botany.http://www.jstor.org Amer. J. Bot. 71(2): 273-280. 1984. AMONG THE DIFFERENT MODES of pollination, bat pollination is probably the most recent in the evolutionary history of angiosperms and is of great importance in tropical ecosystems (Heithaus, Flemming and Opler 1975; Sazima, 1976). Most of the studies on bat pollination (chiropterophily) report the time of anthesis, amount and solute concentration of the nectar produced by the plants and the general ' Received for publication 17 March 1983; revision accepted 5 July 1983.We would like to acknowledge the Sociedad Venezolana de Ciencias Naturales for letting us make use of the headquarters at the Estacion Biologica de Los Llanos in Calabozo, Venezuela. We are immensely grateful to Haydee Farihas for her technical assistance in the field and laboratory. We wish to express our gratitude to Sixto Garcia for the tables and graphs presented in the manuscript and to the following students for their help in the field: Z. ABSTRACTThe breeding system and floral characteristics of Baukinia ungulata L. were investigated during two consecutive flowering cycles (Dec-Feb). This tree grows in savannas in small sized patches scattered in the grassland and in larger tree clusters called "mata." Anthesis occurs between 17:00 and 19:30. Anther dehiscence and nectar secretion begin one hour after anthesis. Nectar is produced for 10 to 12 hr with an average sugar concentration of 13.5%. Flowers in small trees secrete nectar at a higher rate than do flowers in bigger trees. Two phyllostomatid bat species, Glossophaga soricina and Phyllostomus discolor, visit the flowers of B. ungulata between 20:00 and 4:00. Visits of variable duration occur every 1 to 2 hr. In general, the times of greatest nectar production of highest solute concentration of the nectar and of most frequent visits by the main pollinators coincide. The flowers are significantly dimorphic in pistil length, and ovules are abortive in the short-pistil flowers. Artificial self-and cross-pollinations in longand short-pistil flowers show that B. ungulata is genetically self-incompatible and functionally andromonoecious.