THE ROLE OF MUCILAGE IN THE GERMINATION OF CUIPO, CAVANILLESIA PLATANIFOLIA (H. & B.) H. B. K. (BOMBACACEAE), A TROPICAL TREE

The mucilage in the fruits of cuipo, Cavanillesia platanifolia, is an adaptation to the unpredictable rainfall at the beginning of the rainy season in the tropical moist forests of central Panama. It allows cuipo to germinate rapidly following rains in the early rainy season, before other potential competitors. Mucilage comprises 27% of the mass of the large ( 3.5 g) winddispersed fruits of cuipo. Water uptake capacity of cuipo fruits (7 g water/g fruit in 10 min) was greater than that of the diaspores of five wind-dispersed species sympatric with cuipo. Diaspores of these five species lost all their absorbed water within 2 days but cuipo retained it for 2 wk. Experiments using two mucilage treatments (seeds within fruit and seeds alone) and three watering treatments (daily, weekly, and biweekly) indicated that 1) neither mucilage nor watering treatments altered percent germination; 2) the presence of mucilage-rich fruits lessened the effects of drought on seedling development and decreased the degree of wilting; and 3) both treatments interacted in their effects on percent and cause of mortality. IN THE TROPICAL moist forests of central Panama, seeds of 75% of the woody species germinate in the first 3 mo of the 8-mo-long rainy season (Garwood, 1983). Within this period, there is considerable variation among species in the temporal pattern of germination, including differences in the length of time between the beginning of the rainy season and the beginning of germination and the degree ofsynchronyofgermination(Garwood, 1982). To understand this interspecific variation, the germination response of each species must be evaluated in terms of the selection pressures acting on the timing of germination. In this tropical forest, the primary factor selecting for germination early in the rainy season has been the competitive advantage accruing to earlier germinating seedlings, at least for species dependent on gaps in the forest canopy for establishment (Garwood, 1979, 1982). Factors selecting against early germination are the unpredictability of rainfall in the early rainy season and constraints on the timing of seed dispersal (Garwood, 1983). l Received for publication 23 July 1984; revision accepted 5 February 1985. I thank C. K. Augspurger, C. C. Horvitz, N. Brokaw, and two anonymous reviewers for comments on the manuscript and the Smithsonian Tropical Research Institute for making the facilities of Barro Colorado Island available. This study was supported by a National Science Foundation Graduate Fellowship and Doctoral Dissertation Research Grant (DEB 76-81976) and the Coulter and Hutchinson Funds of the University of Chicago. In this report, I evaluate the germination response of Cavanillesia platanifolia (H. & B.) H. B. K. (Bombacaceae), commonly called cuipo, in terms of the conflicting selection pressures on timing of germination. Cuipo is a deciduous canopy tree up to 40 m tall, which flowers in the middle of the dry season and disperses its large ( 12 cm wide), single-seeded, wind-dispersed fruits in the late dry season (Croat,1978). It ranges from Nicaragua to Peru and is common in the tropical moist forests of eastern Panama (Croat, 1978; Hartshorn, 1978). Cuipo is a fast-growing, early successional species (Hartshorn, 1978). Cuipo seedlings, with leaflike cotyledons up to 12 cm long, are the largest phanerocotylar seedling (sensu Duke, 1969) among >200 species examined in a concurrent study of germination (Garwood, 1983). A cursory examination of germinating cuipo fruits revealed copious amounts of wet mucilage, a trait shared with its Brazilian congener C. arborea (Duarte, 1979). In dry fruit, the golden-brown mucilage is concentrated around the seed and extends into the base of the wings. As external mucilages are known to aid in water uptake and retention during germination in some species, particularly in dry habitats (Harper and Benton, 1966; Mayer and PoljakoSMayber,1975; but see Grubert,1974), I wished to evaluate the role of mucilage in the germination response of cuipo. Since external mucilage is rare in species with large or winddispersed diaspores, in trees, and in tropical