Pollination

• Title

  Pollination

• Description

  The morphology and development of the flowers of C. guianensis have been well illustrated by Thompson (1921). In addition to being showy, its flowers are strongly and pleasantly aromatic. Ormond et al. (1981, 1982) determined that the osmphores of this species are most abundant on the hood filaments and less common on the adaxial surface of the petals. Oil droplets are found in the cells of the epidermal and stomatal guard cells of the osmophores. In addition, the cells of the mesophyll of these structures are rich in starch. After anthesis, oil droplets and starch are not nearly as plentiful. The internal temperature of the flowers at anthesis is seven degrees above the ambient temperature. Ormond et al. (1981, 1982) suggest that the oil droplets volatilize to yield the pleasant aroma and that the starch in the adjacent cells provides the energy for this process. The aroma, at least in some individuals, is dominated by monoterpene hydrocarbons (Knudsen & Mori, 1996).The apices of the hood filaments are yellow, a color that attracts bees. Moreover, UV light is only reflected from the hood and has perhaps evolved to attract pollinators to rewards found in that part of the flower. The sweet aroma of these flowers is strongest in the early morning and bees have been recorded visiting the flowers as early as 5:00 a.m.; the petals and androecium drop from the flowers around 12:00 p.m. (Ormond et al., 1981). Yarsick et al. (1986) report that the flowers of a cultivated Venezuelan tree open gradually between 7:00 and 8:30 p.m. with the hood and ring anthers opening simultaneously. These authors confirm observations that the hood anthers are larger than the ring anthers - the former have an average of 2,850 tetrads and the latter 450 monads per anther. The flowers of C. guianensis are without nectar and are mostly visited by bees in search of pollen. Xylocopa brasilianorum has been reported to be the principal pollinator outside of the native range of C. guianensis in the botanical garden of the Museu Nacional in Rio de Janeiro. Dahlgren (1924) states "The large black bumble-bees that visit the flowers force their way under the tip of the hood, between the two sets of stamens, and become thoroughly dusted with pollen." It is likely that the bees observed by Dahlgren also represent a species of Xylocopa. Yarsick et al. (1986) report bees, wasps, and a flower fly visiting the flowers, but note that Xylocopa frontalis and a species of Bombus appear to be the most efficient pollinators. Carpenter bees of the genus Xylocopa have also been observed pollinating cultivated trees of C. guianensis in India (Aluri & Reddi, 1990). Ormond et al. (1981) have demonstrated that bagged flowers do not set fruit nor do the flowers in the bags end up with pollen on their stigmas. They demonstrated that two percent of artificially self pollinated flowers produce fruit whereas artificially cross-pollinated and naturally pollinated flowers yield 6% and 4% fruit respectively. In their study, they observed a tree flowering out of phase with the other trees set fruit, and there are other unsubstantiated reports of cultivated trees growing in isolation producing fruit. Thus, it appears that C. guianensis is self-compatible, but that there are more fruits set if out crossing takes place. Poiteau (1825) observed that the pollen of the hood was larger and yellow and that of the staminal ring smaller and white. Thompson (1927) was the first to suggest that pollen from the hood does not germinate on the stigma, but it was Jacques (1965) who first observed that the ring and hood pollen are different. This observation has been confirmed by Mori et al. (1980), Ormond et al. (1981), and Yarsick et al. (1986). These authors have demonstrated that the pollen of the staminal ring is found in monads and has a smooth exine while that of the hood is in tetrads and has a rougher exine. Ormond et al. (1981) add that the ring pollen is covered with droplets of a lipid substance whereas that of the hood is dry. Examination of the stigma shows that monad pollen dominates the surface while tetrad pollen is rarely found there. The ring pollen shows a high degree of both in vitro and in vivo germination whereas not a single pollen grain from the hood anthers germinates (Mori et al., 1980; Ormond et al., 1981). In conclusion, it has been clearly demonstrated that there are two types of pollen in the flower of Couroupita guianensis and that the most likely pollinators are species of large bees, such as Bombus and Xylocopa, which are rewarded for their efforts with non-germinating pollen that they collect from the hood.