Fragmentation Experiment and Model for Falling Mercury Drops

The experiment consists of counting and measuring the size of the many fragments observed after the fall of a mercury drop on the floor. The size distribution follows a power-law for large enough fragments. We address the question of a possible crossover to a second, different power-law for small enough fragments. Two series of experiments were performed. The first uses a traditional film photographic camera, and the picture is later treated on a computer in order to count the fragments and classify them according to their sizes. The second uses a modern digital camera. The first approach has the advantage of a better resolution for small fragment sizes. The second, although with a poorer size resolution, is more reliable concerning the counting of all fragments up to its resolution limit. Both together clearly indicate the real existence of the quoted crossover. The model treats the system microscopically during the tiny time interval when the initial drop collides with the floor. The drop is modelled by a connected cluster of Ising spins pointing up (mercury) surrounded by Ising spins pointing down (air). The Ising coupling which tends to keep the spins segregated represents the surface tension. Initially the cluster carries an extra energy equally shared among all its spins, corresponding to the coherent kinetic energy due to the fall. Each spin which touches the floor loses its extra energy transformed into a thermal, incoherent energy represented by a temperature used then to follow the dynamics through Monte Carlo simulations. Whenever a small piece becomes disconnected from the big cluster, it is considered a fragment, and counted. The results also indicate the existence of the quoted crossover in the fragment-size distribution.Comment: 6 pages, 3 figure

Work in Darkness: How Hawk Moth Produce Mangabas (Hancornia speciosa, Apocynaceae) in Brazilian Cerrado

Mangaba (Hancornia speciosa, Apocynaceae), native to Cerrado in Brazil, is a tropical fruit crop, consumed mainly as juice. Supply, however, does not satisfy the market because mangabas are still harvested mainly in natural populations. Thus, the species has a great potential as future fruit crop. Recently, first experimental mangaba orchards arose in agricultural research stations in northeastern Brazil. We analysed floral biology and breeding systems, determined effective pollinators and evaluated the pollination success of mangaba in natural environments and experimental orchards. Furthermore, we evaluated environmental demands of effective pollination. Hancornia speciosa is a self-incompatible tree with nocturnal flowers. Insects with long mouthparts of more than 30 species, especially nocturnal hawk moths (Sphingidae), visited the flowers. The flowers exhibit a precise pollination apparatus, which optimises pollen transfer between flower and pollinator. During a flower visit, almost half of exogenous pollen grains adhering to the proboscis are deposited on the stigma surface. While the pollination mechanism avoids self-pollination, mass-flowering promotes geitonogamy. A pollination experiment with nylon threads simulating consecutive flower visits within a crown revealed that there is no fruit set after the third consecutive flower visit. Nevertheless, all groups of flower visitors with long mouthparts were effective pollinators and, mangaba plants, in general, benefit by a high pollinator abundance and diversity. Fruit set in the studied populations were strongly pollinator limited, and the mangaba berries showed a high variation in size and weight. Seed number was directly correlated to fruit weight. An optimised pollinator mediated flow of cross pollen, thus, is responsible for large fruits. Our data suggest that fruit set could be increased two to three times with better pollination service. The study indicates that pollinator management implies management of the surrounding vegetation of the mangaba orchards guaranteeing a diversified environment. Strong pollinator populations require sphingophilous plants in the surroundings of the plantation that provide nectar when mangaba does not flower. Moreover, their oligotrophic caterpillars need specific species of host plants to survive. Orchards with clonal mangaba plants will result in low fruit set