Although widespread among fungi, lichens, liverworts, and mosses, seed dispersal mechanisms operated by rain are unusual among flowering plants. Generally speaking, two mechanisms are involved in seed dispersal by rains: splash-cup and springboard. Here we describe a new seed dispersal mechanism operated by rain in a Neotropical rainforest herb Bertolonia mosenii Cogniaux (Melastomataceae). The study was carried out at the lowland Atlantic rainforest, Parque Estadual Intervales, São Paulo State, Brazil. To evaluate the role of raindrops dispersing the seeds, 32 plants were assigned to two groups: i) treatment group were covered with translucent plastic shelters 10-20 cm above the infructescences; ii) control plants were left uncovered. When fruiting finished, five fruits per plant were harvested and the seed number was counted. Seed dispersal distances were measured under laboratory conditions, and the fruit maturation schedule was monitored. Treatment and control plants differed significantly in the number of seeds per fruit (mean ± SD = 412.4 ± 186.1 and 52.4 ± 70.7, respectively; Mann-Whitney tests: U = 269.9, p < 0.001), suggesting that raindrops represent the mean by which seeds are released from fruits. The seed dispersal distances ranged from 1.3-29.5 cm (n=97) and from 1.9-35.3 cm (n=100) when water-drops fall from 1 m and 2 m height, respectively. Mature fruits were produced during the wettest months, insuring seed dispersal by rain. We experimentally demonstrate that rain is necessary to release the seeds from the capsules in an innovative way we named squirt-corner seed dispersal mechanism: when a raindrop strikes the mature fruit the falling water forces the seeds outward to the angles of the triangular capsule through which the seeds are released. As far as we know squirt-corner represents a new rain-operated seed dispersal mechanism, and a novel seed dispersal mode for Melastomataceae, and for flowering plants from Neotropical forests.

Key words: Atlantic forest, Bertolonia mosenii, hydrochory, Melastomataceae, seed dispersal