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Brief Communications |
Departamento de Botânica, Plant Phenology and Seed Dispersal Group, Universidade Estadual Paulista, Caixa Postal 199, 13506-900 Rio Claro-SP, Brazil
Received for publication May 10, 2001. Accepted for publication August 14, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSTUDY SITE AND PLANT...MATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Key Words: Atlantic forest • Bertolonia • Brazil • hydrochory • Melastomataceae • seed dispersal
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSTUDY SITE AND PLANT...MATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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; Van der Pijl, 1982
). Some desert plants and herbs from temperate regions apparently use rain as a seed dispersal mechanism (Brodie, 1951
; Savile, 1953
; Friedman, Gunderman, and Ellis, 1978
). Generally speaking, two mechanisms are involved in seed dispersal by rains: the splash-cup and the springboard (sensu Brodie, 1955
). In the former, raindrops are caught by cup-shaped capsules, and the seeds they contain are thrown away with the splashing water. In the springboard or catapult mechanism, the fruit or persistent calyx tube (or gemmae in the case of Kalanchoe tubiflora; Brodie, 1955
) is attached to the stem by a resilient pedicel, which is bent downwards by the pressure of a falling raindrop. When the pressure is released, the pedicel springs back into its normal position shooting the seeds away from the plant. In this paper we describe a new seed dispersal mechanism operated by rain in a Neotropical rainforest herb, Bertolonia mosenii Cogniaux (Melastomataceae). We experimentally demonstrate that rain is necessary to release the seeds from the capsules through what we call the "squirt-corner" seed dispersal mechanism. In addition, dispersal distances of seeds were measured under laboratory conditions. Fruiting phenology was also monitored and compared to the seasonal distribution of rains. As far as we know, the squirt corner is a novel mode of seed dispersal for the family Melastomataceae and for flowering plants occurring in Neotropical forests.
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STUDY SITE AND PLANT SPECIES |
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TOPABSTRACTINTRODUCTIONSTUDY SITE AND PLANT...MATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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4000 mm of rain annually (Morellato et al., 2000
). Although rains are well distributed throughout the year and in no month is there <100 mm, rains are more intense and frequent from September to March (Fig. 1), a period that also corresponds to the hottest season (Morellato et al., 2000
). Old-growth forest predominates in the study site, with an open understory and a canopy 25–30 m high (Morellato et al., 2000
).
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). Bertolonia species are small herbs (sometimes creeping) and have long been used as ornamentals (Bailey, 1947
). Their fruits are so distinctive from other Melastomataceae that they attracted the attention of early botanists (De Candolle, 1828
; De Chamisso, 1834
). The fruit is prominently triquetous (Figs. 2–3); the triangular, dry capsule dehisces in three valves with persistent ovary apex that are only seen from a polar angle (Figs. 4–5; Baumgratz, 1991
). Such an unusual fruit received its own category (bertolonid fruit) in the classification system of Baumgratz (1989)
for fruits of Brazilian melastomes. Wind was thought to be the dispersal agent for the seeds of Bertolonia (Baumgratz, 1991
).
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). It is a common species at the study site, where it usually forms patches on the forest floor. It also grows on rocks, along riverbanks, and on fallen and standing (live or not) tree trunks. Each plant bears 1–24 pediceled capsules (N = 37; x [±SD] = 7.9 ± 4.9) whose triangular, raindrop-capturing surface measures 4.0–6.2 mm on its side (N = 29, x [±SD] = 4.7 ± 0.5; Figs. 2–4). The fruits are upright positioned and arranged sequentially along the infructescence stem in a manner that avoids the superposition among them (Fig. 2). As a consequence, no fruit obstructs the passage of a falling raindrop to the fruit immediately below it (Fig. 3). Seeds are tiny, measuring from 0.3 to 0.7 mm in length (Baumgratz, 1991
), and are covered by the ovary apex in the mature fruit (Figs. 4–5). Each fruit may contain >700 seeds (see below) arranged along the three placental axes (Fig. 6). Besides sexual reproduction, B. mosenii can also propagate via vegetative growth. |
MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONSTUDY SITE AND PLANT...MATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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To evaluate the role of raindrops in dispersing the seeds, 32 additional plants were marked in January 2000 immediately prior to fruit maturation (Fig. 1). These plants were randomly assigned to two groups, treatment and control, with 16 plants each. Plants in the treatment group were covered with plastic shelters positioned 10–20 cm above the infructescences. The shelters, made of translucent plastic and totally open on the sides, prevented raindrops from contacting the fruits. Control plants were left uncovered. In April 2000, following the fruiting period, five fruits of each plant in both groups were harvested and the number of seeds was counted.
Seed dispersal distances were recorded by dropping water from a dropper on mature fruits in the laboratory. To facilitate locating seeds following simulated raindrops, we placed a white cloth underneath the plants. Drops of water were allowed to fall from heights of 1 m (in 17 plants) and 2 m (in another 10 plants) on the fruits. The seed displacement distances were then measured.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONSTUDY SITE AND PLANT...MATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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). Treatment and control plants differed greatly in the number of seeds their fruits contained (mean ± SD = 412.4 ± 186.1 seeds/fruit and 52.4 ± 70.7 seeds/fruit, respectively, N = 80 fruits in both cases; Mann-Whitney tests: U = 269.9, P < 0.001), suggesting that raindrops effectively release seeds from fruits. Additionally, control plants had a much greater variation in mean number of seeds per fruit than treatment plants (coefficients of variation [CV] = 135.0% and 45.1%, respectively). Such variation probably reflects individual differences in seed dispersal success, which depend upon the arrangement of the canopy and subcanopy foliage above the plants. In fact, for plants positioned under certain herbs in the Marantaceae, whose broad leaves intercept raindrops, we observed germinating seeds sprouting directly from capsules, thus indicating unsuccessful dispersal.
The average distance of seed dispersal was 8.5 cm (SD = 6.2 cm, range = 1.3–29.5 cm, N = 97) when water drops fell from 1 m height and 12.7 cm (SD = 8.9 cm, range = 1.9–35.3 cm, N = 100) from 2 m height above the fruit. Dispersal distances from this experiment likely represent only a minimum estimate, as under natural conditions raindrops may fall from the forest canopy (20–30 m), likely producing a much wider range of dispersal distances.
In Bertolonia mosenii, the seeds are covered by the persistent ovary apex (Figs. 4–5). When a raindrop strikes the mature fruit, the water droplet forces the seeds outward to the angles (corners) of the triangular capsule and the seeds are released. Unlike the splash-cup mechanism, where the seeds exposed on the bottom of the cup are released by the splashing action of the raindrop (Brodie, 1951
), in B. mosenii, seeds are "squirted" through the fruit corners. Seed shape also differs between splash-cup fruits and Bertolonia. In the former, the seeds are lenticular, while the Bertolonia seeds are obovate to clavate, favoring its displacement through the fruit corners. Differences in fruit and seed structure, as well as mechanisms for releasing seeds, warrant a new denomination for Bertolonia: squirt-corner seed dispersal mechanism.
Given that all Bertolonia species possess triangular fruits similar to those of B. mosenii (Baumgratz, 1991
), we suggest that raindrops are the seed dispersal agent initiating the squirt-corner mechanism for all members of the genus. Dispersal by raindrops should also be investigated for other melastomes in the genus Salpinga (S. margaritacea and S. longifolia), which also possess bertolonid fruits (Baumgratz, 1989
; Barroso et al., 1999
) and may share a similar seed dispersal mechanism.
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FOOTNOTES |
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2 Author for reprint requests (pmorella{at}rc.unesp.br
).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONSTUDY SITE AND PLANT...MATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Barroso G. M. M. P. Morim A. L. Peixoto C. L. F. Ichaso 1999 Frutos e sementes morfologia aplicada à sistemática de dicotiledôneas. Editora da Universidade Federal de Viçosa, Viçosa, Brazil
Baumgratz J. F. A. 1989 Morfologia dos frutos e sementes de Melastomataceae brasileiras. Archivos do Jardim Botânico do Rio de Janeiro—Série Botânica 27: 113-155
Baumgratz J. F. A. 1991 O gênero Bertolonia Raddi (Melastomataceae): revisão taxonômica e considerações anatômicas. Archivos do Jardim Botânico do Rio de Janeiro—Série Botânica 30: 69-213
Brodie H. J. 1951 The splash-cup dispersal mechanism in plants. Canadian Journal of Botany 29: 224-234
Brodie H. J. 1952 Nature's splash guns. Natural History 61: 403-407
Brodie H. J. 1955 Springboard plant dispersal mechanisms operated by rain. Canadian Journal of Botany 33: 156-167
De Candolle A. P. 1828 Mémoire sur la famille des Mélastomatacées. Treuttel et Wurtz, Paris, France
De Chamisso A. 1834 De plantis in expeditione speculatoria romanzoffiana et in herbariis regiis berolinensibus observatis. Melastomataceae americanae. Linnaea 9: 383-385
Friedman J. N. Gunderman M. Ellis 1978 Water response of the hygrochastic skeletons of the True Rose of Jericho (Anastatica hierochuntica L.). Oecologia 32: 289-301[CrossRef][ISI]
Morellato L. P. C. D. C. Talora A. Takahasi C. C. Bencke E. C. Romera V. B. Zipparro 2000 Phenology of Atlantic rain forest trees: a comparative study. Biotropica 32: 811-823[ISI]
Mori S. A. J. J. Pipoly 1984 Observations on the big bang flowering of Miconia minutiflora (Melastomataceae). Brittonia 36: 337-341[CrossRef][ISI]
Savile D. B. O. 1953 Splash-cup dispersal mechanism in Chrysosplenium and Mitella. Science 117: 250-251
Van der Pijl L. 1982 Principles of seed dispersal in higher plant, 3rd ed. Springer-Verlag, Berlin, Germany
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