Shower power: Raindrops shoot seeds out with a splat

Brazilian botanists have caught Mother Nature playing with squirt guns.

SQUIRT CORNERS. The rain forest Bertolonia mosenii forms seeds in triangular brown capsules (above), which have open corners. Each capsule (below, left) reveals hundreds of seeds if someone peels the top open (at right, below). Pizo

W. L. Daltro

When raindrops hit the triangular seed capsules of a Bertolonia plant, water floods internal channels, then squirts out the capsule corners carrying seeds with it, report Marco A. Pizo and L. Patrícia C. Morellato of Universidade Estadual Paulista in Rio Claro. In the January American Journal of Botany, they say they’d never heard of such a seed-dispersal mechanism.

Rain-powered dispersal turns up in plenty of fungi, lichens, liverworts, and mosses, say the researchers. Flowering plants, however, rarely depend on a shower to spread their seeds.

Before the recent find, scientists knew of only two basic designs for the few flowers recognized as rain-sown. Some, such as the pearlwort Sagina, form seeds in raindrop-size cups. Droplets landing send up a splash and with it go the seeds. Other flowers, such as some kalanchoes, balance their seeds on miniature springboards. Rain bends the springboards down, and when they pop back, off fly the seeds.

Pizo and Morellato found the new seed-dispersal design in Bertolonia mosenii,

a Brazilian flower in the same family as North America’s meadow beauty. Some botanists have proposed that the open-cornered seed capsules in the Bertolonia genus lets the wind carry the seeds.

Think rain instead, says the Brazilian team. In a lowland nature reserve in southeast Brazil, Pizo checked 20 flowers monthly for a year. Seeds matured mainly in February and March, the rainiest period at the study site. They grow to less than 1 millimeter in length, so a raindrop flood can easily wash them out of their capsule. One 5-mm-wide capsule contains some 700 seeds.

Researchers sheltered some plants with little plastic roofs 10 to 20 centimeters above the capsules. At the end of the test, the roofed capsules hadn’t managed much seed dispersal. They contained roughly eight times as many seeds as those exposed to rain did.

In the lab, researchers mimicked falling rain by squeezing a dropper 1 or 2 meters over seed capsules. Water droplets sent seeds squirting out corner openings 8 cm and nearly 13 cm, respectively.

Plant reproductive biologist Spencer Barrett of the University of Toronto says that seed dispersal is “relatively poorly understood.” Yet its perils have huge consequences for a species. Many dispersal mechanisms dump many seeds into hostile territory. Also, plants that don’t spread widely face a risk of inbreeding.

Barrett says he’s not too surprised that a novel means of dispersal has turned up. “The more you look in the tropics, the more you find odd, new reproductive mechanisms,” he says.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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