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Spring 2012

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

Snowflake images courtesy of the Jericho Historical Society, Jericho, Vermont

April 10, 2012

Explosive bittercress fruits

Most people associate early spring with flowers and tender vegetation, not ripening fruits. But for hairy bittercress, which began flowering in early February, it's seed-slinging time!

Anyone pulling bittercress from the garden this time of year has experienced the half-foot-tall weed's assault of seeds. Botanists use the term "ballistic seed dispersal" to describe the explosive nature of ripened bittercress fruits.

The pods, or siliques, consist of a central membrane flanked by two outer strips, or carpels, with about 30 seeds in between.

As cells on the outer surface of each carpel dry out, they shrivel, creating tension on the exterior face of the carpel. Meanwhile, the inner surface of the carpel is kept moist and plump by adjacent mucilage.

When cells at the bottom end of the carpel finally give way, the carpel snaps into an upward curl, propelling seeds as far as 16 feet — an effective strategy for spreading into new areas.

Bittercress reaps another benefit from ballistic seed dispersal: An insect climbing onto a ripe silique, with the intention of making a meal out of it, may suddenly find itself catapulted several feet away, far from its would-be meal.

SOURCE: "The mechanism for explosive seed dispersal in Cardamine hirsuta (Brassicaceae)," American Journal of Botany

Hairy bittercress silique and seeds, Cardamine hirsuta