At the Speed of Pollen

Photo by Roger Irwin
Bunchberry

Too often, innovations that have existed in the plant world for millennia are attributed to the human mind. A study of the bunchberry (Cornus canadensis), found in woodlands throughout New England, proves us wrong yet again. It seems that bunchberries have been using the catapult, a discovery credited to humans, for much longer than we have.

Bunchberry, a relative of the flowering dogwood tree (C. florida), uses its catapulting mechanism to spread pollen grains far and wide. The process is triggered either by the petals opening or by an insect landing on the plant. As the flower opens, pollen launches from the plant in less than 0.5 milliseconds.

Using high-speed video, Dr. Joan Edwards and her colleagues from Williams College recorded and timed the opening of the flower. The bunchberry moves faster than the one hundred milliseconds it takes a Venus flytrap to close around prey, affording it the title of “world’s fastest plant,” according to some reports.

The bunchberry catapult functions like a medieval trebuchet, using a fulcrum, or elbow, to attain greater distance and a strap to enhance throwing power. The pollen is attached to the throwing arm, ready to fly. The power behind the catapult lies in the release of stored elastic energy, like a coiled spring unsprung. Once an insect lands or wind opens the flower, the arm, held in place by a thin vascular string, is released and pollen is launched up to 2.5 centimeters into the air. The height, more than 10 times that of the flower, ensures that pollen will be caught and carried away by the wind, which can take it as far as 3 feet away. The grains accelerate at 2,400 times the force of gravity and reach speeds of 7 miles per hour. During the pollen’s flight, it experiences 800 times the force that astronauts feel during takeoff. 

Bunchberry is found in woodlands throughout northern North America, and in the mountains to the south. It grows in shady areas with moist, well-drained soils, thriving in cool woods and on mountain slopes. The plant is usually between 2 and 8 inches tall, with white petals (called bracts) that appear from May to July. Although the bracts resemble petals, they encircle a dense cluster of tiny white or yellow-green flowers. Surrounding the flower is a whorl of four to six leaves. Small, bright red berries appear in the fall, taking the place of the flowers. The berries are edible, but the seed is large, leaving only a small amount of fruit.

The catapulting mechanism aids in the reproduction of the bunchberry by allowing it to cross-pollinate with other plants through two possible methods. First, when an insect triggers the explosion, pollen sticks to its hair and body. When the insect lands on a different flower, the pollen attaches to the new stigma. Smaller insects that do not travel from flower to flower in succession, such as ants, will not trigger the reaction. Second, and more commonly, the flowers can open on their own and allow the wind to carry pollen to other flowers.

To see the bunchberry’s catapulting action firsthand, take a walk through the woods next spring. Be sure to bring along a magnifying glass and a twig to trigger the explosion, but don’t get too close! 

 
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