It’s easy to assume plants just sit there and do little, while animals run around busying themselves. Not the case: Plants often are on the move, or at least their seeds are. A plant doesn’t need legs, flapping wings, or a brain to travel beyond home to propagate. It just needs something or someone to help.
Water can be involved in dispersing seeds—from water lily seeds floating on a lake to coconuts caught in ocean-crossing currents. And animals often play a role, with birds carrying them on feathers and feet, mammals on feet and fur, and we humans in our pants cuffs, and on our farm machinery, cars, trains and planes. But riding the wind is usually the best way to go.
Airborne seeds spiral down from trees, drift serenely in light breezes over meadows, or catch air currents and fly high over continents, even oceans.
For air travel to be most successful, a seed first must have a slow enough descent from the plant, so it can be caught by wind and carried afar to fertile soil. It must be dispersed far enough from its parent so as not to compete for space and nutrients.
Some seeds fly high. Others are so small they mingle with dust in the high atmosphere circling the globe. The smallest of these are orchid seeds weighing less than a microgram each, but the 2 million tufted seeds from a single 10-inch cattail head are only slightly larger.
We’re familiar with the plumed parachute seeds: dandelion in spring, milkweed in autumn. There are dozens of others, such as thistle’s starburst, goldenrod’s tufts, dogbane, wild artichoke and goatsbeard.
More than most plants, dandelions have traveled and populated northern and southern latitudes at every longitude. They are everywhere. And they owe this success to their parachute seeds, which have been known to fly at 18,000 feet.
Before the trip, a maturing dandelion seed develops spiny projections at its top that push upward to become the flight apparatus, or pappus.
The seed itself is suspended on a vertical shaft below its fanned-out parachute. When full-fledged, the pappus responds to humidity, opening when air is dry and closing when damp. With low humidity and a wind of less than 2 mph, a dandelion seed can travel any distance, for any length of time. In high humidity, the pappus closes, and the seed drops to the ground for possible germination.
And if a dandelion seed falls on water, it rests on the surface where the pappus serves as a sail, responding to the slightest breeze, driving the seed ashore.
At this time of year, it’s milkweed seeds that take off. They explode from tightly packed pods and drift off in airborne clouds over meadows. A milkweed seed usually detaches from its pappus less than 200 yards after taking flight, an event you’ll notice if you look closely. After the seed drops, the drifting parachute, relieved of its burden, ascends higher in the air.
Large single-winged seeds represent another means of aerial dispersal. White pine and maple seeds are two examples, yet they differ aerodynamically. After detachment from a cone, a white pine seed drops in a rapid, irregular, fluttering fall, while the more refined maple seed makes a more leisurely descent.
The little maple whirlybirds flying through autumn’s leaf swirl represent a sophisticated means of aerial dispersal. Every fall, after seeing them spin down, I toss a few into the air to watch them gyrate back to earth. Each seed rotates precisely according to aerodynamic principles.
Maple seeds, called samaras, develop in pairs, with separate wings sticking out in opposite directions. They are twins. They hang clustered together, waiting for inevitable dispersion. Only when the wind blows, or the attachment weakens, do they break apart and take flight. And there they are … a sudden gust, and the sky is filled with a cloud of twirlers, thousands of them airborne and away to their fates.
Note how the wing emerges from a maple samara at an almost imperceptible pitch from the heavy oval seed. The wing has a reinforced leading edge, while the trailing edge is tissue-thin and curved outward to offer a better sail. Once free in air, its rotation is rapid, either clockwise or counter-clockwise.
Dispersal by wind does not always require a flight plan. I’ve noticed small seeds dancing over the surface of ice-crusted snow, blown down from the hill above. Sixty years ago a biologist in North Dakota measured the speed and distance seeds moved over snow crust. The seeds, he found, that traveled best—and those that move in winter here in northern New England—were mostly small and spherical. Seeds rolling over the snow are our miniature equivalents to the tumbleweeds of the Western Plains.