Q: Why did the witch hazel cross the street?
A: To grow on the other side.
Of course shrubs can’t really cross the street, but their seeds can. The seeds of witch hazel are shot out of capsules with such force that they travel up to 40 feet. We tend to think of seed dispersal as a passive enterprise, with a germ of the next generation carried aloft on tiny parasols, hitchhiking with burrs that are stuck to fur or feather, or dropping onto the surface of water to drift away and sprout in distant soil. But witch hazel is one of many kinds of plants and fungi that can actively expel their seeds or spores a considerable distance by employing various forms of compression chambers, coiled green springs, and catapults.
Some years ago, I was leading teachers through the Philbrick-Cricenti bog in New London, New Hampshire, on a summer field trip. After marveling at the carnivorous pitcher plants, sundews, and bladderworts, we sat down along the trail to eat our own lunch near a large patch of sphagnum moss. When the sunlight beamed through a hole in the tree crowns and shone on the moss’s tiny red spore cases, they began to blow their little caps off. Each capsule emitted an audible “pop” as it spewed a minute puff of spores into the bright air. Sphagnum spore cases shrink as they mature, increasing the pressure inside to as high as five atmospheres until they flip their tops and spill the spores. This is equivalent to the pressure that an ocean diver experiences at a depth of 165 feet.
Certain kinds of mushrooms also use pressure to shoot their spores out, in this case from between their gills. This helps to mobilize the spores and move them up out of the still air near the ground and into the wind. Cup fungi grow spores in structures that absorb moisture as they mature, increasing the pressure inside. Eventually, and often en masse, these structures erupt and release a smoky cloud of spores that waft on the breeze.
Jewelweed, or “touch-me-not,” is perhaps the most well known seed thrower. The orange or yellow, cornet-shaped flowers form thin, inch-long seedpods. Gently pinch the end of one of the ripe, swollen pods, and five tensile plant springs instantly separate and unfurl to fling the seeds a good distance. Henry David Thoreau aptly described this phenomenon in an essay titled, The
Dispersion of Seeds. “Touch-me-not seed vessels, as all know, go off like pistols at the slightest touch, and so suddenly and energetically that they always startle you, though you are expecting it.”
The round-leaved yellow violet squeezes its seeds in a way that is similar to how we shoot watermelon seeds by pinching them between the tips of our fingers until they fly. As the violet’s three-parted seed capsule ripens, then dries, it gradually compresses the seeds until they are spit out, one at a time, with some traveling nearly 10 feet.
Certain members of the pea family also sling their seeds. As the pods of wild vetch mature and dry, the inner lining shrinks more than the outer layer, which generates a tension between the two layers. Eventually, this tension causes the pod to split lengthwise down the sides, and each half of the pod snaps into a coiled shape, flipping the seeds outward. Soak the halves of the open pod in water, and they will resume their original shape.
Another legume, called birds-foot trefoil, forms beautiful, pea-like yellow flowers in mid-summer. Flowers mature into narrow, inch-long, brownish black seedpods that radiate out from the stem in a pattern that resembles the toes on a bird’s foot. Each ripe seedpod springs open to widely disperse its 10 to 20 seeds. In most areas, birds-foot trefoil is an innocuous denizen of roadsides and neglected areas. In some places, however, the plant is considered an invasive species, thanks to its form of seed dispersal, which is so effective it can overrun a fallow field.
The short, leggy witch hazel tree inhabits the forest understory as an integral part of the plant community. Autumn is a good time of year to look for the ripened seedpods that hang on branches alongside the fragrant yellow flowers. These unique blossoms display spidery petals when the sun shines, then protectively close up during cold nights. Should you decide to prune a sprig of witch hazel and take it indoors to enjoy, place it in an empty vase in the kitchen. As the fruit begins to warm and dry, pressure will build until the two seeds burst out of each capsule with a shot. Maybe one of the black-coated seeds will land on your plate. Then you can crunch it open to taste the pistachio-like flavor inside.