When I lived in Venezuela in my 20s there was a tree near our house that really took the prize for saying “Stay away!” The trunk of this tree, Hura crepitans, known as the sandbox tree, was covered in big nasty thorns, giving it one of its nicknames, monkey no-climb tree. The sap was extremely poisonous and caused a rash and temporary blindness if you got it in your eyes. But another nickname was the dynamite tree. The fruits of this tree looked like those small pumpkins people might use as a centerpiece on the Thanksgiving table, with ribs running from the stem to the base. The little pumpkin-like fruits ripen and dry in the sun and then suddenly – BAM - the fruit explodes! It splits along those ribs in an explosion with the force and noise of a pistol going off. The seeds fly over 150 miles per hour and fall up to 60 feet away.
We don’t have anything around here to rival the Dynamite tree, but several local plants also have exploding fruits, albeit on a more modest scale. They might not rival the fireworks on the fourth, but they are fun to see. Jewelweed, violets, sorel and mistletoe all have seed pods that burst when ripe. How and why would they do this?
The why seems straightforward. Seeds that explode out of seedpods will find themselves far away from the parent plant where there won’t be much chance of that icky inbreeding with mom and dad. The babies also won’t compete with the parents for light, water and nutrients. Also, the jettisoned seeds might find themselves in a new area where that plant hadn’t grown before. The downside is that seeds might end up someplace completely unsuitable to the plant. And, speaking evolutionarily, plants count on their offspring thriving after the parents are gone. But, birds kick their babies out of the nest, so why not plants.
Jewelweed, or Impatiens capensis, is the poster child for exploding seed pods around here. Also known as Touch me Not, the plant is very common in the Northwoods, growing in slightly wet forest openings with good sun, or along lake shores. Plants are about 2-5’ tall with bright orange and red-speckled, trumpet-like flowers. They are pollinated by bees, hummingbirds or butterflies and, once fertilized, the plant produces inch-long seed pods with 2-5 seeds inside. The pod becomes slightly inflated as it ripens, and you can see the darkening and ripening seeds inside. When fully ripe, especially if something brushes against the pod, the whole pod explodes, and the seeds are sent flying. This is where the name Touch me Not comes from. Look for ripening pods in mid-to late summer, and if possible, introduce a child to the joy of giving a ripe pod a slight squeeze and watch their delight when they feel the gentle burst of seeds in their fingers.
Violets, such as the common blue violet or the yellow forest violet, both common throughout Wisconsin, also have exploding seed pods. The drying pods open along three seams creating three little “canoes” filled with seeds. Then, as they dry out more, the sides of the little canoes squeeze the seeds, like squeezing watermelon seeds with wet fingers, and the seeds go flying out. Another common plant, wood sorel, Oxalis stricta, has three-parted leaves that you could easily confuse with clover. But these plants have bright yellow flowers and are quite weedy in yards and waste places. The plant produces erect, candle-like fruiting capsules that, when ripe, burst open and expel thousands of seeds far from the parent plant.
Dwarf mistletoe, Arceuthobium pussilum, is another plant with exploding fruits. But this plant, a parasite on black spruce trees in northern Wisconsin bogs, causes the host tree to form tight clusters of branches and needles called a witch’s broom. While most species of mistletoe around the world are at least somewhat parasitic, this mistletoe is a true parasite, with scale-like leaves that do not photosynthesize. It produces root-like structures that penetrate the host tree, so it can suck water, sugar and nutrients directly from the host. Mistletoe plants produce separate male and female flowers, and following pollination and fertilization, produce a berry-like fruit. Pressure builds and builds in the berry, and finally the seeds erupt from the fruit with explosive force, traveling up 40 feet. A sticky coating on each seed helps the seeds stick to a new host and start another generation.
These plants might not have the wizardry of fireworks on the fourth, but it must be quite a show for the local ants and chipmunks!
