It’s the time of year when our family loves to gather and see how prolate spheroids travel through the air, end-over-end.
Yes, our children are still talking about last December’s “Double Doink,” but I am not speaking about the NFL kicking game. Instead, we grab jewelweed (Impatiens capensis) seed pods, and carefully touch them so that we can watch the seed eject.
Now the NFL football did evolve to its prolate spheroid shape: It began life as a soccer ball and moved toward a rugby ball shape, in other words from a coconut to a watermelon. But even though the game was promoted as a means to toughen America’s youth in a time of extended peace, 15 deaths in one year was considered too violent. So the forward pass was promoted, and the ball became the shape of a jewelweed seed, not for end-over-end rotation, but for the spirals that an overhand toss produces.
So why did the jewelweed move from a game of fumbled seeds to an aerial attack? As with humans, the answer seems to lie in genetics. It is best for jewelweed to breed outside of its immediate family. In other words, if the “apple doesn’t fall far from the tree,” the resultant “inbreeding” is not in the plant’s best interest. In other words, natural selection means that the jewelweed plants that are best suited for survival are those that stray a bit from the “apron strings.”
Now, there is another evolutionary wrinkle. Natural selection also “figured out” that if the plant could toss its seed into a stream—jewelweed is a streamside plant—and if the seed could float. ... Well, it would be like joining the Navy and finding access to a much larger gene pool than just the next town over.
So a study published by Marika Hayashi and others, presents the following findings about jewelweed. First, if it is well-watered, jewelweed’s ejection mechanism exceeds the energy storage capacity of both spring steel and elastin. Second, to eject its seeds, jewelweed it takes a common plant stem structure and inverts it. Third, jewelweed launches its seeds exactly how physics would predict.
Nevertheless, its energy transfer is low. The implications of Hayashi’s study are that the energy transfer would be better if jewelweed’s seeds were BBs. But BBs don’t float. So because jewelweed’s natural selection “chose” both Air Force and Navy, it sacrificed some flight ability for sailing ability.
Of course, footballs both fly and float. But while the NFL’s prolate spheroid has evolved toward the throwing game’s spiral flight, I wonder why the jewelweed still “prefers” end-over-end flight.
As for me, I am glad that the watermelon—or for that matter coconuts—have not realized that ballistic dehiscence could be an efficient way of spreading their seed away from their parent plants.
Joshua Arp is an ISA-certified municipal specialist, Clarks Summit’s municipal arborist and an operator of an organic lawn and landscape maintenance business. Reach him at firstname.lastname@example.org.