Wednesday, July 27, 2016

Exploding seeds, or the magic of ballistochory

The beautiful lemony flower of a Pale Jewelweed, Impatiens pallida, dangles beneath the succulent foliage. We have one other native species in this genus (in Ohio), the Spotted Jewelweed, I. capensis, which has orange flowers. Both are very common in moist woods, often in semi-deep shade. Bumblebees are major pollinators, and hummingbirds are also smitten with the flowers.

Once successfully pollinated and seeds have developed, the trick is to find a way to disperse those seeds reasonably far from the parent. That's how plants maintain and/or expand their populations, and they've got a big bag of tricks to abet their migrations. Some plants have sticky fruit, and mammals unwittingly move them about. Other fruits float, and are readily dispersed by water. Yet others are windborn. Ants and other insects cart away seeds. There are many other mechanisms of seed dispersal, but we're going to talk about ballistochory.

It's not hard to see where the common name "jewelweed" comes from. The water-resistant leaves are waxy, and cause liquid to bead up into beautiful gemlike drops.

But it is the fruit of Impatiens that we're here to discuss. That would be the cylindrical misshapen package hovering over the leaf on a slender pedicel. This fruit is ripe, or close enough, and within are three or four seeds. The outer shell of the fruit is comprised of five elastic evanescent strips called valves. Thin bands of weaker tissue bind the valves together. Inside the fruit, the seeds are held in place by a specialized piece of tissue called the replum.

I stimulated dehiscence, or "popping" of the fruit seen in the previous image. Impatiens capsules are like little grenades ready to explode when ripe, and the slightest touch pops them open. In order to keep all the parts upon explosion, one must cup the fruit within a closed fist.

The lumps on the left are two of the seeds. On the right is the tangled mass of spent valves. During the growth process, the inside and outside of the valves are subjected to different tensions, so that the outer and inner walls have decidedly different stresses placed on them. When the fruit is ripe and finally dehisces, the thin veins of tissue holding the valves together abruptly rip. This causes a nearly instantaneous release of pressure in the valves, causing them to instantly revert to an unstressed state. In the process, the seeds are hurled forcefully outward, sometimes flying a few feet. The effect is similar to popping a fully expanded balloon, and the subsequent instant shriveling of the rubber shell.

Impatiens fruit are ripening nicely about now, and you can test explosive ballistochory seed dispersal for yourself. Nest time you see a big fat fruit, just tap it and watch the botanical bomb go off. Better yet, cup the fruit tightly in your hand to capture the seeds. Then eat them. They taste just like fresh walnuts.

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