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Specifics on explosive seed dispersal

Posted: August 25, 2011 - 3:25pm
The long spike of a geranium fruit has seeds around the base. They are flung forcibly outward when the fruit dries, in a form of ballistic dispersal.   Photo courtesy of Bob Armstrong
Photo courtesy of Bob Armstrong
The long spike of a geranium fruit has seeds around the base. They are flung forcibly outward when the fruit dries, in a form of ballistic dispersal.

As we wended our way up Gold Ridge, en route to Gastineau Peak, we noted some quivering leaves near the ground. Pretty soon, we could discern a slender, furry body moving deliberately from plant to plant. Then a small head with a white eye-ring poked up near a stem of northern geranium, nipped off the fruit, and munched up the seeds. This red squirrel was a long way from any trees, but it was systematically depleting the geranium seed crop, just as if it had come there for that purpose.

The fruit of northern geranium is so distinctive that the plant is sometimes known as “cranesbill.” It consists of a relatively tall spike (roughly an inch tall), at the base of which there are up to five attached knobs, each containing a seed. When the fruit matures and dries, a hinge at the top of the spike loosens abruptly. The spike then splits into several longitudinal sections that flip upward very rapidly. This action elevates the knobs almost explosively, flinging the seeds outward (like an underhand, backhand throw). I know from experience with another species of geranium that the force of the throw is enough to fling the seeds at least twenty-five feet, if unimpeded.

This mode of seed dispersal is called “ballistic dispersal.” It is found locally in lupine, whose pods twist open forcefully, violets and impatiens, which is sometimes called “touch-me-not” for the way in which the seed capsules pop open. The champion of the ballistics mode, however, is probably a tropical tree called Hura crepitans. The tree makes rock-hard fruits a couple of inches in diameter; the fruit splits into sections (like an orange) when it is thoroughly dry. With a mighty pop, the seeds fly in all directions at high velocity, rattling against the surrounding vegetation. The trick is to bring a few fruits home, put them on the kitchen table to dry, and wait. Then, when no one is mentally prepared, they will go off, startling the daylights out of every creature within earshot, and ricocheting all around the room.

That was a long digression from Gastineau Peak, but I couldn’t resist recalling the fun that Hura’s fruits provided. That was many years ago — these days they might just give me a heart attack.

The route up Gold Ridge provided a good floral show at the higher elevations, including gentians, monkshood and some still-flowering geraniums. In some dense salmonberry thickets we heard muttering and clucking and peeping, so we knew there had to be a family of grouse lurking under the leaves. Only after we passed by did the hen and big chicks flutter up and over the trail into the brush on the other side.

From Gastineau Peak we looked down into Icy Gulch, where three mountain goats reclined on a green knoll. There was a stiff, chilly breeze that sent us into the lee of a small side ridge for lunch, but the ravens were enjoying it thoroughly, showing off their aerobatic maneuvers.

On another day, a friend and I came down the Fish Creek trail, just because we hadn’t walked it for a long time. Aside from a prodigious mudhole filled with a deep, sloppy, viscous mess (which we reduced somewhat by using some primitive engineering), the most notable observation was the horde of pink salmon thronging the stream up the barrier falls. The banks were littered with long-dead chum salmon, largely intact except for missing eyeballs. The ravens had been foraging selectively for the choice bits of fat that pad the eyes. Dozens of ravens were still there, including what sounded like over-grown but lazy juveniles clamoring for food delivery from their parents. But no eagles.

Strangely, there was no bear sign along the trail, despite the dense crowds of pinks, until we reached the highway bridge. There we found one bear scat — full of blueberry remnants. This begs the question: Why were bears seemingly ignoring the stream full of salmon?

 

Mary F. Willson is a retired professor of ecology.

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