Summer ending…

…and musings on seed dispersal

Summer seems to be closing down all too quickly. Bird song is past, replaced by the chips and squeaks of juvenile birds scuttling in the underbrush. Even the late-hatching mallard ducklings have their full-on adult plumage and no longer hang out with Mama all the time. Fireweed is already done blooming in most places (in very early August, yet!), a sure sign that fall is upon us.

Here and there we can see some late-season wildflower stragglers, putting out their last flowers of the year. However, a few wildflowers are just getting started: the little blue-flowered felwort (a.k.a. star gentian) along the Boy Scout trail in the grassy meadow near the slough; the purple-flowered northern gentian and the sky-blue gentian on Gold Ridge.

For many plants, this is a time for dispersal of seeds (although cottonwoods and most willows accomplished this earlier in the summer). Mature seed capsules have opened on fireweed, releasing the seeds with their fluffy, white parachutes to float on the breezes. Lupine seed pods have begun to pop open on warm days, scattering the ripe seeds that pitter-patter as they fall through the surrounding vegetation. All over town, the non-native mountain-ash offers its orange, fleshy fruits to willing avian foragers that gobble up the free lunch and excrete the seeds elsewhere.

The whole point of seed dispersal is to send a plant’s offspring away from the mother plant, landing in new sites where they may be able to germinate and grow. If all the seeds just dropped at mama’s feet, so to speak, the competition among those densely packed seedlings would be ferocious, and seed-eaters would be likely to come and demolish the lot in one go. So there are advantages to traveling, but it is always a sort of lottery: most seeds and juvenile plants die, landing in a bad site or picked off by a predator. On the whole, however, it seems to be better to disperse than to congregate.

Plants have evolved many different ways of dispersing their seeds. Here is just a sample, from species here in Southeast.

–By wind: Some seeds bear devices that can lift them on a puff of wind: the fluffy parachutes of fireweed, willow, cottonwood, and dandelion; the propeller-like blade of a twirling maple seed; the small flat wing around the seeds of alder, spruce, hemlock, and rattlebox. Orchid seeds are as small as fine dust, because they (unlike other seed plants) contain no stored material to nourish seedlings; so they easily waft away on a breeze.

Fireweed seeds. Photo by Bob Armstrong

–By water: The seeds of the yellow pond lily are surrounded by a buoyant matrix that keeps the seeds afloat for a few days.

–By animal consumers: The succulent, fleshy fruits of blueberry, salmonberry, twisted stalk, mayflower, lingonberry, devil’s club, and crabapple are eaten by birds and bears, which digest the fruit and excrete or regurgitate the seeds.

–By animals that pick up seeds on fur or feathers (or socks): the prickly seed-heads of avens, the spiky seeds of some grasses, the micro-hooks on capsules (and stems and leaves) of bedstraw and the seeds of sweet-cicely.

–By explosive opening, ballistically shooting out the seeds forcefully: Lupine pods snap open; touch-me-not capsules fly open at a touch, expelling the seeds; mistletoe seeds are expelled with force (and may also sometimes get stuck on passing animals). Wild geranium holds its seeds in five small cups at the base of a long ‘crane’s bill’ with a hinge at the tip; when the mature seed container is dry, the hinge pops opens suddenly and flings seeds vigorously (with a backhand toss). Long ago, I measured the distance achieved in this way, for the eastern species of geranium, and found that seeds could travel at least thirty feet from a plant no more than twenty inches tall. Not bad.

–By shaking: Fern-leaf goldthread bears a whirligig of capsules, each with an opening near the end. If the stem or the capsules are jostled at just the right time and in the right way, a seed flies out. Foamflower puts its seeds in capsules that look like old-fashioned sugar-scoops, with the bottom part longer than the top part. Again, the right kind of jostling releases a seed, which gets an extra impetus from the lever-like action of the lower part of the scoop. Seeds of chocolate lily are stacked tightly in their capsules; the capsules split open and the seeds can be shaken out (they also have small wings that might give them a little glide).

–By unknown means: Some plants produce seeds with no evident means of dispersal, either on the seed itself or on the mother plant—no edible fruit, no hooks, no wings, no ballistics…These species generally have shorter dispersal distances than those with specific dispersal adaptations, so their seeds are likely to be more clustered than those of the other plants; but what are the ecological consequences of their more clustered distribution? Seed shakers probably do a little better. Ballistically dispersed seeds achieve a somewhat wider distribution, and both wind- and animal-dispersed seeds can travel far.

There are other ways for seeds to get around. As Darwin pointed out, they may ride in the mud on the feet of ducks. They may travel in the guts of herbivores that eat the greenery but don’t digest the seeds. Floods may wash them way downstream. These, however, are largely serendipitous events, not specific adaptations resulting from evolution; they may nevertheless be quite effective in moving seeds around.

The pattern of seed distribution around a parent plant is called a ‘seed shadow’. Most seed shadows exhibit a concentration of seeds relatively close to the parent with a long ‘tail’ of fewer seeds, extending to greater distances. Most studies of seed shadows have focused on distances that include the majority of the dispersed seeds. But the tail of the distribution cannot be ignored. It is much more difficult to measure, because there are fewer seeds at greater distances from the parent, and some of the distances can be very long indeed (miles!). But it is those far-travelling seeds that make it possible for plants to colonize new areas.

Here is a little natural history game to play, if you are inclined, as you walk the trails. Try to find as many seed dispersal mechanisms as you can; there are differences among habitats. Can you find plants that have other dispersal mechanisms, and how do they work? Think about other factors that influence the distance that seeds travel (such as the height from which the seed is released). And special kudos to anyone who can send me a good local photo of the seed container of northern geranium after it has flung out the seeds.

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