We’re deep into seed season on the Vineyard. Searing yellow goldenrod flowers have turned into drab puffs of drying seed. The elongated seed pods of butterfly-weed split and disgorge a flurry of fluff. Acorns roll underfoot like marbles. As plants die (if they’re annuals) or drift into winter dormancy (if they’re perennials), seeds are the legacy they leave, all that’s left to show for the year’s productivity.
Seeds look simple and inert, but they’re remarkable creations. In one way or another, all seeds need to disperse from the parent plant, sprout at the right time, and then sustain the young plant until it can produce its own food. But plants have evolved an astonishing array of ways to achieve these basic results.
One of the most important traits of seeds is that, unlike mature plants, they can move. Rooted in the ground, plants eventually succumb to competitors or changing conditions. Seeds counteract this fate by colonizing new places. One very common way to disperse is on the wind; many seeds (think of dandelions and milkweed) are attached to bundles of fiber light enough to drift, sometimes for miles, on the lightest breeze. Other plants have evolved mechanical means to ensure at least a little mobility for their seeds. The seed pods of many species build up internal strain as they dry, and when they’re fully ripe, they release that tension by bursting and scattering the seeds they contain.
Many other plants rely on an assist from animals to move their seeds around. The most obvious instances are berries, which are nothing more than seeds packaged in a layer of edible, often tasty flesh. In the hours that a winterberry or a bayberry spends in the digestive tract of a bird, it can be transported many miles, the berry’s flesh helping fuel the journey. Voles, mice, and squirrels also move huge quantities of seeds around; some of what they gather and cache goes uneaten and has a chance to germinate. Surprisingly, even the humble ant is involved in a similar partnership with many plant species. Some species of violets, for instance, produce seeds coated with sugars, which hitch a ride from foraging ants.
Relying on such imprecise means of travel, plants need to produce large numbers of seeds because only a few will stumble onto a spot offering suitable conditions. The lucky ones that do still need to pick the right time to germinate — late enough in the spring to avoid freezing, but early enough so the new plant can get established and store resources to survive the winter. In general, the cues that prompt wild seeds to sprout are the same ones familiar to every gardener: temperature, moisture, and, sometimes, brightness or daily duration of light. And to avoid premature germination in late fall or during a winter thaw, seeds of flowers both wild and cultivated often need a period of cold conditions before they are primed to sprout.
But plants have also evolved ways to postpone germination until conditions signal a moment of opportunity. Germination of many grassland wildflower seeds, for example, is promoted by exposure to smoke (or, more precisely, to certain chemicals contained in smoke). In effect, when the seeds smell smoke, they know that established plants around them have burned, increasing the odds that seedlings will have access to sufficient light and water.
Once they’ve sprouted, young plants need a helping hand until they’ve developed leaves and pigments to carry out photosynthesis. Seeds generally meet this need by containing stored starch or oil that the seedling can draw on for energy. In the case of, say, an acorn, each potential oak tree is packaged with enough food to send down a substantial tap root. But resources stored in the seeds are resources the parent plant can’t use for itself. Each species, therefore, has evolved its own way to balance investment in seeds with the need to reproduce against slim odds.
A few of our most interesting plants minimize the need for resources in their seeds by tapping another plant’s roots. Known as “hemiparasites,” such plants usually have a close relationship with just one or a few host plants. Sandplain gerardia, for example, an annual that is surely the most rare plant found on the Vineyard, apparently depends on a link to little bluestem grass in order to survive.
Seeds are essential for plants but important, too, for what they offer other wildlife. Deer fatten on acorns; migrant sparrows flock to patches of weeds (ragweed seeds are a favorite among these birds); meadow voles collect and store a wide range of grass and wildflower seeds. Just as human well-being depends on the size of the harvest, the natural world flourishes or withers depending on the year’s productivity. Seed season is a time that shapes the future.