Wandering through a meadow, a child stops by a jewelweed plant, on the lookout for its plump seed pods. As she reaches in and touches one, it suddenly bursts open and ejects a seed, startling and delighting her. This child is inadvertently helping with an important part of a plant’s life cycle, the dispersal of its seeds. Producing seeds for the next generation is only part of a plant’s job. It needs those seeds to reach a place where they can grow. Spring-loaded seed capsules like those of jewelweed are just one of the many fascinating mechanisms plants use to disperse their seeds.
A seed is the fertilized, ripened ovule of a cone-bearing plant (gymnosperm) or a flowering plant (angiosperm). The seed contains an embryo, the tiny beginning of a new plant, and it is usually equipped with at least some nutrients for the embryo, stored in starchy tissue called the cotyledon. Some plants, like corn, iris, and lily, are referred to as monocots, having just one cotyledon. Beans and maples are examples of dicots, plants that produce seeds with two cotyledons. Together, the embryo and cotyledon(s) are enclosed and protected by a seed coat. In suitable conditions the embryo begins to grow, and its parts absorb water and grow quickly. The root (radicle) extends down into the soil to absorb water and nutrients, the stem (hypocotyl) grows upward, and the leaves (plumule) open to capture sunlight and make food for the plant by photosynthesis.
In conifers, the seeds are formed inside the cones, which protect but don’t fully enclose them. In the flowering plants, which dominate the world’s vegetation, seeds form and ripen inside the flower’s ovary, which then matures into a container called a fruit. When we think of a fruit, we usually picture the soft, fleshy varieties that satisfy our palates, like apples, melons, or berries. However, many plants produce dry, woody or papery fruits. A fruit may enclose just one seed (like a sunflower seed) or many (like a milkweed pod.) If a whole group of fruits grow close together, as in Queen Anne’s lace or goldenrod, we call it a seed head. Nuts like acorns and beechnuts are seeds enclosed in woody fruits. Plants in the legume family, like peas and beans, enclose their seeds in pods, arranged in a single row. Looking at a bean or pea we can see a slight depression, called the hilum, where it was attached to the pod. As the pod dries out, it splits along a seam to release the seeds. Valued as a food source for humans and livestock, other plants in the legume family include alfalfa, peanuts, lentils, clover, and soybeans.
Seeds need to move away from the parent plant to avoid competition for sunlight, water, nutrients, and space, and they have many ways to travel. Seeds and their containers use wind, water, animals, or mechanical means to reach new locations. Some plants use force to eject their seeds from spring-loaded containers. The capsules of violet, columbine, witch hazel, and jewelweed burst open when they mature, propelling the seeds through the air. Milkweed pods split along one edge, exposing a spiral of tightly-wound seeds that unfurl a few at a time. Some seed containers are attached to a long, flexible stalk that sways back and forth in the wind. Columbine and campion capsules open at the top and allow the wind to shake the seeds onto the ground.
The seeds of many grasses, wildflowers, shrubs, and trees are dispersed by wind. They have different adaptations for being carried by the air. A milkweed seed is covered in hundreds of long, thin hairs that serve as a parachute. These hairs repel water, so they also keep the seed from sinking if it should land on a pond or stream. Willows produce fuzzy seeds in catkins that, like milkweed, can sail on wind or float on water. Dandelion seeds glide with a parachute of fuzzy hairs, as do thistle and aster seeds. Larger wind-borne seeds of maple, ash, elm, and pine have papery wings that spin in the wind, slow their descent, and allow them to glide as they drop.
Plants that grow in or around water produce seeds well equipped for a wet journey with air pockets and specialized seed coats to increase buoyancy. Water lily seeds develop a jelly-like covering that helps the seed to float. The jelly soon dissolves, and the seed sinks to the bottom where it germinates underwater. Coconuts are buoyant seeds that travel vast distances across oceans, sprouting along shorelines far away from the parent tree. On a smaller scale, cranberries grow in bogs where uneaten berries drop off the plant and may float away in high water.
Animals help disperse the seeds of many plants. Soft and fleshy fruits appeal to the senses of mammals, birds, and even turtles with their bright colors, sweet or rich tastes, and fragrant smells. Though these fruits are digested, the seeds pass through digestive tracts and survive, due to their tough seed coats. Blueberries, raspberries, chokecherries, sumac, and dogwood are a few examples of plants that attract animal dispersers with their tasty, nutritious berries. Some wildflower seeds like bloodroot, trillium, and Dutchman’s breeches have a fleshy structure called an elaiosome attached to the seed coat. Ants eat the oil- and protein-rich elaiosome and then discard the seed in an underground “compost” pile, an ideal place for the seed to germinate. Some nuts and seeds can’t survive being eaten and instead count on being collected and carried away by animals but forgotten – in a chipmunk burrow or a hole made by a crow, jay, or squirrel. Those that escape being a meal germinate the next spring.
Seeds do not have to be eaten for animals to disperse them. Hitchhiking seeds catch rides on fur, feathers, or clothing. Burdock burrs are covered in barbs, Queen Anne’s lace, agrimony, and enchanter’s nightshade use bristles, and beggar ticks have two barbed spines for hitchhiking. Dispersal by animals allows seeds to efficiently travel great distances away from the parent plant. Because it’s only by chance that a seed lands in a good place to grow and mature, plants produce seeds and fruits in prodigious quantities and in so doing feed all kinds of animals.
As agriculturalists, gardeners, and travelers, humans disperse seeds too. Throughout history, humans have introduced exotic seeds as food and ornamental plants. Clovers and pasture grasses were brought to North America for grazing and hay; burdock, dandelion, and lamb’s-quarters were intended as food for humans but most gardeners now see them as weeds. Apple trees, native to the Caucasus, have found North America a hospitable climate, and many grow wild. Queen Anne’s lace was introduced from Europe for medicinal uses and has since escaped cultivation and colonized roadsides and other open areas. Some species that escape cultivation become nuisances or even threaten the native ecosystems of their new homes.
Plant breeders often select plants to create a product that appeals to costumers. Bananas, seedless grapes, and seedless watermelon are examples of fruits that are cultivated so they do not produce seeds. Without seeds, these varieties cannot reproduce in the wild, nor can gardeners save the seeds. Bananas and grapes grow from cuttings of the original plant, while seedless watermelons are hybrids between two varieties that make a sterile fruit.
We all have memories from childhood of helping a plant disperse its seeds, like blowing away dandelion seeds or popping jewelweed. Seed-bearing plants have evolved diverse adaptations to ensure that their seeds and fruits travel. Whether by wind, water, or animal, each seed and fruit is well equipped for its journey.
Suggested Reading
Brown, Lauren. Weeds in Winter. New York: W. W. Norton, 1976.
Holland, Mary. Naturally Curious. North Pomfret, VT: Trafalgar Square, 2010.
Magee, Dennis W. and Ahles, Harry E. Flora of the Northeast: A Manual of the Vascular Flora of New England and Adjacent New York. Amherst, MA: University of Massachusetts Press, 1999.
Scott, Jane. Field and Forest: A Guide to Native Landscapes for Gardeners and Naturalists. London: Walker, 1992.
Nearby Nature 