They Get Around

If you’ve ever had to remove burrs from your pants after a hike or from your dog’s ear after a walk, you know what I’m talking about. The different ways plants have evolved for dispersing their fruits and seeds are truly remarkable. All for the sake of reproducing their species, seeds are adapted in different ways to travel by land, air, and sea—and in some cases, to even launch themselves on their own incredible journey through the digestive tracts of birds and other animals.

There are several ways seeds are dispersed: by hitchhiking on fur or clothing; by becoming airborne; by exploding from pods; by being carried in water; by being digested by birds, fish, and other animals; and by being buried.

While walking along the edge of my property a few days ago, I brushed up against some plants and found several “hitchhikers” attached to my jeans. They were Stick-tights (Desmodium cuspidatum). Their small, flat, green triangular seeds, which have tiny hooks that attach to the fur of many mammals, eventually find a new place to germinate. Devil’s Beggar-ticks (Bidens frondosa), with its two-pronged hooks, is another hitchhiker.

Stick-Tights seeds (Desmodium sp.) on a sneaker. Notice the small hairs. Photo by Jud McCranie / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0).

We all have blown the ripe seeds of Dandelion (Taraxacum officinale) or thrown the ripe seeds from milkweed pods to watch them float away. A “parachute” attached to your seed can certainly spread you far. Other seeds have wings that act like whirligigs or helicopters, such as the double-winged samaras of maples (Acer spp.) that spin to get away from the parent tree and hopefully finding good ground in which to germinate. There is a seed attached to each wing.

The orchid family has the world’s smallest seeds. Seeds such as these from Vanilla (Vanilla planifolia) don’t have an endosperm with food reserves so they need to disperse in the air and reach soil that contains fungi, which supply resources for germination and growth. Fingers and photo by Patrick Sweeney.

Researchers have documented incredible long-distance dispersals of plants and fungi on the feathers of birds. Microscopic spores of mosses, liverworts, and fungi have been found on the primary feathers of three bird species: Semi-palmated Plover (Charadrius semipalmatus), Red Phalarope (Phalaropus fulicarius), and American Golden-Plover (Pluvialis dominica). These birds migrate from the Arctic to South America. Those spores were still alive when the birds arrived at their destinations. When the birds molt their feathers the spores contact the ground and germinate.

Native to Connecticut, Jewelweed, also called Touch-Me-Not (Impatiens capensis), has a more explosive tendency. As the fruits dry out, mechanical energy stored in valves in the fruits cause them to coil inward, collapse the fruit pods, and eject the seeds. If you touch the slender seed pods, which are ripe now, you will see the seeds explode out of the pods, sometimes traveling up to 8 feet (2.5 meters) or more. Jewelweed spreads easily as a result. And it’s a good thing, because the flowers are an important source of nectar for migrating hummingbirds. The ripe white berries of dwarf mistletoes (Arceuthobium spp.), native to the western United States and Canada, also explode, ejecting seeds at an average speed of 60 miles (96.5 kilometers) per hour and scattering them as far as 50 feet (about 15 meters). The Eastern Mistletoe (Phoradendron leucarpum), native to southern North America is not nearly so dramatic. Its seeds are dispersed when birds eat its fruit, leave seeds in droppings and also wipe the sticky seeds from their bills on tree branches, where they can germinate.

The seed of the Coconut Palm (Cocos nucifera) can float in saltwater for up to a year before seeding itself on a southern sandy shore. The “seeds” of the Red Mangrove (Rhizophora mangle) actually germinate and become seedling propagules before dropping off the parent tree. They are dispersed by water and eventually embed themselves in the shallows.

Patrick Sweeney, Senior Collections Manager for Botany at the Yale Peabody Museum, is holding a Double Coconut or Coco de Mer (Lodoicea maldivica) the world’s largest seed. There are various theories as to why the seed evolved to be so large. The giant seeds drop near the parent and have the food resources to grow in the shade and develop a long petiole to reach light. Photo by Larry Gall.

There is a well-known relationship between the Yellow-rumped Warbler (Dendroica petechia) and the flowering shrub Small Bayberry or Wax Myrtle (Morella caroliniensis). When these warblers eat these berries, the birds’ gizzards scarify the seeds and their droppings help to fertilize them, ensuring germination. These birds were once called Myrtle Warblers.

Besides eating the blue, berry-like cones of the Eastern Red-Cedar (Juniperus virginiana), Cedar Waxwings (Bombycilla cerdrorum) love to eat the red berries of Mountain-Ash (Sorbus sp.), thereby spreading the tree’s seeds. Photo by Mike’s Birds from Riverside, CA, US / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.0).

Other seeds such as acorns rely on animals like squirrels, chipmunks, and even crows to bury them. If the animal does not return to its stash, the seeds may germinate. To attract wildlife, plants like blackberries and raspberries surround their seeds with a brightly colored and sweet tasting pulp. Even ants get into the act. The seeds of the spring wildflower Bloodroot (Sanguinaria canadensis) have elaiosomes, fatty attachments that attract ants. The ants will carry seeds to their nest and eat the elaiosomes. The seeds then have a fertile place to germinate.

Seed of Jeffersonia diphylla (Twinleaf; Berberidaceae) with a white or yellowish appendage called an elaiosome. This fat body attracts ants, which carry the seed to their nest. Seed dispersal by ants is known as myrmecochory. Photo by Hans Stuessi / CC BY (https://creativecommons.org/licenses/by/4.0).

Why do these seed dispersal adaptations exist? There are several reasons. Often, the survival rate of a seed improves away from the parent plant, because it competes less with the parent and, in cases where predators are attracted to a clump of newly germinated plants clustered around a parent plant, it is less likely to be eaten. These adaptations allow plants to reach specific habitats that are more favorable for survival, a hypothesis known as directed dispersal. Adaptations that favor dispersal on a more distant scale allow plants to colonize new and vacant habitats or regions, reducing a plant’s risk of predation and increasing its chances for survival.

So, the next time you spy a Jewelweed “popper” go ahead, give it a pinch and watch those seeds fly! You’ll be helping seeds do what they were meant to do—and besides, it’s fun.

Published by Jim Sirch

Jim Sirch is the author of Beyond Your Back Door, a weekly blog about nature in your neighborhood. He is also Education Coordinator for the Yale Peabody Museum, a UConn Master Gardener and board member of his local land trust. As a trained naturalist, he brings a deep understanding of geology, plants and wildlife and how they interact within a particular ecosystem. He holds a B.S in Forestry from West Virginia University, a B.S. from Miami University in Science Education; and an M.S. in Environmental Studies Administration from Antioch University. He is also the 2014 Sigmund Abeles Award recipient from the Connecticut Science Teachers and Supervisors Association for outstanding science teaching and professional development.

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