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Seeds of milkweeds take flight on fine hairs.

Seeds of milkweeds take flight on fine hairs.


Seed Dispersal

The Myriad Ways Plants Have Evolved to Survive and Thrive


Humans use words like inventive and resourceful to describe ways in which plants do various things. Of course, plants don’t possess intelligence or will, so these words can’t be used to describe their capacity to survive and thrive. These mechanisms are actually the result of millions of genetic experiments to stay alive in a given situation. Keep in mind the Earth has not stood still since plants first made their first successful attempts at living on land about 425 million years ago.

Some of the most amazing plant stories have to do with their ability to reproduce. A key to their successes is the discovery of sex. Yes, the mixing of genes from plant to plant can increase the possibility that subsequent offspring have a gene package that may be more suited to prevailing conditions and that the resulting daughter plant will survive, breed, and increase the population in a more positive way.

The result of sexual reproduction in flowering plants is, of course, a seed or seeds. These packages of genetic material have the ability to lie dormant for a while (sometimes a very long while) until conditions are ripe to begin a new cycle of life. Seeds come in all sizes, from the nearly microscopic seeds of lettuces (more akin to grains of sand), to the biggest of all, the double coconut palm (a giant two-lobed coconut that rivals Coco’s behind). All seeds must leave their mother and find their ways to another spot and hopefully germinate, grow into another mature plant, and repeat the dance to send their offspring out, too.

That dispersal of seeds has taken myriad paths. There are seeds encased in flotation devices like the husk of a coconut, capable of island hopping across the ocean. Some come attached to tufts of fine hairs that can waft on the slightest breeze like those of milkweeds, some bromeliads, and the famous kapok tree (Ceiba). There are seeds that have papery wings like those of elms and maples that also catch the wind and whirl down far from their mother tree. Many seeds come wrapped in attractive and edible casings so that animals will find them tempting and disperse them; think apples, berries — all those tasty fruits.

Dispersal can take on other dimensions, too. There are a number of plants whose ovaries respond to touch or heat to explosively expel the seeds inside. Many species of pines do not release the nuts from their cones until and unless they undergo a wildfire. The common garden impatiens produces seeds in an inflated pod that splits suddenly when disturbed, ejecting its many tiny seeds all at once. Some euphorbias and an interesting member of the cucumber family, Echinocystis lobata, do the same thing.

Once the seeds have left the mother ship, they encounter challenges to establishing wherever they land. To ensure success, there are other adaptations for these difficult times. For example, members of the geranium family have seeds that are sharply pointed on one end, but have a relatively long “tail” (called an awn) that is hygroscopic, meaning that it readily absorbs moisture. When the seed leaves the fruit, the awn dries unevenly into a tight spiral. Once in contact with moist soil, it rehydrates, undoing the spiral and driving the seed into the ground. Other seeds are clothed with barbs to latch onto the fur of passing animals, or, in the case of a few aquatic plants, they may be covered in a sticky gel that glues them, at least temporarily, to duck feet. It just goes to show you what a few million years of experimentation can produce.

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Virginia Hayes, curator of Ganna Walska Lotusland, will answer your gardening questions. Address them to Gardens, The Independent, 122 W. Figueroa St., S.B., CA 93101. Send email to vahayes@lotusland.org.

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