seed dispersal -- 11/10/21

Today's selection -- from Guns, Germs and Steel: The Fates of Human Societies by Jared M. Diamond. Plants distribute their seeds in many different ways: wind, water, and the guts of animals:

"Let's begin by looking at domestication from the plant's point of view. As far as plants are concerned, we're just one of thousands of animal spe­cies that unconsciously 'domesticate' plants.

"Like all animal species (including humans), plants must spread their offspring to areas where they can thrive and pass on their parents' genes. Young animals disperse by walking or flying, but plants don't have that option, so they must somehow hitchhike. While some plant species have seeds adapted for being carried by the wind or for floating on water, many others trick an animal into carrying their seeds, by wrapping the seed in a tasty fruit and advertising the fruit's ripeness by its color or smell. The hungry animal plucks and swallows the fruit, walks or flies off, and then spits out or defecates the seed somewhere far from its parent tree. Seeds can in this manner be carried for thousands of miles.

"It may come as a surprise to learn that plant seeds can resist digestion by your gut and nonetheless germinate out of your feces. But any adventurous readers who are not too squeamish can make the test and prove it for themselves. The seeds of many wild plant species actually must pass through an animal's gut before they can germinate. For instance, one Afri­can melon species is so well adapted to being eaten by a hyena-like animal called the aardvark that most melons of that species grow on the latrine sites of aardvarks.

Epilobium hirsutum seed head dispersing seeds

"As an example of how would-be plant hitchhikers attract animals, con­sider wild strawberries. When strawberry seeds are still young and not yet ready to be planted, the surrounding fruit is green, sour, and hard. When the seeds finally mature, the berries turn red, sweet, and tender. The change in the berries' color serves as a signal attracting birds like thrushes to pluck the berries and fly off, eventually to spit out or defecate the seeds.

"Naturally, strawberry plants didn't set out with a conscious intent of attracting birds when, and only when, their seeds were ready to be dis­persed. Neither did thrushes set out with the intent of domesticating straw­berries. Instead, strawberry plants evolved through natural selection. The greener and more sour the young strawberry, the fewer the birds that destroyed the seeds by eating berries before the seeds were ready; the sweeter and redder the final strawberry, the more numerous the birds that dispersed its ripe seeds.

"Countless other plants have fruits adapted to being eaten and dispersed by particular species of animals. Just as strawberries are adapted to birds, so acorns are adapted to squirrels, mangos to bats, and some sedges to ants. That fulfills part of our definition of plant domestication, as the genetic modification of an ancestral plant in ways that make it more useful to consumers. But no one would seriously describe this evolutionary process as domestication, because birds and bats and other animal consumers don't fulfill the other part of the definition: they don't consciously grow plants. In the same way, the early unconscious stages of crop evolution from wild plants consisted of plants evolving in ways that attracted humans to eat and disperse their fruit without yet intentionally growing them. Human latrines, like those of aardvarks, may have been a testing ground of the first unconscious crop breeders."

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author:

Jared M. Diamond

title:

Guns, Germs, and Steel: The Fates of Human Societies

publisher:

W.W. Norton & Company, Inc.

date:

Copyright 2005, 2003, 1997 by Jared Diamond

pages:

115-117
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