Seeds Were Flowering Plants' Secret Weapons

A new book explores how flowering plants beat out the competition on ancient Earth.

By Sarah Richardson|Tuesday, November 14, 2006
RELATED TAGS: SEX & REPRODUCTION

Call it flower power. When the forerunners of bloom-producing plants evolved, some 360 million years ago, the world was dominated by drab fernlike plants that reproduced via spores. The eventual emergence of flowering upstarts around 140 million years ago is still what Charles Darwin called an "abominable mystery." But one thing is clear: They crushed the competition. Today spore-bearing plants make up only 3 percent of all plants on land. The reasons for that conquest were unclear until the 17th century, when naturalists like Nehemiah Grew investigated the anatomy of flowering plants using the recently invented microscope. What they learned is that botany is fundamentally about sex, and flowering plants have thrived because they are better at it than spore producers. One of the bloomers' devastating weapons: a beautiful little bundle called a seed.

In a lavish new publication from Firefly Books, titled Seeds, botanist Wolfgang Stuppy of the Royal Botanic Gardens in Kew, England, and visual artist Rob Kesseler document the astonishing complexity of botanical sex. Before most plants can make seeds, pollen must navigate the specialized portal of a flower and fertilize an egg cell at the end. That is relatively easy to understand compared with all that must happen next: producing, nurturing, storing, and dispersing the resulting embryo. Flowering plants have evolved a sheltered three-tissue seed design that packs a lunch (the nutritive tissue called the endosperm) and an overcoat (the hard, protective seed coating) along with the embryo itself. Each component arises from a genetically distinct tissue in the fertilized plant.

Stuppy likens the evolution of the seed to the emergence of hard-shell eggs in land animals. When animals lived only in the ocean, they generally released eggs and sperm to mingle in water, as fish still do today. A hard-shell egg allowed animals to reproduce and thrive far from water, in the much harsher environment of dry land. Likewise, as the world headed into a cooler, drier climate around 250 million years ago, the early seed-bearing plants had a distinct advantage over their simpler, spore-releasing relatives that then flourished in moist, warm swamps.

Seed-bearing plants also figured out better ways to get around. Some seeds sprout improbably elaborate barbs in order to snag a lift on passing animals. A significant number hitch a ride by growing a morsel called a elaiosome that entices ants to carry them off a few feet. Other seeds are textured or buoyant, so they can float away on wind or water.

The human appetite for seeds has resulted in new forms of dispersal as well. Thousands of years ago, people began collecting and cultivating nutrient-rich seeds, like corn, lentils, and oats, for food. Modern farming has singled out certain seeds and made their plants fantastically successful; witness the acres of Triticum aestivum—ordinary wheat—lined up in neat rows across the plains of the United States. Meanwhile, botanists like Stuppy put a new spin on the tradition of seed collecting at the Kew gardens' Millennium Seed Bank, an international repository for plant species threatened with extinction in the wild.

Along with a loss of diversity, each vanished plant will take with it one more clue about how seeds work: "Their method of sexual reproduction is so sophisticated that, even today," Stuppy writes, "scientists do not fully understand why they do it in the way they do."

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