Flower Origins Carry Scent of Mystery

But fossil discoveries put scientists on trail of 'sneaky herb theory'

When the florist hands you your Valentine's Day bouquet, treat those roses with respect. They represent a form of plant life that has outlasted the dinosaurs and changed the face of the planet.

Roses and their relatives also represent one of Earth's most enduring evolutionary puzzles. Since the days of British naturalist Charles Darwin, who looked at flowering plants and found "an abominable mystery," scientists have struggled to answer basic questions: How old are they? What led to their remarkable diversity (between 250,000 and 300,000 species today)? And how did they come to dominate plant life in most places on the planet?

Researchers have been busy looking for clues at both ends of the time line.

In what George Schatz of the Missouri Botanical Garden in St. Louis calls "spectacular finds," paleobotanists during the past decade have uncovered fossils of flowers and flower parts dating back 120 million years. These discoveries have "fueled a resurgence of research into the origin of flowering plants."

Known collectively as angiosperms, flowering plants blossomed in diversity and geographic reach during the middle and end of the Cretaceous period, between 146 million and 65 million years ago. The new fossils have prompted some researchers to revise their views on the ancient angiosperms that lie at the base of this expansive family tree.

Treasure in the mud

Initially, fossils dating back between 93 million and 95 million years pointed to woody magnolia-like plants as the original angiosperms. But during the mid- 1980s, researchers began to find charcoalized remains of tiny herb-like angiosperms that date as far back as 120 million years.

The fossils, which have been found in mud deposits in Sweden, Portugal, England, and and along the Eastern and Gulf coasts of the United States, preserved flowers and flower parts in fine detail and represent a gold mine of data for researchers.

"There's more material than we can possibly work on," says Patrick Herendeen, an assistant professor of biology at George Washington University in Washington.

Their emergence has led researchers such as James Reveal, a botany professor at the University of Maryland at College Park, to look to "the sneaky herbs among the feet of dinosaurs" as the root of today's profusion of flowering plants.

The sneaky-herb theory holds that true angiosperms were weed-like. The majority of these early angiosperms resembled members of the pepper family and dominated forest floors.

With their rapid reproduction rates, the species quickly repopulated and dominated land that had been ravaged by forest fires or other environmental changes. As they spread, they developed their unique "come hither" look as the plants competed for insect or other pollinators.

Despite these insights, looking to fossils for flowering-plant origins may have its limits.

"For a long time, there has been an argument - and it still rages - about pre-Cretaceous angiosperm fossils" and their credibility, Dr. Reveal says.

The problem is that the two key traits of angiosperms - their two-stage fertilization process and the efficient way they convey food to their roots - wouldn't be immediately obvious early in their history, when their physical appearance would still resemble their coniferous ancestors.

Thus, the task of dating has been left to molecular biologists, who use the slow rate of change in DNA molecules as "clocks" to estimate age. Using this approach, your Valentine roses can trace their ultimate roots back some 200 million years.

Today's petals

While some researchers have been looking at fossils, others are looking for clues among today's petals. These efforts got a boost with the rediscovery in Madagascar of a population of plants thought to have changed little since their emergence at least 60 million years ago.

Genetic material from these plants is expected to give molecular biologists new tools to use for establishing the history of angiosperms.

The population, first discovered in 1909, appeared to go extinct. Dr. Schatz reported its 1994 rediscovery in a recent issue of the journal Nature.

The plant, known as Tahktajania perrieri, "is a key discovery because it looks to be the most basal member of one of the earliest families of flowering plants," says Elizabeth Zimmer, a research botanist at the Smithsonian Institution's Laboratory of Molecular Systematics.

Information about the plant's genetic makeup is feeding into a larger set of angiosperm genetic data being compiled by researchers at Washington State University. This should allow researchers to compare traits among flowering plants at the molecular level.