A plant preserved in 100 million-year-old amber has revealed the oldest evidence of sexual reproduction in a flowering plant, U.S. and European researchers say.

Encased within the piece of amber is a cluster of 18 tiny flowers from the Cretaceous Period, with one of them in the process of making some new seeds for the next generation, they said.

The now-extinct plant from the mid-Cretaceous, when flowering plants were changing the face of Earth -- adding beauty, biodiversity and food -- is exhibiting the reproduction process "angiosperms," or flowering plants, still used today, Oregon State University biologist George Poinar said.

Microscopic examination of the plant by OSU researchers and German colleagues revealed pollen tubes growing out of two grains of pollen and penetrating the flower's stigma, the receptive part of the female reproductive system.

"In Cretaceous flowers we've never before seen a fossil that shows the pollen tube actually entering the stigma," he said. "This is the beauty of amber fossils. They are preserved so rapidly after entering the resin that structures such as pollen grains and tubes can be detected with a microscope."

Much Cretaceous plant life was composed of conifers, ferns, mosses and cycads, but flowering plants were beginning to emerge while dinosaurs still dominated the planet.

"The evolution of flowering plants caused an enormous change in the biodiversity of life on Earth, especially in the tropics and subtropics," Poinar said.