Image copyright Other Image caption The flowers, from an extinct plant species, are incredibly well preserved

The oldest evidence of sexual reproduction in a flowering plant - dating back 100 million years - has been found in Burma.

The team discovered a cluster of 18 tiny flowers in a piece of amber; one of them was in the process of making new seeds for the next generation.

Flowering plants caused an enormous change in biodiversity on Earth.

A US-German team has published findings in the Journal of the Botanical Institute of Texas.

"The main aspect of this discovery is that it presents a new look at the biodiversity of early flowering plants and the evidence of reproduction is interesting," co-author Prof George Poinar, from Oregon State University (OSU), told the BBC Tamil Service.

The perfectly-preserved scene is part of a portrait created in the mid-Cretaceous Period when dinosaurs walked the Earth and flowering plants were in the process of changing the face of the Earth forever, Prof Poinar explained.

Image copyright Other Image caption The researchers have also discovered early bees trapped in the Burmese amber

The fossils were formed when flowing tree sap covered the specimens, beginning the long process of turning into a semi-precious gem. The amber fossil is so well preserved that detailed study can be carried out on the flowers and their characteristics.

"This allows us to see the flower from different angles and different stages of development," Prof Poinar said.

Even more remarkable is the microscopic image of pollen tubes growing out of two grains of pollen and penetrating the flower's stigma, the receptive part of the female reproductive system.

This sets the stage for fertilisation of the egg and would begin the process of seed formation, had reproduction not been interrupted by the fossilisation process.

"In Cretaceous flowers, we've never before seen a fossil that shows the pollen tube actually entering the stigma," said Prof Poinar.

"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."

The pollen of these flowers appears to be sticky, suggesting it was to be carried by a pollinating insect.

Image copyright Other Image caption The evolution of flowering plants transformed biodiversity on Earth

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

New associations between small flowering plants and insects, as well as other animals, resulted in the successful distribution and evolution of these plants through most of the world today.

The broad mechanisms of reproduction in plants have remained unchanged for some 100 million years, the study suggests.

"Our study on insects from the ambers in Burma shows very primitive forms as well, for instance the bees that we found still have characteristics of wasps, and we know bees evolved from wasps," Prof Poinar told the BBC.

He says the amber fossils of Burma, also known as Myanmar, are a treasure trove for researchers.

The newly described but now extinct genus and species of flower, from the country's Hukawng Valley, has been named Micropetasos burmensis.

Scientists say the other interesting aspect is the fact that these tiny flowers cannot be placed into any present day family.