Pollen Tubes (IMAGE) Oregon State University Caption The pollen tubes penetrating the stigma on this ancient flower are the only known fossil of this type, showing the process of sexual reproduction in a flowering plant. Credit (Photo courtesy of Oregon State University) Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.