Tracheobionta plant fossil. Tracheobionta is the subkingdom of plants that contain vascular tissues, xylem and phloem. They are commonly known as the vascular plants. Fossil forms of Tracheobionta are well represented because the tough, lignified cell walls of the xylem preserve well. This fossil dates to the Carboniferous period, 300 million years ago. Found in Montceau-les-Mines, Saone et Loire, France.