Final answer:
Cooksonia is the oldest-known plant considered to have xylem, dating back to the Silurian period, around 430 million years ago. Although there is no definitive proof from the fossils, Cooksonia is broadly recognized as an early vascular plant.
Step-by-step explanation:
The oldest plant known to possess xylem tissue is Cooksonia. Cooksonia pertains to an ancient genus of plants that thrived during the Silurian period, around 430 million years ago. Paleobotanical research suggests that this genus likely contains the first type of vascular plants, which includes structures resembling xylem. Despite the lack of definitive proof from Cooksonia fossils to confirm the presence of vascular tissues, it is generally accepted that Cooksonia is among the earliest forms showing traits consistent with vascular plant development.
Investigation into fossilized material from the Rhynie chert of Scotland has provided substantial evidence of these early vascular plants. Club mosses, particularly from the phylum Lycopodiophyta, are another significant group of ancient vascular plants that existed during the Carboniferous period. However, these are younger than Cooksonia and are not considered the earliest forms with xylem.