They are significantly larger than those of the protoxylem and secondary xylem. Metaxylem has wider vessels and tracheids than protoxylem. Metaxylem develops after the protoxylem but before secondary xylem. Arrows indicating protoxylem strands at tip of primary xylem. Each stem contained vascular tissue with one or two strands of protoxylem. Primary xylem maturation exarch, with protoxylem strands at tips of xylem ribs. The protoxylem was probably inside the xylem, maturing from the inside out. In appearance protoxylem is usually distinguished by narrower vessels formed of smaller cells.