Evolutionary history of plants phloem on the outside. Since xylem cells comprise dead, lignified tissue, subsequent rings of xylem are added to those already present, forming wood. The first plants to develop secondary growth and a woody habit, were apparently the ferns, and as early as the Middle Devonian one species, "Wattieza", had already reached heights of 8 m and a tree-like habit. Other clades did not take long to develop a tree-like stature. The Late Devonian "Archaeopteris", a precursor to gymnosperms which evolved from the trimerophytes, reached 30 m in height. The progymnosperms were the first plants to develop true wood, grown from
APA Division of Clinical Neuropsychology later conference at Houston, Texas, where Division 40 was represented along with other neuropsychology-related organizations, produced the formal criteria for training and education in the field. Division 40 was one of the sponsoring organizations of the Houston conference. Division 40 has evolved over time, forming four advisory committees on science, education, practice and public interest, mirroring the overall structure of the APA. It is one of the three largest divisions of the APA. The division had 636 members in 1981, 1785 members in 1985, 3880 in 1995, and 4348 in 1998. Currently it has around 5000 members, indicative of growth