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
Passum sub iugum Passum sub iugum The passum sub iugum ("pass under the yoke") was a practice in ancient Italy where defeated enemies were made to pass beneath a yoke constructed of spears either to humiliate them or to remove blood guilt. The practice of "passum sub iugum" was a ritual humiliation of enemies practiced by the people of ancient Italy. Both the Romans and the Samnites forced the captives of their defeated enemies to pass under a yoke formed from spears. The practice is said to have originated as a form of expiation. According to Livy, Horatius killed his own sister because