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
7 Seeds are listed below. A main topic of "7 Seeds" are the four seasons, since every team is named after one of them: Spring, Summer A/B, Autumn and Winter. The team members were chosen into their teams based on which seasons their names matched best. Almost every "7 Seeds" chapter arc is named after one of the 24 solar terms consisting of the 72 pentads (Shichijūni kō). The Rainwater chapter, for example, completely consists of Shichijūni kō. "7 Seeds" is written and illustrated by Yumi Tamura and published by Shogakukan. It began serialization in the November 2001 issue of the monthly