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 7 Seeds When astronomers predict that the Earth will be hit by a meteorite, the world leaders meet to develop a plan for human survival called the Seven Seeds project. Each country will cryonically preserve a number of healthy young people, which will allow them to survive the devastation of the impact. After a computer determines that Earth is once again safe for human life, it will revive each group. The Japanese government creates five groups of survivors named Winter, Spring, Summer A, Summer B, and Fall. Each group consists of seven members, who are not told about what will