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
Stars named after people program, it was common for astronauts to be trained in celestial navigation, and to use a list of naked-eye stars which to take bearings. As a practical joke, Gus Grissom gave names to three stars on this list, which were references to the three Apollo 1 crew: The names stuck, perhaps "in memoriam" for their deaths in the Apollo 1 fire, and were used through the rest of the program. Unknown to Grissom, these stars already had traditional names; however, those were not generally used, allowing the three new names to make their way into other records. Today, they are