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
The Seeds of Doom The Seeds of Doom The Seeds of Doom is the sixth and final serial of the 13th season of the British science fiction television series "Doctor Who", which was first broadcast in six weekly parts on BBC1 from 31 January to 6 March 1976. The serial is set in Antarctica and England. In the serial, the plant collector Harrison Chase (Tony Beckley) seeks to retrieve a malignant alien plant called a Krynoid for himself. In Antarctica, British scientists Charles Winlett and Derek Moberley discover a pod buried in the permafrost and take it back to their camp. John Stevenson, the