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
Sub-provincial division other, regular prefecture-level divisions, which are completely ruled by their provinces. However, they are marked the same as other provincial capitals (or prefecture-level city if not provincial capital) in almost all maps. The original 16 municipalities were renamed as sub-provincial municipalities on 25 February 1994 by the Central Organization Committee out of prefecture-level municipalities. They are mostly the capitals of the provinces in which they are located. Currently, there are 15 sub-provincial municipalities after Chongqing was designated direct-control: Chongqing was formerly a sub-provincial municipality of Sichuan until 14 March 1997, when it was made an independent municipality by splitting it