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 out of Sichuan altogether. Xinjiang Production and Construction Corps also has the powers of a sub-provincial city. Chengdu is the largest sub-provincial municipality; has a population exceeding that of the independent municipality of Tianjin while, both Harbin and Chengdu have a bigger area than Tianjin. Additionally, the head of Pudong New Area of Shanghai and Binhai New Area of Tianjin, which is a county-level district, is given sub-provincial powers. The National Joint Conference of Sub-provincial City People's Congress Standing Committee Chairpersons () are attended by the chairpersons and vice-chairpersons of all sub-provincial cities. It was proposed by the Guangzhou Municipal