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
Evolutionary history of plants is not a viable food source for fungi, herbivores or detritovores, so is prone to preservation. It is also robust, so can withstand pressure and display exquisite, sometimes sub-cellular, detail. All multicellular plants have a life cycle comprising two generations or phases. The gametophyte phase has a single set of chromosomes (denoted 1"n), and produces gametes (sperm and eggs). The sporophyte phase has paired chromosomes (denoted 2"n), and produces spores. The gametophyte and sporophyte phases may be homomorphic, appearing identical in some algae, such as "Ulva lactuca", but are very different in all modern land plants, a condition known as