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
Archaeological sub-disciplines these are not areas of study in their own right, and are only methods to be used in larger projects. Post-excavation analysis also makes use of a wide variety of further techniques. Archaeological sub-disciplines As with most academic disciplines, there are a number of archaeological sub-disciplines typically characterised by a focus on a specific method or type of material, geographical or chronological focus, or other thematic concern. Certain civilizations have attracted so much attention that their study has been specifically named. These sub-disciplines include "Assyriology" (Mesopotamia), "Classical archaeology" (Greece and Rome), "Etruscology" (Etruria), and "Egyptology" (Egypt). Another main division of