<P> The early Devonian landscape was devoid of vegetation taller than waist height . Greater height provided a competitive advantage in the harvesting of sunlight for photosynthesis, overshadowing of competitors and in spore distribution, as spores (and, later, seeds) could be blown for greater distances if they started higher . An effective vascular system was required in order to achieve greater heights . To attain arborescence, plants had to develop woody tissue that provided both support and water transport, and thus needed to evolve the capacity for secondary growth . The stele of plants undergoing secondary growth is surrounded by a vascular cambium, a ring of meristematic cells which produces more xylem on the inside and phloem on the outside . Since xylem cells comprise dead, lignified tissue, subsequent rings of xylem are added to those already present, forming wood . </P> <P> 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 . </P> <P> 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 a bifacial cambium . The first appearance of one of them, Rellimia, was in the Middle Devonian . True wood is only thought to have evolved once, giving rise to the concept of a "lignophyte" clade . </P> <P> Archaeopteris forests were soon supplemented by arborescent lycopods, in the form of Lepidodendrales, which exceeded 50m in height and 2m across at the base . These arborescent lycopods rose to dominate Late Devonian and Carboniferous forests that gave rise to coal deposits . Lepidodendrales differ from modern trees in exhibiting determinate growth: after building up a reserve of nutrients at a lower height, the plants would "bolt" as a single trunk to a genetically determined height, branch at that level, spread their spores and die . They consisted of "cheap" wood to allow their rapid growth, with at least half of their stems comprising a pith - filled cavity . Their wood was also generated by a unifacial vascular cambium--it did not produce new phloem, meaning that the trunks could not grow wider over time . </P>

Where do lycophytes horsetails and ferns fit in the evolutionary history of the plants