Tree form

The early Devonian landscape was devoid of vegetation taller than waist height. Without the evolution of a robust vascular system, taller heights could not be attained. There was, however, a constant evolutionary pressure to attain greater height. The most obvious advantage is the harvesting of more sunlight for photosynthesis – by overshadowing competitors – but a further advantage is present in spore distribution, as spores (and, later, seeds) can be blown greater distances if they start higher. This may be demonstrated by Prototaxites, thought to be a late Silurian fungus reaching eight metres in height.[58] To attain ‹See Tfd›arborescence, early plants had to develop woody tissue that provided support and water transport. The stele of plants undergoing "secondary growth" is surrounded by the vascular cambium, a ring of 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. The first plants to develop this 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.[59] 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,[60] reached 30 m in height. These progymnosperms were the first plants to develop true wood, grown from a bifacial cambium, of which the first appearance is in the mid Devonian Rellimia.[61] True wood is only thought to have evolved once, giving rise to the concept of a "lignophyte" clade. These Archaeopteris forests were soon supplemented by lycopods, in the form of lepidodendrales, which topped 50m in height and 2m across at the base. These lycopods rose to dominate late Devonian and Carboniferous coal deposits.[62] Lepidodendrales differ from modern trees in exhibiting determinate growth: after building up a reserve of nutrients at a low height, the plants would "bolt" to a genetically determined height, branch at that level, spread their spores and die.[63] They consisted of "cheap" wood to allow their rapid growth, with at least half of their stems comprising a pith-filled cavity.[32] 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.[verification needed] The horsetail Calamites was next on the scene, appearing in the Carboniferous. Unlike the modern horsetail Equisetum, Calamites had a unifacial vascular cambium, allowing them to develop wood and grow to heights in excess of 10 m. They also branched multiple times. While the form of early trees was similar to that of today's, the groups containing all modern trees had yet to evolve. The dominant groups today are the gymnosperms, which include the coniferous trees, and the angiosperms, which contain all fruiting and flowering trees. It was long thought that the angiosperms arose from within the gymnosperms, but recent molecular evidence suggests that their living representatives form two distinct groups.[64][65][66] The molecular data has yet to be fully reconciled with morphological data,[67][68][69] but it is becoming accepted that the morphological support for paraphyly is not especially strong.[70] This would lead to the conclusion that both groups arose from within the pteridosperms, probably as early as the Permian.[70] The angiosperms and their ancestors played a very small role until they diversified during the Cretaceous. They started out as small, damp-loving organisms in the understory, and have been diversifying ever since the mid[verification needed]-Cretaceous, to become the dominant member of non-‹See Tfd›boreal forests today.