The greening of land
The Devonian Period marks the beginning of extensive land colonisation by plants. With large land-dwelling herbivores not yet present, large forests grew and shaped the landscape.
Many Early Devonian plants did not have true roots or leaves like extant plants, although vascular tissue is observed in many of those plants. Some of the early land plants such as Drepanophycus likely spread by vegetative growth and spores. The earliest land plants such as Cooksonia consisted of leafless, dichotomous axes and terminal sporangia and were generally very short-statured, and grew hardly more than a few centimetres tall. Fossils of Armoricaphyton chateaupannense, about 400 million years old, represent the oldest known plants with woody tissue.
By the Middle Devonian, shrub-like forests of primitive plants existed: lycophytes, horsetails, ferns, and progymnosperms evolved. Most of these plants had true roots and leaves, and many were quite tall. The earliest-known trees appeared in the Middle Devonian. These included a lineage of lycopods and another arborescent, woody vascular plant, the cladoxylopsids and progymnosperm Archaeopteris. These tracheophytes were able to grow to large size on dry land because they had evolved the ability to biosynthesize lignin, which gave them physical rigidity and improved the effectiveness of their vascular system while giving them resistance to pathogens and herbivores. These are the oldest-known trees of the world's first forests. By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of many plant groups and growth forms has been referred to as the Devonian Explosion or the Silurian-Devonian Terrestrial Revolution.
The 'greening' of the continents acted as a carbon sink, and atmospheric concentrations of carbon dioxide may have dropped. This may have cooled the climate and led to a massive extinction event.
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