The theory that the leaves (megaphylls) of ferns and seed plants evolved by the modification of terminal branches (telomes) of stems. It envisages that firstly, instead of the primitive equal (dichotomous) branching of the stem, there developed a main axis with lateral side branches. Next, each lateral branch system was restricted to one plane, instead of forming a three-dimensional pattern. Lastly, the spaces between the telomes in each lateral branch system were gradually filled in by webbing consisting of thin sheets of photosynthetic parenchymatous tissue. There is fossil evidence for this sequence of events occurring in certain trimerophytes and progymnosperms, notably in trees of the genus Archaeopteris, of the late Devonian period. Megaphylls are unique to the euphyllophyte group of vascular land plants.