Taxonomic and Evolutionary Implications of Rhizome Structure and Secondary Thickening in Iridaceae

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Abstract
Rhizome structure was observed in most rhizomatous genera of the Iridaceae to determine its taxonomic applications within the family and to determine the evolution of the secondary thickening meristem (STM) within the group. An STM occurs in all species of Patersonia examined, similar to that described in Nivenia, Witsenia and Klattia, but in no other genera of Iridaceae. Most genera have a primary thickening meristem (PTM) near the stem apex, which produces some primary stem vasculature, adventitious roots and in some species, notably in Orthosanthus chimboracensis, an extensive pericyclic vascular plexus. The results show that the 2 subtribes Aristeineae and Sisyrinchiineae are closely allied, and the STM may have evolved from a more extensive PTM or may be retained as a primitive character.