Morphology and development of the subterranean organs of the achlorophyllous Sciaphila polygyna (Triuridaceae)

The subterranean organs of the achlorophyllous Sciaphila polygyna (Triuridaceae) are described, depicted, and structurally explained for the first time. Unlike other Triuridaceae, the subterranean system of S. polygyna appears as a complex star‐like structure of short, but thickened, roots as well as scale leaves and shoots. A complete series of sections revealed the following construction. In the axil of a scale leaf at a shoot of first order, a side shoot of second order as well as a pair of endogenous shoot‐borne roots arise. This side shoot of second order also develops a scale leaf very early in ontogeny, which again gives rise to a side shoot of third order and a pair of shoot‐borne roots. Other scale leaves at shoots of any order may also bear shoots and root pairs. This growth pattern occurs in a very close manner without internode elongation, resulting in the clumped, star‐like appearance. The structures described superficially resemble the root systems of many mycoheterotrophic plants from other families. Comparisons with respect to how they develop, however, show that these similar root systems can result from distinct developmental patterns, suggesting independent evolutionary pathways and a considerable evolutionary pressure towards abbreviated and thickened roots in mycoheterotrophic plants. Possible advantages as well as evolutionary implications of the structures described are discussed. © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 146 , 295–301.