Subterranean axes in tribe Diurideae (Orchidaceae): morphology, anatomy, and systematic significance

The morphological and anatomical nature of perennating storage organs of the predominantly Australasian orchid tribe Diurideae (Orchidoideae: Orchidaceae) as well as anatomical concepts in tribes Orchideae and Diseae of Orchidoideae have been problematic for 150 yr, reflected in conflicting or vague terminology and questions about polystely and even monophyly of Orchidoideae. From a representative survey of underground organs of 145 species in 37 ingroup genera (Diurideae) and two outgroup genera (Spiranthes and Disa), the so-called "root-stem tuberoids" are here interpreted as root tubers (except for the stem tubers of Rhizanthella) borne on either droppers or stolonoid roots. All root tubers examined are bounded by a 1-4 layered velamen and exodermis, whereas droppers and stolonoid roots may have velamen-exodermis or a simple epidermis depending on the taxon and often bear multiseriate or uniseriate trichomes associated with mycorrhiza (as do roots of some taxa). The "polystely" reported in tubers of many Orchideae also occurs in tubers of many Diurideae but represents only dissection of the siphonostele into 2-13 traces. Cladistic analyses of data show extraordinarily high levels of homoplasy in characters related to root, dropper/stolonoid root, and tuber, so that these characters alone are of limited usefulness.