Aestivation organ structure in Drosera subgen. Ergaleium (Droseraceae): Corms or tubers; roots or shoots?

Abstract

The nature of the subterranean aestivation organ in Drosera subgen. Ergaleium is reinvestigated and expanded across a wider range of taxa. The structure is confirmed to be anatomically and developmentally a stem tuber, and the adventitious growths arising along the subterranean stem are confirmed as roots. The enveloping, multi-layered, tunicate, corm-like sheath seen in some species is of epidermal and not of leaf origin, so that the structure is a tuberous condensed rhizome and not a corm. Nevertheless, the resulting structures appear to function ecologically as corms, the sheaths possibly protecting the dormant tubers from dehydration, and/or the abundant droserone pigment in the sheath tissues acting to deter fungi and other organisms. Similarly, the adventitious root-like structures on the stems are found to be anatomically roots, albeit lacking a developed root cap, and to possess well developed, epidermally derived root hairs, contrary to the suggestion of some studies that the 'hairs' were ectomycorrhizal hyphae. However, many Ergaleium species have no sheaths, or use remnant stem and/or root tissues, and some other species appear to show sheath-related hyphal associations, and highly reduced roots with few or no obvious root hairs. Thus, the ecological strategies involving these structures remain an area for further study.