Subterranean structures and mycotrophy of the achlorophyllous Dictyostega orobanchoides (Burmanniaceae)

Plants of Dictyostega orobanchoides arise from about 1 mm thick rhizomes, w hich are densely covered by sessile, imbricate, peltate scale leaves. The r esulting interfoliar spaces are inhabited by fungal hyphae up to 6 mum thic k, often developing vesicle-like bladders. The fungus also colonizes the ti ssue of the scale leaves, inter- as well as intracellularly, forming vesicl es but no arbuscules, and it even penetrates the vascular bundles of the le aves. The rhizome itself does not become infected. The 200 mum thick roots emerge from the rhizome and have a 2-layered cortex and voluminous rhizoder mis, which both are delicate and often disrupted or missing. In contrast, t he strongly reinforced, tertiary endodermis and the central cylinder are du rable and have a considerable tensile strength. Although the roots grow thr ough the hyphal masses in the interfoliar spaces when emerging from the rhi zome, they only become infected from the rhizosphere, A collar of rhizomoge nous tissue hinders the interfoliar hyphae from direct contact to the roots . Only within the rhizodermis, the mycorrhizal fungus builds coils of heter omorphic hyphae. arbuscule-like structures, and vesicles. Hence, the mycorr hiza in D. orobanchoides is assigned to the arbuscular mycorrhiza. It is hy pothezised, that the ephemeral mycorrhizal tissue combined with the durable vascular system of the roots is a strategy to avoid the high costs of prot ecting the large rhizodermal surface area. The rhizomogenous collar is expl ained as an extra protection to the tender, young roots, when emerging from the rhizome. The necessity to include other subterranean plant organs alon g with the roots in future mycorrhizal studies is emphasized.