Subterranean structures and mycorrhiza of the achlorophyllous Burmannia tenella (Burmanniaceae)

Plants of the myco-heterotrophic Burmannia tenella Benth. form star-shaped root systems consisting of 0.7-2 mm thick, succulent, brittle roots, reachi ng lengths of up to 3 cm. In cross section the roots consist of an epidermi s, about10 layers of parenchymatous cortex cells, an endodermis with U-shap ed secondary cell wall depositions, and a very reduced central cylinder wit h two to fivecentral xylem elements and two opposite phloem strands, surrou nded by a pericycle of relatively large cells. Based on the thick, aseptate , intracellularly coiled hyphae, arbuscules, and the frequent vesicles, the fungal association of B. tenella is considered to be a Paris-type arbuscul ar mycorrhiza. The morphological and anatomical structures of the root are discussed in the context ofthe mycorrhizal dependency of B. tenella. In som e root samples, a second fungus with septate hyphae colonized the cortex in tracellularly. This fungus restricts the spread of the aseptate symbiont wi thout causing morphological changesto the cortex cells.