Subterranean structures and mycotrophy of the achlorophyllous Triuris hyalina (Triuridaceae)

Triuris hyalina Miers, an unusual achlorophyllous plant, was investigated f or subterranean morphology, root anatomy, and mycotrophy. Stems with scale leaves extend subterraneously to a depth of 15 cm. Pairs of adventitious ro ots develop at the scale leaves and clumps of apparently radiating roots, f ormed by accumulations of side shoot and scale leaf developments, occur. Ro ots consist of epidermis, short cell exodermis, three distinct layers of co rtex parenchyma, endodermis, and an extremely reduced central cylinder with one or two central tracheidal xylem elements. The fungus associated with T . hyalina roots exhibits thick-walled, 6-9 mu m thick, aseptate external hy phae. It penetrates the epidermis by developing appressoria and enters the cortex solely through the short cells of the exodermis. In the cortex cells , the aseptate hyphae start to coil. In the outer cortex layer, hyphae are thin, frequently branched, and most densely coiled. In the middle cortex la yer they are thicker and less densely coiled, and mostly appear degenerated to clumps of amorphous fungal material. The inner cortex layer rarely beco mes colonized. Vesicles occur in the outer and the middle cortex layers. Th is mycorrhizal pattern is interpreted as an adaption to attain a sustainabl e use from the endophyte. It is suggested that the mycorrhiza in Triuris hy alina be interpreted as a type of arbuscular mycorrhiza (AM). Implications for systematics and ecology are discussed.