Imhof, S. (1997):

Root anatomy and mycotrophy of the achlorophyllous Voyria tenella Hooker (Gentianaceae)

Bot. Acta 110: 298-305.

Abstract:

The central cylinder of the root of Voyria tenella consists of up to ten central, non-lignified, tracheidal xylem elements surrounded by some parenchymatic tissue and 5-7 groups of phloem. A pericycle could not be discerned. Even though the endodermis carries a faint suberin lamella it can not be discerned anatomically without special staining. The cells of the 1-3 cortex layers next to the endodermis are elongated longitudinally, the subsequent cortex parenchyma is multilayered and consists of isodiametric cells. The cells of the 2-3 layered outer dermal tissue are smaller than those of the adjacent cortex, their walls carry a suberin lamella and the outermost of them constantly scale off. The dermal tissue is interpreted as a multilayered exodermis.
The fungal colonization in roots of Voyria tenella remarkably differs from any known mycorrhizal pattern. After having penetrated the dermal tissue, the always intracellularly growing hyphae head straight towards the inner cortex layers, where they spread along the central cylinder. Ramifications from these inner spreading hyphae then colonize the cortex parenchyma from the inside and they develop dense hyphal coils. Eventually, the coiled hyphae swell and collapse, resulting in amorph clumps of fungal material. This mycorrhizal pattern is referred to as an intraradical fungus garden.
Arguments are given to call the mycorrhiza in Voyria tenella a specialized arbuscular mycorrhiza. Phylogenetic and ecological implications of the observations and the results are discussed.

Key words: Voyria, Gentianaceae, arbuscular mycorrhiza, mycotrophy, heterotrophy, root anatomy, phylogeny, evolution.