Many myco-heterotrophic plants have thick, succulent, brittle roots, and keep the primary root structure. Looking at the species of Voyria (Gentianaceae) we may infer the evolutionary steps towards a condensed root system, resulting in small, star like root balls (e.g. Voyria tenella). However, clumped or star like root systems are also present in Triuris hyalina (Triuridaceae), Burmannia tenella and Afrothismia winkleri (Burmanniaceae), three families which are not at all related to each other. Hence, condensed root systems seem to be advantageous for myco-heterotrophic plants, and in fact, a globose root structure combines a maximum of cortex space for the indispensable root fungus with minimum distances for matter transport. Closest to this theoretical optimum are the tubers of some Thismia secies (Burmanniaceae; e.g. Goebel & Süssenguth (1924), Beiträge zur Kenntnis der südamerikanischen Burmanniaceen. Flora 117: 55-90). But also the species presented here show features that approach this theoretical aim, to a different extent and by distinct pathways. On the other hand, little subterranean surface area decreases the probability of becoming infected by an appropriate fungus. This disadvantage is encountered by adaptations concerning the mycorrhizal pattern. Distinct compartimentation of the mycorrhizal tissue allows a sustained use of the fungus: the fungus survives in certain parts but becomes digested in others.