Imhof, S. (2010):

Are monocots particularly suited to develop mycoheterotrophy?

In: Seberg, O., Petersen, G., Barfod, A., Davis G. J.: Diversity, Phylogeny and Evolurion in the Monocotyledons. Aarhus University Press, Kopenhagen, pp.11-23.

Abstract:

More than 400 plant species are mycoheterotrophic, of which by far the majority are monocotyledons (88%) – a remarkable statistic, considering monocots account for only about 23% of all flowering plant species. Which factors make monocotyledons so appropriate for this mode of life? Subterranean organs of several non-related mycoheterotrophic plants share common features, which in turn can be interpreted as either supporting, or even being prerequisites for, the mycoheterotrophic habit. These are: (1) herbaceous habit, (2) a maintained and often voluminous primary root cortex parenchyma, (3) a tendency for a star-like organisation of the root system, mostly with short and thick or long and thin roots, and (4) an often complicated mycorrhizal colonization pattern. Monocots meet these putative demands for mycoheterotrophy better than other angiosperms for the following reasons: (1) Monocots are primarily herbaceous. (2) Monocot roots lack secondary growth, which normally obliterates the primary cortex in dicots. (3) The secondary homorhizy of monocots predetermines a  starlike root system, which in dicots must be achieved by reductions of allorhizy. (4) Monocots are able to protect their central cylinder through a comparatively inexpensive tertiary endodermis. (5) Due to their predisposition for shoot-borne roots and multilayered cortices, monocots are more prone to develop voluminous and diverse subterranean organs (rhizomes, scale leaves, roots), which in turn promote the development of complex colonization patterns.