Monotropa uniflora in the understorey of an Abies religiosa forest, El Chico National Park, Hidalgo, Mexico. Thanks to Sarah Pierce for the ID.

On numerous occasions on our trip through the coniferous forests of Mexico on the #PinaceaeGo project, we’ve encountered a ghostly pale plant on the forest floor. Depending on where in the world you’re from, it might look very familiar; or totally unlike anything you’ve ever seen before. That in itself is part of the mystery surrounding this plant.

Monotropa uniflora is variously known as the ghost plant, corpse plant, or Indian Pipe. As its appearance suggests, it is parasitic, and does not contain any chlorophyll of its own for photosynthesis. Unlike most parasitic plants, however, it doesn’t obtain its energy from other plants. Instead, it is an unusual example of a mycoheterotroph* — it steals sugars from mycorrhizal fungi in the soil which are themselves symbiotic partners of trees. In other words, the trees choose to associate with the fungi, providing them with sugars in return for soil nutrients. Monotropa hijacks this arrangement and takes from the mycorrhizae while offering nothing in return.

This indirect theft is what allows it to survive even in dense, dark forests, and to be associated with a range of forest types. While most parasitic plants target particular plant species, M. uniflora is highly specialised on a few species of mycorrhizal fungi from the Russulaceae. All these features set mycoheterotrophs apart from more common parasitic plants such as dodder (Cuscuta spp.) or broomrapes (from my favourite plant family, the Orobanchaceae**), which are direct parasites on other plants.

Confusion over the taxonomy of Monotropa has been long-standing. Prior to Linnaeus, a related species Monotropa hipopitys was actually included in the genus Orobanche. Early botanists must have thought that parasitism ranked as a higher criterion for organising plants than flower traits. For a while it had its own family, the Monotropaceae, but it is now incorporated into the Ericaceae, along with heathers and Rhododendron, which superficially look nothing like it.

Another of its bizarre features is what biogeographers refer to as a disjunct distribution. It occurs in all sorts of places worldwide, in Russia, North and South America, but with large gaps between them. Genetic evidence suggests that these are distinctive, but not enough to call them different species. How did they get to such widespread locations? Was it through their dust-like seeds, or linkages in previous climates, or human transportation? Even where it is found, it’s never very common, and appears only in certain seasons.

Global distribution of Monotropoideae (including Monotropa uniflora) from Kathleen Kron’s Ericaceae site.

It’s not often that one can come across the same distinctive species in so many places around the world. How did a species of heather come to evolve into a fungal parasite, to develop such a strange form, and to spread itself quite so far? Wherever you find it, it’s always going to be special and mysterious.

* Here’s a pretty old review of this group of plants from 1994. If another has been published since then I’m unaware of it, and suggests that there might be scope for an update.

** One day I hope to retire to a herbarium and prepare a full review of the Orobanchaceae, so long as no-one else gets there first. I collect them on sight, wherever I am in the world. This is probably a pipe-dream. I’m never going to retire…