Epifagus virginiana is a parasitic plant that grows on the roots of American beech trees. Also known as "beechdrops", clumps of their brown stems can be found protruding from the forest floor, reaching up to 30 cm tall and lined with purple-white flowers. To most people, they look like just another ordinary plant amidst the undergrowth. But Epifagus lacks a key component which is usually a defining characteristic of plants - chlorophyll, the pigment which allows plants to harness solar power. Instead, the way this plant obtains its nutrients is via an underground tuber attached to the roots of its host. The beechdrop is a very discerning parasite - as its name indicates, it usually goes after beech trees, but sometimes they switch up their target and end up engaging in botanical cannibalism.
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| Left: The stem and flowers of beechdrops, Epifagus virginiana, Centre: a beechdrops tuber, with adventitious root (ar) indicated, Right: two beechdrops tubers linked via a parasitic connection. Photos from Fig. 1 and Fig. 2 of the paper. |
A botanist named Dr. Luiza Teixeira-Costa was conducting a study on these parasites in the mixed forest at Powdermill Nature Reserve in Pennsylvania, where there is a dense population of beechdrops. After carefully excavating a dozen of those parasitic plants, she noticed one of the specimens was composed of two Epifagus plants locked in a peculiar pairing. When she looked at that specimen more closely, she found that the pair consisted of two beechdrops tubers clinging tightly to each other, as if one of the plant was parasitising its fellow parasite.
To see if there's an actual parasitic connection between the tubers and that this was not two plants that had wrapped around each other via happenstance, Dr. Teixeira-Costa tested whether fluid could be passed between the two plants by injecting one of them with a special tracking solution, and examining it with micro CT scan to create a 3D image of the plants' internal structure. The scanning revealed that there is continuity in the vascular tissue that connects the two plants. It would be like if you find a pair of animals that have joined together and are connected via their circulatory systems. But that was one specimen - perhaps this was just a freakish one-off occurrence?
In order to get a better picture of this phenomenon, Dr. Teixeira-Costa examined the collections of Epifagus specimens at the Meise Botanic Garden and the Harvard University Herbarium, to see if there are more of such pairings. Out of 150 Epifagus that were in those collections, four of them were composed of a pair of E. virginiana plants attached to each other, in a similar way to how these parasites would usually attach to their host.
In addition she also searched through the online digital archive of herbarium specimen images from 233 herbaria via the SERNEC portal. Out of the 3097 Epifagus specimens in those collections, she found 52 specimens that showed potential signs of parasitising a fellow beechdrops. All those specimens had been collected from across the plant's distribution range in the United States. What this exhaustive search revealed is that while such cases of beechdrops-on-beechdrops is not a frequent occurrence, it is not negligibly rare either. Furthermore, it is widespread and not restricted to just one particular region.
Orobanchaceae plants such as Epifagus are usually able to recognise its own kind and avoid this kind of friendly misfire - so what made some beechdrops turn on one of their own? One possible scenario might involve an Epifagus plant attaching itself to a beech tree root, and was followed by the seed of another Epifagus which responded to the stimuli given off by the tree root. But instead of latching onto the beech tree root, it ended up parasitising one of its fellow Epifagus that was already there.
These examples of intraspecific parasitism have been described as "botanical version of cannibalism" and have been documented in a range of other parasitic plants. If anything, compared with the beechdrops, other parasitic plants seem to be far less discriminating about parasitising their own kind, particularly among mistletoes and dodders. In the world of botanical parasites, sometimes a parasitic plant's worst enemy is a fellow parasite.
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