What is the mystery of mistletoe cactus?

By Chelsie Vandaveer

March 3, 2005

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Biogeographers study the dispersal of life-forms around Earth—how things got where they are, how they changed along the way. They include in their studies both the extant (living) and the extinct. Some plants like bracken (Pteridium aquilinum) and whisk fern (Psilotum nudum) are cosmopolitan (worldwide) and have changed very little over the eons. The theory is that these evolved long before the breakup of the supercontinent, Pangaea. Other plants are limited in distribution; these are considered to have evolved after the separation of the continents.

Cacti are one of the limited-distribution families—everyone 'knows' that cacti (family: Cactaceae) are New World plants. Though mostly thought of as inhabiting deserts, many thrive in areas of high rainfall. Some climb as vines, others are epiphytes. Since there are no cacti fossils and the plants have limited ranges, they are considered recently evolved.

Plant of the Week 02/14/2005

Mistletoe Cactus (Rhipsalis trigona)
Plant of the Week 02/14/2005

Among the epiphytic cacti are the 35 or so species of 'mistletoe cacti' (Rhipsalis Gaertner). Their center of diversity is South America, primarily Brazil. But among the Rhipsalis, one species is a biogeographical mystery—Rhipsalis baccifera.

Rhipsalis baccifera subspecies baccifera is native to Florida, the Caribbean islands, Central America, and South America. Those in South America have 22 chromosomes (n=11, 2n=22), but those in Central America and the Caribbean are tetraploids with 44 chromosomes. Rhipsalis baccifera ssp. shaferi is diploid and native to Argentina, Paraguay, and Bolivia. Rh. baccifera ssp. cleistogama was only recently discovered in Bolivia and has flowers that never open, but still produce seeds. Rh. baccifera ssp. hileiabaiana is also a new subspecies and is found only in the very rainy areas of Bahia, Brazil.

But the mistletoe cactus is not just found in the Americas and no one has a very good explanation for its distribution. Rhipsalis baccifera ssp. erythrocarpa is found in the mountains of East Africa and is a tetraploid. Rh. baccifera ssp. mauritiana, another tetraploid, is native to tropical Africa and scattered across the islands of the Mascarenes, Comores, Seychelles, Madagascar, and Sri Lanka. Rh. baccifera ssp. ftdauphenensis, another new subspecies, is found around Ft. Dauphin, Madagascar. But the strangest is Rh. baccifera ssp. horrida of Madagascar and these are either tetraploids or octoploids (88 chromosomes).

The anomalous distribution has been 'blamed on' birds, but birds don't migrate east and west. (And surely any seeds would have cleared their digestive tract long before reaching the east coast of Africa.) Some think that lovesick sailors (mistaking the cactus for mistletoe) may have carried sprigs and cast them ashore when they reached new islands. And maybe, the mistletoe cactus has simply been around for a very long time.

The Flora of Zimbabwe has a great photograph of the mistletoe cactus taken by Bart Wursten. To view the photograph, click on the link:

Click here to view the photograph

(Compiled from: "Rhipsalis", Hortus Third, Staff L.H. Bailey Hortorium, NY State College of Agriculture and Life Sciences, Cornell University, Macmillan, NY, 1976; "The Rhipsalis Riddle - or the day the cacti came down from the trees", Dr. Phil Maxwell, New Zealand Cactus and Succulent Journal, 1998, reprinted on the internet and various other pages of www.rhipsalis.com; W3TROPICOS, Jim Solomon, Missouri Botanical Garden and "Rhipsalis", A-Z Encyclopedia of Garden Plants, Christopher Brickell and Judith D. Zuk, eds, American Horticulture Society, Dorling Kindersley, NY, 1996)

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