Mistletoe Cactus

When it comes to mistletoe cactus, Rhipsalis baccifera, there's not much "there" there, to steal a phrase from Gertrude Stein. It certainly is not a show-stopping plant. It doesn't even look like a cactus. Its white berries and slender stalks give it a very superficial resemblance to mistletoe, but mistletoe has leaves and is a parasite, and not even distantly related to cacti.

Pendant Stems of Mistletoe Cactus

Rhipsalis baccifera is an epiphyte that has adapted to the extremes of sea-level mangrove swamps and the high altitudes of the cloud forest, and everything in between. The general assumption among botanists is that cacti are exclusively New World plants, and have evolved relatively recently. Mistletoe cactus fits that theory, being native to 2 counties in southern Florida, the Caribbean, eastern Mexico, Central America and throughout much of tropical South America. But it also is widespread, and considered native in tropical Africa, South Africa, Sri Lanka, Madagascar, and parts of India. How it got there  has had scientists scratching their heads for generations.

  Rhipsalis the largest genus of epiphytic cacti, and Rhipsalis baccifera has numerous subspecies. The classification probably needs cleaning up, but the fact remains that the species is highly polymorphic (appears in many different forms). The vast differences in geography and habitat would account for these differences. It even varies on the cellular level, with most New World species being diploid, while Old World species are commonly tetraploid. 

 Like other so-called "airplants," it uses its roots mainly for anchoring and stabilizing rather than for absorbing water and nutrients. It sometimes also grows on rocks. Trailing stems of my plants have rooted in the upper layers of potted plants standing lower on my shelves, which makes me think that it would creep happily in a layer of humus or porous light soil at ground level. Rhipsalis baccifera, subsp. baccifera is the form found in Florida and much of the New World tropics.

The stems of the Florida native are about the diameter of cooked spahgetti, hence its second common name, "spaghetti cactus." Stems of other subspecies may be thicker, angular, or flattened. The stems can grow up to 9 meters long, though they generally are shorter. My plants have stems only about 2-one-half feet long, but that is because I haven't found a good place for them, and they don't grow as lushly as they should.

The stems hang downward and may branch into multiple stems, which also branch. They are tender and succulent, though become somewhat woody at the base. The main feature that distinguishes cacti from other spiny species is the presence of an areole, a generally raised, cushion-like structure from which the spines arise. The stems of mistletoe cactus have only rudimentary areoles, which tend to disappear with age. With a hand lens you can see a few hairs emanating from these very basic structures. New growth is reddish, and quite spiny/hairy. 

Areoles on Opuntia (Prickley Pear  Cactus)

In all the years I've had my plants, I've never managed to catch them in bloom. That's mainly because I've just left them under a tree or bush and forgotten about them. In fact, I'd forgotten I even had them until  Hurricane Irma in 2017 exposed one clinging for life in a crotch of a defoliated 7-year apple (Genipa clusiifolia). By all accounts the flowers are small and insignificant, though with the aid of a hand lens, "insignificance" sometimes can spring into beauty. Translucent whitish berries are borne directly on the stems or on a very short stalk. Birds eat the berries, helping to disperse the species. 

Adventitions Roots, New Growth, Rudimentary Areoles

In fact, the plant is so unassuming, yet vexing in its variability, that a specialist named Ken Friedman wrote, "So many species are named R. baccifera that it is almost impossible to tell an original. Four or five  growing in my greenhouse  have different vegetation although the flowers are similarly inconsequential. If anything, they are large weeds that take up more room than they are worth." 

So how did this modest plant colonize such a large portion of the globe? The theories that exist are not exactly convincing on their own. 

The least likely theory is that the  plant was spread by the shipping trade in the 15th century and onward. But the plant is widespread into interior regions of the Old World, and not limited to the port areas. That kind of spread normally wouldn't occur in a matter of hundreds of years. Even more problematical is the fact that Old World subspecies differ significantly from the "ur-type," R. baccifera, and such evolution normally requires eons, not centuries to occcur. The plant also is prominent in Ayurvedic medicine, which some believed began as early as the Bronze Age. It is possible that it was a later addition, but it still takes a long time for something to become entrenched in regional  medical lore.

The next theory involves continental drift. This theory holds that the plant was well-established before the breakup of the supercontinent Gondwana. The fly in the ointment here is that Gondwana probably began to break up around 180 million years ago. If cacti as a group didn't evolve until some 30 million years ago, as is generally accepted, that leaves a huge gap. In that case Rhipsalis would have to be an atypical, extremely ancient genus. Since cacti leave few, if any, fossil records, nobody really knows when Rhipsalis evolved, but it probably was well after Gondwana's destruction.

That leaves us with birds, which we know are frequent vectors of plant dissemination. It certainly makes sense for the distribution within the New World. Many of Florida's native plants were brought by birds  from the Caribbean and the Bahamas. A problem with this theory is that most birds migrate north to south or the reverse. I'm not aware of any seed-eating birds that currently migrate over the Atlantic from the Americas  to Africa and beyond. That would be a vast distance for a seed-eating bird to traverse, and also a vast distance for a seed-eating bird not to poop. We don't yet have any fossil record of any ancient bird prototype that would have been up to the trip either.

The "answer," if there is one, is probably a combination of continental drift and the birds. The breakup of Gondwana didn't exactly occur overnight, but over the course of millions of  years. In fact, we are still moving. What eventually became Africa and South America would have been much closer at varioius points in geological history than they are today, and there most likely would have been islands and mountaintops that are now submerged or destroyed. Then island-hopping by birds would make sense.

Personally, I rather like the "space aliens" theory put forward by "Laidback Gardener" Larry Hodgson, who suggests that, "millions of years ago, space aliens moved the plant around, just to mess with scientists trying to understand how R. baccifera got around." Why not?

Further Reading:

“The Cactus That Traveled the Globe.”Larry Hodgson. Http:/laidbackgardener.blog/2018/12/03/the-cactus.://

Rhipsalis baccifera (JS Meuller) Stern. In Cactus Journal (Croyden) 7:107 (1939). With comments and photographs by Ken Friedman. www.Rhipsalis.com/species/baccifera.

“I Havana a Clue How I Got Here; Cactus Goes for a Drink in Cuba, Wakes Up in Cape Town.” Jan. 27, 2014 by alieyres. http://blogs.reading.ac.uk/2014/01/rhipping-yarns.