Tristerix Species, Mistletoe

Per Jan Emming of Destination:Forever Ranch and Gardens, a 40 acre desert botanical garden and sustainable living homestead Arizona:
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"Mistletoes are a group of plants spanning several independently-evolved families which have developed the ability to parasitize other plants, tapping into their vascular systems to withdraw water and nutrients provided by their host for their own survival purposes. There are many hundreds of mistletoe species worldwide, parasitizing virtually all major vascular plant families. This includes the cactus family, which is generally mistletoe-free save for one major exception in Chile: Tristerix aphyllus.

Tristerix is capable of penetrating the thick skin of at least two cactus species, Trichocereus chilensis and Eulychnia acida, and sending thin rootlike structures through several inches of nutrient-poor, watery cortex tissues to tap into the nutrient-rich vascular bundles in the interior of the cactus stem. Most mistletoes are incapable of either feat should a seed land upon the spiny skin of a cactus, yet somehow Tristerix manages to accomplish both. How does it do this?

The answer has been looked into by my friend Jim Mauseth, a University of Texas botany professor who along with a Chilean colleague named Gloria Montenegro studied this situation in the mid 1990s. A simplified version of the process is that a sticky Tristerix seed is deposited upon the cactus' skin by a feeding or defecating bird, and that the small seed spends virtually all of its very limited resources upon the process of splitting open the epidermis and sending a few cells of its own down into the watery, nearly nutrient-free cortical cells that the cactus uses to store moisture for survival in prolonged dry spells. These tiny Tristerix cells somehow manage to stay alive and nearly undetected by the cactus' defense mechanisms, threading their way down deeper into the cactus in search of the more deeply buried phloem tissues which carry much larger amounts of sugar and other nutrients.
Once the Tristerix cells finally reach the vascular bundles, they sink in and start to divide and grow. Keep in mind that at this point there is absolutely zero indication that there is anything happening inside the cactus' body, and that the entire Tristerix "plant" is comprised of little more than a handful of invisible cells completely buried within the cactus. This really stretches the definitive limits of what we might be able to call a "plant", but there you have it. Once tapped into the cactus' vascular system, the Tristerix grows larger and eventually sends a bulbous knob of cells about the size of a pea back outwards towards the epidermis of the cactus, where flowers buds form just beneath the skin. These knobs eventually burst forth through the cactus' skin, much like the famous creature in the movie "Alien" does, and flower buds are formed and open.
The stems and flowers of Tristerix are red, tubular, and showy and are clearly adapted to attract hummingbirds for pollination. Once the flowers are fertilized, white berries follow that ripen to pinkish red. They are consumed by other birds who feed upon fruits, and the process can begin again when the birds deposit the seeds upon different cacti.

In Chile, many but not all larger columnar Trichocereus and Eulychnia cacti have at least one or two Tristerix mistletoes, and some are quite heavily infested. While the mistletoe infestations must take some toll upon the cacti, they clearly manage to survive for many decades with their parasitic problems and are not killed. True parasites evolve to never kill their hosts, unlike predators, because doing that would also be fatal to the parasite; therefore they have a vested interest in not withdrawing too many resources from their hosts and causing their mortality, which is why parasitic organisms are usually fairly careful in the metabolic demands they make.

This phenomenon of mistletoes upon cacti does not occur in North America at all, although a few very rare and probably incidental cases of mistletoes in the genus Phoradendron and Psittacanthus have been observed growing upon cacti in Mexico. It is certainly not typical, unlike the case of Tristerix. However another type of mistletoe with an almost identical lifestyle and reproductive process named Viscum minimum (in an entirely different plant family) parasitizes columnar Euphorbias in African countries, which is a fascinating example of convergent evolution. Enjoy these photos of the fascinating biology of this unique plant!

Tristerix aphyllus is a leafless parasite that survives almost entirely within the body of its cactus host. Only when it is time to flower and reproduce does the Tristerix erupt out of the skin of the cactus it normally lies hidden within. In this case, it seems to sprout primarily out of the areoles, which are the buds of the cactus from which spines, flowers, and new branches also emerge.

The specific name aphyllus means "leafless", a reference to the fact that the mistletoe lacks those, unlike other mistletoe species (such as those seen upon oaks, for example) which have permanent leaves and do some photosynthesis of their own. The red stems of Tristerix are totally free of chlorophyll and the parasite is therefore 100% dependent upon the cactus for all of its food and water needs. Being leafless also helps the cactus survive, since if the mistletoe were to be lush and leafy it would utilize too much water during desert droughts, deplete the cactus' water reserves, and possibly kill them both. This situation would serve neither plant, so the Tristerix has lost all of its leaves as an adaptation to conserve limited moisture and help both the host and the parasite survive.

The showy red flowers of Tristerix aphyllus are clearly adapted to pollination by hummingbirds. The bright scarlet color and tubular shape are well understood to be highly attractive to these birds, which are found naturally only in North and South America. The exserted stamens are positioned to dab pollen from the yellow anthers onto the foreheads of visiting hummers probing the floral tube for nectar. The pistil, located in between the stamens, is also positioned to receive the pollen from a different plant when the hummer visits, ensuring genetic outcrossing.

The last photo of mine shows Tristerix aphyllus growing on a Eulychnia acida. Tristerix aphyllus also grows upon Eulychnia acida cactus plants in Chile. The immature berries are greenish at first, then become white or pale pink as they grow, eventually ripening to pinkish-red. That is when fruit-feeding birds of many species will eat them. Most mistletoe berries are quite sticky and birds will wipe their beaks and behinds onto many surfaces in order to clear them away, and a certain percentage of those sticky seeds will end up on new cacti to perpetuate the life cycle.

Trichocereus chilensis is a widespread cactus of central Chile, occurring in the region around the capital city of Santiago and northwards as it starts to become Atacama Desert, where it is eventually replaced ecologically by the more xeric Eulychnia acida. This T. chilensis is growing amongst the Chilean wine palms (Jubaea chilensis) of La Campana National Park about 75 miles northwest of Santiago.