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Vegetative Vampires

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By —— Bio and Archives November 8, 2017

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Few of the over 4,000 species of known parasitic plants are of economic importance. Those that are however can cause 80 percent crop losses.

Some of the most infamous of these are the Dodders, Cuscuta, ironically of the of the Morning Glory (Convolvulaceae) family. Common names for various species indicate the hatred held for string-like stems: Devil’s Guts, Devil’s Hair, and Witch’s Hair. In medieval Europe, dodder was sometimes viewed as evil transformation of normal grape wine species. As late as 1831 the presence of dodder in crops was connected to the appearance of the comet in the previous year. Like other fully parasitic plants, it achieves its vampire act by means of haustoria, penetrating the host plant to suck its sap. Victims along with grape vines may include coffee shrubs, soybeans, asparagus, melons, chrysanthemums, petunias, garlic, oak trees, and tomatoes. Not all dodders achieve their evil design on tomatoes. As Susan Milius explained in Science News, a tomato plant poked by a haustorium of C. reflexa, however, panics. A patch of cells on the stem elongate and bursts, forming a scab that stops the intruder. The haustorium stalls and eventually dies.

At least one vegetative vampire continues to form part of widespread celebrations. The Druids are said to have used a golden sickle to cut bunches of shrubby mistletoe from the oaks on which it grew. The falling plants were caught in woolen sheets to prevent them touching the ground. Today in northwest Europe it is commercially harvested from apple trees, shipped at no little expense to markets there to be sold to hang in home hallways to bring luck and perhaps a kiss to the inhabitants around Christmas time. A strange practice for what is hemiparasite: parasitic under natural conditions and also photosynthetic to some degree

Sir Thomas Stamford Raffles is remembered today as the founder of Singapore and its famous Raffles Hotel. Whether he would have considered it a similar honour to have 28 species of parasitic plants named after him is another matter. Rafflesia species are hosted by lianas of the genus Tetrastigma, native to Southeast Asia, from Thailand, Malaysia, Indonesia and through to the Philippines. One species of Rafflesia may not have much else going for it, but does bear the world’s largest flowers. A full metre across, they usually emerge from its host’s roots, but sometimes high on the vine’s stem. These odiferous flowers are pollinated by flies and the resulting seeds distributed by ants. Nonetheless, Rafflesia are the state flowers of Indonesia, the Sabah State of Malaysia and Thailand Surat Thani province.

Perhaps not as spectacular or even evil smelling, a surprising number are to be found in North American woods. Beech Drops, Epifagus virginiana, for example, are root parasites exclusive in the wild to American beech Fagus grandifolia. However, a close but alien relative, Orobanche ramosa, or Branched Broomrape, parasites tomatoes and tobacco. One-Flowered Cancer-Root, O. uniflora found in damp roots is also parasitic. Another native of the Broomrape family, Orobanchaceae, is the Squawroot, Conophis americana, often emerging from oak roots, although also from other trees. Unlike the white Beech Drop flowers, those of the Squawroot are yellowish.

In recent years, some question has been raised as to if these plants are truly parasitic, or while non-photosynthesizing, obtain nutrients from fungi which live on other plant roots. They are thus said to be mycoheterotrophic. It is believed that there are at least 400 species of such plants. Amongst them, and until recently similarly classed as parasitic, are the extraordinary underground orchids of Australia, Rhizanthella. The Western Underground Orchid, R. gardneri, is associated with broom honeymyrtle, Melaleuca uncinata; parasitizing the fungi on its roots while pollinated by termites and gnats.

Nor is it only crops and ornamentals that are threatened by these sycophant suckers. It is something of a mystery how Thesium arvense, native to China and Eastern Europe, reached North Dakota, Montana and now southern Alberta. Fifteen years ago, it was discovered in Calgary’s Fish Creek Park. The shrub-like perennial thrives on native plant species in shady woodland habitats—much to their detriment. Alberta is training dogs to sniff it out.

But thanks to the Snotty Gobble the news is not all bad. The native Australian vine, also known to Diggers as the Devil’s Twine, but botanically as Cassytha pubescens, is being examined as a possible biocontrol agent for invasive weeds. Researchers at the University Adelaide are reported to have discovered it is able to kill European gorse, Scotch broom and blackberry while not damaging native shrubs.

Wes Porter -- Bio and Archives | Comments

Wes Porter is a horticultural consultant and writer based in Toronto. Wes has over 40 years of experience in both temperate and tropical horticulture from three continents.