Mistletoe is unusual among weeds: it is not often thought of as a `weed' in the vernacular sense, and it is not listed in the WSSA's Composite List of Weeds. However, it fits our definition-a plant growing where it is not wanted - and it fits the requirements of this column, meaning it has a long and fascinating relationship with man.
TAXONOMY AND NAME HISTORY
Barlow defines "mistletoes" as shrubby parasites which grow attached to tree branches (3). Taxonomically, mistletoes are flowering plants which belong to one of three families-Loranthaceae, Viscaceae, or Eremolepidaceae (4). Representatives of two other families have also independently adopted the mistletoe life habit (4). In traditional taxonomy, all mistletoes were placed in the Loranthaceae - a name conjugated from the Latin lorum, a thong or strap, and the Greek anthos, a flower (9). Some authorities still take the view that other mistletoe groups are merely subfamilies of the Loranthaceae (4). We are perhaps most familiar with species of the Viscaceae (named from the Latin word viscum, discussed below). In this family are placed the two genera native to the U.S. as well as the genus Viscum, the `traditional' mistletoe used by ancient herbalists, worshipped by Druids, and ultimately described by Linnaeus.
The genus Phoradendron comprises the mistletoes found on deciduous trees in the United States. Thomas Nuttal (1786-1859) named this genus from the Greek phor, a thief, and dendron, a tree (10), in reference to the plant's parasitic habit. The mistletoes which sprout in American doorways during the holiday season are P.flavescens (11) throughout the U.S. and P. serotinum in the east (1). The other U.S. genus, Arceuthobium, grows mostly on pines. This genus was named by Friedrich August Marschall von Bieberstein (1768-1826) from the Greek arkeuthos, juniper, and bios, life (10). The arceutho-bios- commonly called dwarf mistletoes-have a serious impact on the American soft wood industry. The mistletoe of European fable is Viscum album L. (1). Viscum is Latin for 'bird-lime' (9), a sticky sub-stance made from mistletoe berries and smeared on branches to catch small birds. Many Latin words sprang from this root, among them the forerunners of our `viscous' and `viscosity.' Album is Latin for `white,' the usual color of this species' fruit. This plant was described by Carolus Linnaeus in 1753. The word `mistletoe' originated as misteltan in Anglo-Saxon, from the diminutive of German mist (dung) and the Anglo-Saxon tan (twig). Apparently it was observed early on that berry-eating birds play a crucial part in distributing the seed of this plant (7). (The taxonomic name for the Mistle Thrush is Turdus vis-civorus. As mentioned, Viscivorus means sticky. Any guesses about the genus name?) LeStrange suggests that the prefix evolved from the Old Dutch mist (birdlime) or mistle (different-i.e., different from the host tree) (12).
FOLKLORE AND MEDICINE
Viscum album was sacred to the Druids of northern Europe, the Ainos of Japan, and certain African tribes (7). One story has it that the True Cross was made from mistletoe wood, and as punishment the plant was banned from the earth. Mistletoe is still called Herbe de la Croix in parts of Brittany, and it has also been known as lignum-crucia (12). (This reminds me very much of a similar story featuring dogwood.) Other common names of the mistletoe include mislin bush, kiss and go, and churchman's greeting (12). Viscum album was the only mistletoe commonly used in medicine (12), but it appeared in the herbals of western and central Europe, China, and the Middle East. The Greek herbalist Dioscorides recommendedpounding and moistening, or chewing, the fruit of Viscum album to make a paste which would reduce swellings and infections (6). Pliny said mistletoe was a cure for sterility and epilepsy, but to be effective it must not be allowed to touch the ground (12); presumably he perceived the plant as charged with divine curative powers which could easily be discharged through casual handling.
DISTRIBUTION AND ECONOMIC IMPACT
Worldwide, mistletoe is represented by about 900 species in 65 genera of Loranthaceae, and 400 species in 7 genera of Viscaceae (3). The U.S. has only two native genera. Mistletoes cause a great deal of damage in forests, orchards, plantations, and ornamentals worldwide. Hawksworth (8) states that mistletoes impair growth, lower host vigor, reduce wood quality and quantity, reduce fruiting, and predispose trees to attack by insects, disease, and fungi. In North America, the dwarf mistletoes (Arceuthobium spp.), which parasitize conifers, are responsible for most of the mischief. Estimates suggest these species cause annual wood losses of about 500 million cubic feet in the U.S. (13) and 150 million cubic feet in British Columbia (2).
BIOLOGY AND HABITAT
Mistletoe is parasitic on trees-our native Phoradendron mostly on deciduous trees, Arceuthobium on conifers. It gains much of its mystique from the fact that it is evergreen, often showing the only green in the dead of winter in deciduous forests. Mistletoe leaves are opposite, simple and entire, and leathery, sometimes reduced to scales. Its flowers are greenish, inconspicuous, and without petals. The fruit is a berry with gluey fluid inside (10). Mistletoe seed is disseminated by various methods and widely diverse agents. In Australia the major means of dissemination is the defecation of mistletoe seed by birds (4). In some species, the birds eat the fruits after discarding the epicarps, and then defecate the seeds on their perch. Also some birds eat only the viscid pulp and do not swallow the seeds. Many species of birds that feed on mistletoe remove the viscid seeds sticking to their bills by wiping them off on a branch (4). Seeds adhere to the feathers of birds nesting in mistletoe plants and are carried about. Some birds eat the whole mistletoe fruits and regurgitate the seeds in pellets, but the seeds seldom, if ever, become established. Seed of fruits falling onto lower branches of trees is believed to be another form of dissemination (4).
Mistletoe seed lack a seed coat, but instead are covered with viscin, a mass of cellulolytic strands in a matrix of pectic substances (4). Viscin bonds the seed to the host and absorbs moisture. The seed of most species germinates in the spring, with the penetration process requiring up to 6 weeks; the germinating radicles (embryo tips) are negatively phototropic and geotropically neutral. Once contact is made with a host surface, the radicular tip cells divide to form a haustorial disk (or holdfast), a loosely organized mound of tissue. The parasitic cells that come in contact with the host surface have dense contents and large nuclei (4). They adhere to the host tissue, pulling apart host periderm and exposing living host cells. Abundant secretions harden eventually to form an air tight seal between the host and parasite. Concurrently the meristem of the parasite develops, becoming the penetration structure, which secretes a substance into the cavity, enabling the endophyte to pass into the host. Tissue penetration undoubtedly involves enzymes and a mechanical force produced by growing and expanding (4).
Most, if not all, of the familiar mistletoe species are poisonous, containing the amines beta phenylethylamine and tyramine (11). Documented cases of poisoning include Phoradendron villosum killing 13 cattle of a herd of 30 in California (14), though livestock usually avoid the plant. Berries of Phoradendron species are usually considered poisonous, though Kingsbury (11) reports only one confirmed fatality-a death following consumption of tea brewed from the berries of P. flavescens (5). Viscum album, the mistletoe of European legend, is also known to be poisonous (11).
1. Bailey, L. H., and E. Z. Bailey. 1976. Hortos Third. Macmillan Publishing Co., Inc., NY.
2. Baranyay, J. A., and R B. Smith 1972. Can. For. Serv. Rep. BC-X72.
3. Barlow, B. A. 1983. Biogoography of Loranthaceae and Vise. In The Biology of Mistletoes, D. M Calder and P. Bemhardt, eds. Academic Press Inc., NY.
4. Calder, D. M. 1983. Mistletoes in focus: an introduction. In The Biology of Mistletoes, D. M. Calder and P. Bernbardt, cds. Academic Press Inc., NY.
5. Cann, H. M., and H. L. Verhulst. 1959. Toxic Hazards at Christmas. Nat. Clearinghouse Pois. Contr. Cent.
6. Dioscorides. 1933. The Greek Herbal of Dioscorides. R. T. Gunther, ed. Hafner Publishing Co., Inc., NY.
7. Durant, M. 1976. Who Named the Daisy? Who Named the Rose? Dodd, Mead and Co., NY.
8. Hawksworth, F. G. 1980. Crop loss assessment. Proc. E. C. Stakman Commemorative Symp. Univ. Wmn. Exp. Stn. Misc. Publ. 7.
9. Jaeger, E. C. 1944. A Source-book of Biological Names and Terms. Charles C. Thomas, Springfield, IL.
10. Jepson, W. L. 1951. A Manual of the Flowering Plants of California. Univ. of California Press, Berkeley.
11. Kingsbury, J. M. 1964. Poisonous Plants of the United States and Canada. Prentice-Hall, Inc., Englewood Cliffs, NJ.
12. LeStrange, R. 1977. A History of Herbal Plants. Angus and Robertson, London.
13. Shea, K. R, and B. Howard 1969. In Western forest pest conditions. Proc. 59th West. For. and Conserv. Assoc., p. 25-32.
14. Wicktor, C. E. 1952. Poisonous Plants. In Annual Report 23, Los Angeles County Livestock Dep.