Broomrapes, Dodders, and Mistletoes

Lake S. Gill.

Parasites are plants and animals that get their living at the expense of other organisms, which are called the hosts.

Usually we think of parasites as lower forms of life, such as fungi and bacteria. A number of flowering, or seed plants, however, are parasitic on vegetation. Parasitism appears in varying degrees in several widely separated botanical families and is regarded generally as a degenerative process in species that once were free-living. A higher plant that has become parasitic does not, as far as we know, return to independence.

The parasite depends physiologically for its existence on the host plant. The host may incidentally supply protection or physical support, although plants that derive only those benefits are not classed as parasites but are called epiphytes. Examples of epiphytes are Spanish-moss and some tropical orchids that attach themselves to trees but are physiologically independent even though they maintain no contact with the soil.

The parasitic seed plants vary widely in their dependence. The more independent ones are referred to as hemi-parasites, half-parasites, or water-parasites. All of them produce chlorophyll and therefore are capable of manufacturing food from carbon dioxide and water, although they depend on the host for certain dissolved minerals and perhaps organic substances. Some absorb from their hosts all the water they need for transpiration and the manufacture of food. The mildest hemi-parasites look like normal green plants growing in soil. They steal their food through root connections, often inconspicuous, from surrounding host plants, which may suffer little or no harm from the arrangement. Notable parasites of this type are the sandalwood tree of India and its near relative Commandra (false-toadflax), a common herb in North America. The mistletoes illustrate the opposite extremes of this group, having lost contact with the soil and being dependent on their hosts for all water and dissolved minerals even though the chlorophyll-containing species can manufacture sugars and starches in their green leaves and stems.

The complete parasites have no chlorophyll and therefore depend wholly on their hosts for nourishment. Plants that have degenerated to this extent are never green. Their leaves usually are reduced to inconspicuous scales and they exhibit a marked modification of their functional root system, which develops entirely inside the host tissues.

Parasitic seed plants important to agriculture, forestry, and arboriculture in North America are relatively few and can be classified broadly in three groups: (1) Mistletoes are green, yellowish, or brownish plants growing on the stems and branches of trees or shrubs. (2) Dodders are slender, twining, orange or yellowish, rarely white or purplish, leafless, threadlike stems often forming dense tangled mats over host plants. (3) Broomrapes are clumps of whitish, yellowish, brownish, or purplish stems that arise from the roots of host plants.

THE NAME MISTLETOE is derived from the Saxon mistl-tan, meaning "different twig" an indication that the ancients recognized it as something apart from the branches of the host tree. It was featured in the Greek, Norse, and Germanic legends as a plant vested with supernatural powers for good and evil. The Druids and other pagan peoples of Europe used it as a sacred emblem in their religious rites. The herbalists of the early Christian era claimed that mistletoe was once a forest tree but became dwarfed out of shame when its wood was used to make the cross at Calvary. They called it guidhel or all heal and prescribed it as an antidote for poisons and a cure for falling sickness and epilepsy. Amulets made of the plant were often worn to ward off disease.

The early American settlers considered our leafy mistletoe of the East to be identical with the Viscum of their homeland and thus it, too, became vested with much of the traditional symbolism that had developed about the European plant. Today it is highly prized for Christmas decorations and during the holiday period it may shed blessings on those who stand beneath it.

About 305 B. C., Theophrastus, the Greek botanist, recorded technical observations which indicated that he recognized mistletoe as a parasitic plant. In the eighteenth century, Carolus Linnaeus, the great Swedish botanist, described and named the principal European species Viscum album. The name was doubtless selected because the white berries of the plant were then used in the manufacture of bird lime. Strictly speaking, this plant is the true mistletoe, although today the name is loosely applied to many members of the botanical family Loranthaceae, which embraces about 900 species, mostly tropical tree-inhabiting hemiparasites. About 35 species are known in temperate North America. Five of these are in the genus Arceuthobium, also called dwarfmistletoe; the remainder are in the genus Phoradendron, often called leafy mistletoe, true mistletoe, or Christmas mistletoe. The latter genus was set apart from the Old World Viscum in 1847 by Thomas Nuttall, first director of the Harvard Botanical Gardens; the Greek origin of the name means "tree thief."

The Phoradendrons attack chiefly broadleaved trees, although in the far West certain conifers, especially juniper and its near relatives, are common hosts.

All species are regarded as half-parasites that produce clusters of green, perennial, jointed stems on the branches and trunks of trees or large shrubs. The stems mostly bear conspicuous deep green, leathery leaves, which persist for several seasons. A few species, notably some of those on conifers and desert plants in the West, are virtually leafless.

The stems are supplied from an absorbing system, which develops in the bark and wood of the host and takes water and whatever else may be required to supplement the food manufactured in the aerial green parts of the parasite. The flowers are dioecious; that is, the staminate or male and the pistillate or female flowers occur on separate plants. They are borne at the base of the leaves where ( among pistillate plants only) the familiar translucent, whitish berries develop in the late fall or early winter. In some western species the berries are straw to pinkish in color and mature in midwinter.

Within the tough outer coat of the berry is a single seed, which is embedded in viscin, a sticky substance. Birds feed on the sticky pulp of the berries. Some of the discarded seeds stick to their bills, feet, and other parts of the body. Thus the seeds are carried to other trees or other parts of the same tree, where they may be brushed off on a branch or twig, germinate, and perhaps develop into a new mistletoe plant. If the berries are left undisturbed they disintegrate and release the sticky seeds, which may fall and adhere to a limb or twig below. Seeds that pass through the digestive tracts of birds may also germinate and start new mistletoe plants.

In most parts of the country the leafy mistletoes are relatively scarce and are regarded more as botanical curiosities than as serious pests. Because of their value as holiday decorations, some interest has been shown in their culture but thus far there have been no practical developments in at field. Most of the mistletoe for the Christmas trade is gathered in the forests of the Southern and Southwestern States where it is abundant in some localities and where it provides off-season income for agricultural workers. The berries of Phoradendron flavescens are a recognized source of a pressor compound (parahydroxyphenylethyl amine) of limited pharmaceutical value.

In certain and or semiarid places in the West, notably in Texas, New Mexico, Arizona, and California, mistletoe has become so abundant as to warrant control measures for the preservation of shade and horticultural trees. The natural woodland species that are most heavily attacked notably juniper and mesquite generally have such low economic value that large-scale control operations cannot be justified except possibly in public parks where the primary aim is to preserve natural vegetation for the enjoyment of generations to come. In some parts of the Midwest, such hardwoods as walnut and elm are sometimes severely parasitized by mistletoe.

The spread of mistletoe can be reduced somewhat by breaking off the pistillate shoots. That must be done periodically because it does not destroy the absorbing system from which the shoots develop; it merely discourages the parasite from producing berries, which may spread about the infected tree or to other host plants. It is practical only if little mistletoe is present and the danger of reinfection from outside is remote.

When infections are moderate, one can prune off the affected limbs and thus free the host plant entirely of the parasite. Sometimes severe trunk infections can be destroyed by removing the invaded bark. If hopelessly infected trees occur near others worthy of preservation, it would perhaps be best to remove the infected ones entirely.

Although a single species of Phoradendron may attack a variety of trees, strong host affinities are usually developed within a species in a given region. The extent to which crossing from one host species to another occurs in nature is not known exactly, although there are strong indications it is quite limited. In regions where mistletoe is a serious pest, it would seem advisable to favor tree species that are naturally immune or highly resistant to attack in that particular area even though they are congenial hosts elsewhere.

The dwarf mistletoes are found only on conifers. In North America they do not attack juniper and related species, which are common hosts of Phoradendron. The mistletoe shoots themselves vary in color from green to yellow or brown. Less conspicuous than those of the true mistletoes, they range from tiny scattered outgrowths about one-fourth inch high to coarse, jointed stems up to 12 inches long. The seeds are borne in explosive, berrylike fruits. At maturity they may be shot 50 or more feet away from the host tree and thus gradually encroach on the surrounding forest. The seeds are covered with sticky viscin and adhere to any surface on which they may alight.

New infections are most likely to develop when the seeds germinate on young twigs of a suitable host plant. In that case an absorbing root penetrates the tender bark and develops a system of strands that attack the living phloem of the host. They absorb from it what is needed for the development of the parasite. Some of the strands reach the cambium layer of the host and are permanently embedded in the wood as it is laid down each year. They retain their connection with the strands in the phloem indefinitely and are called sinkers. After the absorbing system is well established, it produces buds from which shoots develop. That may occur the year after infection or it may be delayed for a decade or more. Flowers are borne on the shoots and, like Phoradendron, are dioecious. Insects carry the pollen from the male to the female flowers and the fruits mature 5 to 16 months later.