Spurge Family

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Spurge Family

Biology of spurges

Economic products obtained from spurges

Horticultural spurges

Spurges as weeds

Resources

Spurges or euphorbs are species of plants in the family Eurphorbiaceae. This is a rather large family of plants, consisting of about 7,500 species and 300 genera, mostly distributed in the tropics and subtropics, but also in the temperate zones. The most species-rich genera of spurges are the Euphorbia with about 1,600 species, and Croton with 750 species.

Most species in the spurge family have a white latex in their stems and leaves that is poisonous if it contacts the eyes or other membranes, or if it is ingested. The seeds are also often poisonous. Even rainwater dripping from the canopy of the manchineel tree (Hippomane mancinella) in the West Indies has enough toxin in it to cause a dermatitis reaction in people standing beneath.

Some species in the spurge family are economically important, either as food plants, ornamentals, medicinals, or weeds.

Biology of spurges

Spurges exhibit a wide range of growth forms. Most species are annual or perennial herbs, in the latter case dying back to the ground surface at the end of the growing season, but regenerating from roots and

rhizomes at the beginning of the next growing season. Other species of spurges are shrubs and full-sized trees. Some species of spurges that grow in dry habitats have evolved morphologies that are remarkably similar to those of cacti (family Cactaceae). In some cases, the similarities between these families can be so great that many of the plants non-botanists believed to be cacti are actually spurges.

When the stems or leaves of most species of spurges are wounded, they weep a white, milky substance known as latex. The latex of spurges can be used to manufacture a natural rubber. Natural rubber is a polymer in its simplest form, derived from a five-carbon compound known as isoprene, although much more complex polymers can also be synthesized. The specific, beneficial function of latex to wild plants has never been convincingly demonstrated, although this substance may be useful in sealing wounds, or in deterring the herbivores of these plants.

The individual flowers of spurges are usually rather small and unisexual. The latter characteristic can occur as separately sexed flowers occurring on the same plant (this is known as monoecious), or as different plants being entirely staminate or pistillate (dioecious). In many spurges, the individual flowers are aggregated together within a compact, composite structure known as a cyathium. In addition, most species of spurges have nectaries that secrete a sugary solution to attract insect pollinators. Some species flowers are highlighted by specialized, highly colorful leaves, giving the overall impression of a single, large flower. The composite floral structure, nectaries, and brightly colored bracts of spurges are all adaptations that encourage visitations by the insect pollinators of these plants.

Economic products obtained from spurges

By far the most important spurge in agriculture is the cassava, manioc, or tapioca (Manihot esculenta), a species that is native to Brazil, but is now grown widely in the tropics. The cassava is a shrub that grows as tall as 16.5 ft (5 m), and has large, starchy root tubers that can reach 11-22 lb (5-10 kg) in weight, and are processed as food. The tubers of cassava mature in about 18 months, but by planting continuously, people can ensure themselves a continuous supply of this important food plant.

The tubers of cassava contain a poison known as prussic or hydrocyanic acid. The varieties known asbitter cassava have especially large concentrations of this toxic chemical. The prussic acid can be removed from the tubers by shredding them into a pulp, and then washing several times with water, or it can be denatured by roasting. The residual material from this detoxification processes is then dried and ground into an edible meal, which can be used to prepare foods for human consumption. This meal is a staple food for many inhabitants of tropical countries, probably totaling more than one-half billion people. Other varieties of cassava, known assweet cassava, have much less of the prussic acid and can be eaten directly after boiling or baking. In North America, cassava is a minor food, mostly being used to make tapioca pudding.

Another, relatively minor agricultural species is the castor bean (Ricinis communis), from which castor oil is extracted. This species is native to tropical Africa, and it can grow as tall as 49 ft (15 m). The fruit of the castor plant is a spiny capsule containing three large seeds, each about 0.8-1.2 in (2-3 cm) long, and with a colorful, brownish-mottled seedcoat. The seeds contain 50-70% oil, which is extracted from peeled seeds by pressing. The oil is used as a fine lubricant for many purposes. Castor oil is also used as a medicinal, especially as a laxative. The seeds of castor bean are highly toxic when ingested.

The para rubber tree (Hevea brasiliensis) is native to tropical forests of Brazil, where it grows taller than 66 ft (20 m). The milky latex of this tree is collected from wide notches that are cut into the bark cambium, so that the latex oozes out and can be collected in a metal cup. The latex is later heated until it coagulates, and this forms the base for the manufacturing of natural rubber, of which the para rubber tree is the worlds most important source.

The latex of the para rubber plant is collected from wild trees in intact tropical forests in Amazonia, and in large plantations established in Southeast Asia, especially in Malaysia and Indonesia. Para rubber trees can be tapped for as long as thirty years, and as much as 6.6-8.8 lb (3-4 kg) of rubber can be produced from each tree per year. The plantation latex is coagulated in factories using acetic and formic acids, and it is then cured by drying and smoking. The raw rubber is later vulcanized (treated with sulfur under heat and pressure) to make a hard, black, elastic rubber useful for manufacturing many products. If especially large amounts of sulfur are used, about 50% by weight, then a very hard material known as vulcanite or ebonite is produced.

Horticultural spurges

Various species of spurges are grown as showy plants in horticulture. Care must be taken with these plants, because their milky latex is very acrid, and can injure skin and moist membranes. The milder symptoms of contact with the latex of spurges include a dermatitis of the skin. The eyes are especially sensitive, and can be exposed to the latex if a contaminated hand is used to scratch an eye. Severe, untreated exposure of the eyes to spurge latex can easily lead to blindness. Spurges are also toxic if eaten, and children have been poisoned and even killed by eating the foliage or seeds of ornamental spurges.

The most familiar horticultural species of spurge is the poinsettia (Euphorbia pulcherrima), a native plant of Mexico. The poinsettia is often kept as a houseplant around Christmas time in North America. This plant has rather inconspicuous clusters of flowers, but these are surrounded by bright red, pink, or greenish-white leaves, which are intended to draw the attention of pollinating insects.

The crown-of-thorns euphorbia (Euphorbia splen-dens) is a cactus-like plant native to Madagascar, with spiny branches and attractive clusters of red-bracted flowers, which is commonly grown as a houseplant or outside in warm climates around the world. Another tropical African species is the naboom (Euphorbia ingens). This is a tree-sized, cactus like plant, with large, segmented and virtually leafless, green, photo-synthetic stems. It is also commonly cultivated in homes and warm gardens. Another unusual species is the pencil cactus (Euphorbia tirucalli), with thin, green, almost-leafless, photosynthetic stems.

The genus Croton has many species that are grown for their colorful foliage in homes and greenhouses, or outside in warm climates.

The castor bean can also be grown outdoors in frost-free regions as an ornamental plant, because of its interesting, large-leafed, dissected foliage.

Spurges as weeds

Many species of spurges have become noxious weeds in agriculture, especially in pastures, because these plants can be toxic to cattle if eaten in large quantities. One example of an economically important weed is the leafy spurge or wolfs-milk (Euphorbia esula). This species was originally native to temperate regions of Europe and Asia, but became an invasive weed when it was introduced to North America. The introduction of this important weed probably occurred numerous times as a seed that was present in the ballast that ships often carried to give the vessels stability when sailing from Europe to North America. This ballast was commonly soil obtained locally in European harbors, and then discarded at American ports upon arrival.

Leafy spurge now has a wide distribution in North America, but it is especially abundant in the Midwestern prairies. This species occurs in a diverse range of open habitats, including agricultural fields and pastures, and grazed and natural prairies. Leafy spurge is a herbaceous, perennial plant that grows an extensive root system that can penetrate as deep as 30 ft (9 m) into the soil. The leafy spurge also produces large numbers of seeds, which are effectively dispersed by various means, including animals.

Leafy spurge is a severe problem because it can poison livestock if they eat too much of this plant. The only exception is sheep, which can tolerate the latex of leafy spurge, especially early in the growing season. The latex of leafy spurge is also toxic to humans, causing dermatitis upon contact, and severe damage to the eyes and mucous membranes if contact is made there. Leafy spurge is invasive in some natural communities and in semi-natural habitats such as grazed prairie, where this species can become so abundant that it displaces native species.

Infestations of leafy spurge have proven to be very difficult to control. Herbicides will achieve some measure of success locally, but this sort of treatment has to

KEY TERMS

Cyathium The specialized, compact clusters of flowers in members of the spurge family.

Dioecious Plants in which male and female flowers occur on separate plants.

Latex This is a white, milky liquid that is present in the tissues of spurges and many other plants.

Monoecious This refers to the occurrence of both staminate (or male) and pistillate (or female) flowers on the same plants.

Rubber This is a tough, elastic material made from the whitish latex of various species of plants, especially that of the para rubber tree of the spurge family.

be repeated, often for many years. Recent investigations have focussed on the discovery of methods of biological control, using herbivorous insects or diseases native to the natural Eurasian range of leafy spurge, which keep this plant in check in its natural habitats. So far, these methods have not proven to be successful.

Various other species of spurges have also become agricultural weeds in North America, although none as troublesome as the leafy spurge. Some additional, weedy spurges include the spotted spurge (Euphorbia maculata) and cypress or graveyard spurge (E. cyparissa), both of which likely became pests after escaping from gardens in which they had been cultivated.

Resources

BOOKS

Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.

Bill Freedman

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Spurge Family

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