Spurge Family

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 as "bitter 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 as " sweet 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 A castor bean plant. Photograph by James Sikkema. Reproduced by permission. (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 world's 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.