Rubber can be obtained from a large number of plants, but almost all commercially produced natural rubber comes from the rubber tree, Hevea brasiliensis. This tree, sometimes called the Par$aA rubber tree, is native to the Amazon Basin but today is grown on plantations in many other tropical areas. The tree grows best in warm climates with an annual rainfall of at least 75 inches (1,900 mm) well distributed throughout the year. The leading rubber-producing countries are Thailand, Indonesia, and Malaysia. Rubber is also produced in India, China, and Sri Lanka; in several African countries; and in tropical America. Brazil, homeland of the rubber tree, still produces rubber, but on a much smaller scale than in the past.

Other natural sources of rubber include the guayule plant; the gutta-percha tree; the Russian dandelion, or kok-saghyz; and the rubber plant.

Hevea brasiliensis is a member of the spurge family, Euphorbiaceae.

Rubber trees are not grown directly from seed, for trees thus grown vary greatly in production of latex, and the average yield is low. To obtain trees of high production, buds from high-producing trees are grafted on year-old seedlings. After the bud has sprouted, the top of the seedling is removed, and the shoot from the bud is allowed to replace it. The resulting tree has the properties of that from which the bud was taken. The best budded trees produce up to 2,000 pounds of dry rubber per acre (2,240 kg per hectare).

Budded seedlings are transplanted at the rate of about 100 to 150 per acre (250 to 370 per hectare). They grow rapidly, and at about six years of age, when their trunks are about six inches (15 cm) in diameter, they can be tapped.

is done with a special tool. A shallow, diagonal cut extending about halfway around the trunk is made in the bark. A spout leads from the bottom of the incision to a cup into which the latex drips. Trees are usually tapped every other day. The first cut is begun about five feet (1.5 m) above the ground.

In making the next cut, the worker first strips off a layer of coagulated (thickened or solidified) latex and then makes a single cut below the previous incision. By the time the channel nears the ground, bark has grown over most of the upper incisions. Work then begins on the other side of the tree. Thus managed, trees produce for 25 years or longer.

Tapping begins early in the morning, when the air is relatively cool. After tapping a tree the worker puts a little ammonia in the cup to keep the latex from coagulating. After the last tree has been tapped, the worker returns to the first tree and collects the latex.

The latex consists of about 35 per cent rubber. The rubber is in the form of tiny particles suspended in a liquid. Latex is prepared for shipment at a central plant, where it is either concentrated or coagulated. It is concentrated by whirling it in a centrifuge similar to a cream separator. A preservative is added to the concentrated latex to keep it in liquid form during shipment in tank cars and ships.

Most latex is coagulated before shipment. This is done by putting it in tanks and adding formic acid or some other acid to it. The acid causes the rubber to separate from the liquid, just as the curd separates from the whey in cheesemaking. The curd of rubber is removed and passed through corrugated rollers, which squeeze it to remove the liquid.

Pale crepe rubber is made by adding a bleach—usually sodium bisulfite—to latex before it is coagulated. Pale yellow in color, pale crepe rubber is used to make articles in which whiteness or transparency is desirable.

Most natural rubber is processed to meet certain specifications and is generally graded and shipped in bales. Some rubber is shipped as ribbed smoked sheets. This rubber is prepared by hanging sheets of rubber in a smokehouse, where they are exposed to the smoke of a wood fire and kept at a temperature of about 120° F. (49° C.) for several days. Smoking preserves the rubber from molds, bacteria, and oxidation.