RUBBER AND OTHER LATEX PRODUCTS
Rubber is obtained from the milky juice, or latex, of various erect or climbing woody plants of the tropics or subtropics. The majority of the rubber plants belong to the Moraceae,Euphorbiaceae, or Apocynaceae. Although well over fifty species are available as sources, only a few have been important commercially and at the present time Hevea brasiliensis stands preeminent. Wild trees were formerly the only source of rubber, but now cultivated Hevea trees, the so-called plantation rubber, furnish about 98 per cent of the supply. Rubber is the most recent of the major crops of the world. The industry is little more than 100 years old, and cultivation has been carried on only 60 years or so. In view of this, the increase in the production of plantation rubber from 800 long tons in 1900 to 305,000 tons in 1920 and 1,395,000 in 1940 must be considered as one of the greatest triumphs in modern agriculture. This great development of the rubber-growing industry has not been without its drawbacks, however. Overproduction has seriously affected the industry financially in recent years, and many attempts have been made at some sort of regulation. The British and Dutch, in particular, have tried to restrict production and exert other methods of control in Malaya, Java, Sumatra, and other plantation rubber centers within their empires. The recent successful development of synthetic substitutes for rubber, after many years of experimentation, may further tend to jeopardize the natural rubber industry. However, while these substitutes are superior for some purposes, such as the conduction of oil, natural rubber is still preferred for tires, which utilize three-quarters of the rubber output.
Latex occurs in special cells or in a series of special vessels which permeate the bark, leaves, and other soft parts of the tree. Usually only the latex from the lower part of the trunk is of importance commercially. Latex is a gummy white liquid full of minute globules. It is a varying mixture of water, hydrocarbons, resins, oils, proteins, acids, salts, sugar, and caoutchoue, the substance used as the source of rubber. The significance of latex to the plant is obscure. It is of some value in the healing of wounds, and it may serve for protection, nutrition, the transport of materials or as a fluid reservoir.
The properties of rubber have long been known. The primitive