RUBBER, OR CAOUTCHOUC. Figs. Rubber, or caoutchouc, is obtained from the milky juice or latex of a considerable number of trees and shrubs, erect or climbing, which inhabit almost exclusively tropical parts, though some are found in sub-tropical regions. These plants belong to the families Erticacece, Euphorbiaeece and Apney naccre. For practical agriculture, however, there are only four of these plants which can be utilized in cultivation, viz., Hevea Rrasiliensis and Manihot Glaziorii of the Euphorbiacere, and Castilloa dastica and Ficus elastica of the Crticacere. The big woody climbers, La ndol phia and Willughbeia, of the forests of Africa and Malaya, do not respond to cultural treatment. Hancornia and various species of Ficus not mentioned above have given such poor results under cultivation that they are not worth the at tention of the agriculturist, though the rubber has value when it can be collected in sufficient quantity. Mimusops globosa, a tree which produces "Balata rubber" (or gutta-percha), is indigenous to South America and the British island of Trinidad, and might be cultivated to any extent. It is a slow growing tree, but to those who can afford to wait it would doubtless be a most profitable investment.
In the four species above mentioned, we have plants of which one or another is suitable for cultivation on a large scale and with good profit very widely in the tropical regions. All are trees of considerable size, and, under suitable circum stances, of rapid growth.
The United States is entirely dependent on imports for its rubber. Crude rubber is the third largest of the tropical imports of this country. The imports for the five years, 1898-1902, were as follows : It is absolutely essential for the agriculturist who intends planting rubber trees, first carefully to select the kind suited for the climate and soil in which he intends to plant. Much money has been wasted by attempting to plant Ceara rubber in the tropical rain-forest region.
Such errors are easily avoidable.
The latex.
The latex, or milk, is a white liquid consisting of water containing proteid matter, sugar and minute globules of caoutchouc or rubber. The art of mak ing the rubber consists in separating the rubber from the water and other con stituents of the latex.
The latex occurs in a series of special vessels (the laticiferous vessels) which permeate the bark of the stem and twigs and also the leaves and other soft parts of the trees. A section of the bark of the Para rubber (Hevea) under the mi croscope shows on the outer surface sev eral layers of hard, thick-walled cells, forming the cork layer ; below this lie layers of thin-walled, long cells, the bast layer, through which run the laticiferous vessels, which are of some length and which branch and join each other at intervals so as to form a network. Below this layer lies the cambium or growing layer of the bark, and below this again the wood. The latex vessels are most abundant near the cambium layer, and run verti cally, parallel with the long axis of the stem.
To get the latex it is necessary to cut the bark in such a way as to cross as many latex-tubes as possible without unduly injuring the tree. A notion holds that the wound should not penetrate the deli cate cambium layer but stop short of it for fear of risking the life of the tree. As a matter of fact, in Para rubber, at least, the risk is small. Many trees have been cut to the cambium and deeper in the Singapore Botanic Gardens, but none have been injured ; reports from Ceylon, however, recommend great care in this respect. Less deep wounds heal more quickly, it is true, but as the greater part of the latex vessels lie very close to the cambium, unless the wound is made to this layer not more than half the available latex can be secured.
Although latex occurs in all parts of the tree, that which is found in the upper branches and twigs is weak and of no value commercially. It is therefore from the lower part of the trunk, and, in Ficus, from the aerial roots also, that the rubber is derived.