ALTHOUGH Hevea brasiliensis is universally recognised as the plantation rubber tree par excellence, several other kinds of rubber-producing plants are used extensively for the same purpose. In spite of its hardiness, the adaptability of Hevea is limited, and in dry climates Manihot Glaziovii is probably to be preferred. In fact, extensive plantations of the last named species have already been opened in the drier regions of Africa as well as in its native country, the Ceara province of Brazil. In the moist climate of West Africa Funtumia is said to be quite as successful in plantations as Hevea, although the evidence upon the subject is somewhat restricted. Finally, in Mexico large plantations of the indigenous Castilloa have been established. Here, however, it is doubtful whether the planters would not have been better advised to introduce Hevea, since the climatic conditions suitable for the two trees are closely similar.
As stated in Chapter I, plants of Castilloa were brought successfully to Kew prior to the arrival of the most important consignment of Hevea, and trials have been made with the former plant in many parts of the tropics, including almost every British tropical colony. So far as we are aware, Castilloa has nowhere been found equal to Hevea in suitability for plantation use, in spite of fairly exhaustive trials in many different districts.
The home of the Castilloa tree is Mexico and Central America. The total export of rubber from Mexico in 1908—1909 was about 1,000,000 lbs. Of this amount, some 40 per cent, is stated to have been plantation rubber. In 1910 the area of rubber plantations in Mexico is believed to have been about 90,000 acres. The greater part of this area was planted between 1897 and 1906. The capital involved, mostly subscribed in the United States, was estimated at about £4,000,000.
So far, these plantations have not by any means fulfilled the expectations of their promoters. A yield of 67 lbs. of dry rubber per acre from six- to eight-year-old trees, although gathered in at a trifling cost, is very small compared with the yield from the plantations of Hevea in the East. The latex from young trees, moreover, contains a high proportion of resin, and the period of waiting for satisfactory crops is consequently a long one.
The Castilloa has generally been closely planted. From 200 to 300 trees per acre is by no means an uncommon estimate. Since the growth of Castilloa in the rich soils of Mexico appears to be considerably more rapid than the average recorded for Hevea on plantations, there can be little doubt that the trees are greatly crowded.
The trees are only tapped at intervals of about four months. The method employed is purely one of incision, and consists in cutting a few slanting or V-shaped channels in the bark. The rubber is usually prepared from the latex by a process of creaming. In this process the latex is diluted with a considerable bulk of water and, after stirring, is allowed to stand in tanks until the whole of the rubber has collected as a cream upon the surface. The clear liquid is then drawn off from below, and the whole process is repeated until dirt and soluble impurities have largely been removed. The rubber is then finally dried and rolled into sheets. Centrifugal machines are also sometimes employed. In this way much time can be saved, and the washing is more completely carried out.
In Ceylon very fine samples of smoked sheet rubber have been prepared from Castilloa latex. The cultivation, never very extensive, has however been almost entirely given up in favour of Hevea. In the West Indies Castilloa is still under trial, but only on a small scale, and the amount of land available for the cultivation is limited. In Jamaica a yield of 200 lbs. per acre is anticipated from Castilloa after the tenth year. This form of rubber has also been planted somewhat widely in German colonies, particularly in New Guinea
In spite of the small yields per acre, Castilloa is still regarded by some writers as being the most suitable form of rubber for plantation purposes in Mexico, owing to the small cost of collection. Although the total yields are small, the yield from a single tapping is very much larger than in the case of Hevea at the same age. The cost of labour is very much higher in Mexico than in either Africa or Asia.