Another interesting source of rubber is the genus Landolphia, of which there are several species noteworthy as being creepers. This genus provides most of the Congo rubber and belongs to the order Apocynaceae. Ficus elastica spoken of above is one of the genera of the order Urticaceae of which another genus is Castilloa also already mentioned as found in Mexico and one of the very first plants to attract attention as a rubber producer.
Rubber, though found to a slight extent as a solid deposit in the woody fiber of certain species, is almost entirely obtained from the latex of the rubber-bearing plants. The latex is a fluid usually more or less viscous which is carried in vessels, the laticiferous vessels, lying in the inner bark just a little outside of the cells which carry the sap. The caoutchouc itself is in globules of microscopic or sub-microscopic size, being from 1/50,000 inch to 1/6,000 inch in diameter, and forms an emulsion with the suspending liquid. A familiar example of latex is the exudation of the milkweed. The function of latex in the plant itself is unknown. It may be an excretion, it may be intended for the preservation of the tree from attack by fungus or insects or other enemy. The process of raw rubber manufacture consists in the collection of the latex and the coagulation from the serum of the emulsified particles. In tapping the trees the essential thing is to cut deep enough into the bark to sever the laticiferous vessels, but not to cut into the cambium, the living layer of cells from which both the wood and the bark of the tree are produced. In the Amazon Valley this is usually done by a small axe, the incisions being of a V shape, the first being made at a height of about six or seven feet. Later incisions are made at intervals of about two inches below the previous ones, till the base of the tree is reached. Then tappings are begun on the other side of the tree in the same order as before. The latex is collected in a small cup fixed to the tree by moist clay and is removed from time to time. Five pounds of latex is considered a large amount from one tree during the season. The latex is gathered from the cups into a pail and is cured by the smoke of a fire rich in tarry and acid matter. A long wooden rod has rubber latex poured over it and the thin layer which sticks to the rod is dried in the smoke. Over the sheet thus formed is poured more latex, which is also dried in the smoke. Thus layer after layer is produced till a ball weighing from twenty to one hundred pounds is obtained. Thus is made raw fine Para rubber. Some of the latex coagulates on the tree, forming a scrap rubber which is collected and compressed into irregular masses called "negroheads."
It is not my purpose to describe all the processes through which raw rubber passes before it appears in the shape of golf balls or automobile tires or in any of the many forms in which it comes into commerce, but a very brief outline may be given. First, the raw rubber is cut into
