Page:Popular Science Monthly Volume 85.djvu/452

448 different knives have been invented for the purpose. The half-herring bone method is considered the better as being less severe on the tree.

A very curious phenomenon was observed in the early experiments on tapping. It was found that if a second incision was made in the bark of Hevea, near one cut a couple of days previously, there was a greater flow of latex than if the second incision were made at a distance, say, on the other side of the tree. More than that, the latex flowed more freely than on the first incision. In a particular experiment on four trees, tappings were made at intervals of five days, and the volume of latex increased from 61 c.c. at the first tapping to 449 c.c. at the fourteenth, when the series was ended. In view of the fact that the latex from later tappings is thinner than that from the first, another series of experiments was made on ten trees which were tapped every day for a fortnight and the rubber content of each tapping determined. This rose from 64 oz. on the first day to 33 oz. on the fourteenth. Within limits a thin latex is the most satisfactory, the latex from the first incision often being of little use because it coagulates before reaching the proper receptacle and so gets mixed with the bark of the tree and other foreign.matter. Sometimes drip pans are fastened to the tree above the incisions, and water dropping upon tire incisions prevents the latex drying on the tree.

The peculiar action of Hevea owing to which subsequent tappings near the previous incisions produce a greater flow of latex is called "wound response," and no other rubber-bearing plants show wound response in anything like the degree shown by Hevea; in fact, it may be doubted whether the phenomenon occurs in the other genera at all. As compared with Hevea, Castilloa gives a greater flow of latex on the first incision, some five or six times as much. But if, after a couple of days, a further incision is made near the former one, little or no latex flows from it, while, as we have seen, there is in the case of Hevea a greater supply than before, roughly about twice as much, which persists through subsequent tappings. Accordingly, in tapping trees a very thin paring (about one twentieth of an inch) is removed each day or each alternate day. As the first incisions are made about a foot apart, it takes some two hundred and forty parings before the bark is all removed from this part of the tree, and as by the half-herring-bone system only about a quarter of the tree is tapped it takes about four years to remove all the bark and by that time operations can be begun again on the new bark that has formed in the meantime.

The arrangement of laticiferous vessels in Castilloa is different from that in Hevea; in the former the vessels all connect in a somewhat similar manner to that of veins and arteries in the body. Hence, when the vessels are cut, there is likely to be a drain from a large area. In Hevea the tubes arise from a breaking down of cell walls which occur from time to time and so the latex does not flow out so freely at first.