The evidence already given is sufficient to show that the amount of latex which can be removed from a Hevea tree in one year, must be very large in comparison with the quantity which was present in the vessels prior to the commencement of tapping. In further illustration of this point we may take the case of an exceptionally large and vigorous tree, the yield from which was separately recorded for a considerable period.
This particular tree is one of those originally planted at Henaratgoda in 1876. Its girth at three feet from the ground was 102 inches in December 1908 when the experiment was begun; and it had increased to 115 inches in December 1911, at which date the available records come to an end. The tree therefore continued its normal rate of growth of upwards of three inches per annum throughout the whole course of the experiment
The tree was tapped daily on the three-V system described above, and in the space of a little more than two years, four similar areas of bark had been completely tapped. The renewed bark on the first area was then retapped, and was nearly completed at the end of December 1911. During this last period tapping was carried out daily during alternate months, with monthly intervals of rest. The yield of dry rubber was as follows:
No. of Tappings
Total Rubber produced, grammes
Average per . Tapping, grammes
I. on renewed bark not completed
In three years the total yield was nearly 240 lbs. of dry rubber. This was contained in about 70 gallons of latex, or nearly 20,000 cubic inches. The total area of the bark tapped during this period was about 10,000 square inches. The thickness of the laticiferous bark was about half an inch, so that the volume of the bark actually subjected to tapping was about 5000 cubic inches. It is by no means an easy matter to estimate the volume of latex vessels contained in a given volume of bark, owing to the marked shrinkage of the former when the bark is removed from the tree. It will be safe however to assume that the total volume of the latex vessels is not more than one-tenth of the total volume of the bark, and most writers upon the subject have given a considerably lower estimate. We have therefore obtained in three years 20,000 cubic inches of latex by tapping an area of bark which did not contain more than 500 cubic inches of latex at the beginning of the experiment. The problem before us is to account for the remaining 19,500 cubic inches of latex.
The tree is about 80 feet high, and at about ten feet from the ground it divides into three main branches each about four feet in girth. In comparison with the girth at the base of the trunk, the crown of the tree is by no means extensive. In fact the volume of the bark per foot of altitude probably falls off rather than increases as we pass upwards, since although the total circumference of the branch system increases, the thickness of the bark rapidly diminishes. We may therefore assume with some degree of confidence that the total volume of the laticiferous system of the whole tree does not exceed 5000 cubic inches, and is probably very much less. Even if this very liberal estimate be adopted, the whole system must have been emptied twice over in the space of three years. At the end of this period the vessels were still full of latex, and the freedom of flow had increased rather than diminished.