With moderate and careful tapping no limit can at present be set to the period during which a similar yield will continue to be obtained from Hevea. By the end of April 1912, the tapping of the same seventy trees had been continued without intermission for nearly four years. The annual yields obtained are summarised in the following table:
1908 (7 months)
1912 (4 months)
Average annual yield for 4 years
1 For 4 months.
The result is here calculated in the form of the weight of dry rubber which would be obtained from an acre planted at the same distance as the actual trees under experiment. After continuous tapping for nearly four years the trees, especially those tapped at the longer intervals, were yielding rubber in considerably larger quantities than the average amount for the first year. It must be pointed out that the trees in question were upwards of 25 years old, and had not been regularly tapped before the experiment began. They are also very closely planted, namely at a distance of only 12 x 12 feet.
Here we appear to have evidence of a very real and prolonged response on the part of the trees to certain stimulating causes. Among these, two definite stimuli may probably be distinguished, firstly the removal of the latex, and secondly the irritation due to the wounding of the bark. In the case of young and vigorously growing trees a further reason for the increase in yield is apparent, namely the rapid increase in the total volume of the bark. In the case of the trees used in the foregoing experiment, the average increase in girth was less than one inch per annum, so that the increase in volume of the bark was very small in comparison with the amount present at the beginning of tapping. In trees growing under more favourable conditions the increase is often three or four times as great. In the case of young trees planted at wide intervals no limit can at present be set to the period during which a gradual increase in yield may be shown, so long as the tapping is not severe enough to cause definite injury.