If a tree of Castilloa or Funtumia is tapped, and the wounds are reopened after an interval of a few days, or if the bark is again tapped after a short interval in the neighbourhood of the original cuts, little or no latex is obtained at the second tapping. The bark is milked almost dry at a single operation, and the latex tubes are not completely refilled for several months1. In the case of Hevea and Manihot, on the other hand, a good yield is again obtained after an interval of only a single day. These facts have long been known to the collectors of wild rubber. The Hevea trees in the forests of Brazil are tapped repeatedly during a single season, method was given up about the middle of 1911, owing to the accumulation of evidence that the use of the pricker led to damage of the trees. The result of subsequent tapping in which the paring knife only was used, confirmed this impression and led to a general increase of yield.
The use of the pricker may be regarded as having accentuated the damage done to the bark of the trees tapped at shorter intervals.
1 If indeed they ever recover. It is equally likely that much of the latex obtained at later tappings is derived from new latex tubes budded off from the old ones.
whereas in Central America it has frequently been the custom to cut down the Castilloa trees in order that the whole of the available rubber may be obtained at one time.
With the establishment of plantations, opportunities for more precise observations soon arose. Among the earliest experiments dealing with "wound response" were those of Willis and Parkin, who first made use of this expression. These experiments were carried out in Ceylon and may here be recorded on account of their historical interest.
In 1897 Willis tapped a number of Hevea trees of about two feet mean girth at intervals of a week with the following result
More elaborate experiments were carried out by Parkin at Peradeniya in 1899, and the result of one of these is recorded in the following table.
It will be seen that after 14 tappings made at intervals varying from three to seven days (average interval 5 '6 days), the yield had increased by over 600 per cent. The increased yield thus recorded was ascribed to the effect of wound response.
There seems to be little doubt that the increase recorded by Parkin was an exceptional one, and that rather too much weight has been laid on the phenomena observed. It is quite possible that part of the increase was due to climatic conditions, and this view is supported by the -rainfall returns during the period of the experiment, which have fortunately been preserved. They are as follows: