3 1 6 Whitby . — Variation in Hevea brasiliensis . 
half the circumference, reopened every morning) and had had the same 
tapping history. They had been tapped for two successive years on the 
two halves of the ‘basal’ section of the trunk, i.e. the portion extending 
vertically from, say, 20 in. to 4 in. from the ground. In the succeeding year 
of tapping, when the observations were made, they were tapped on the 
section of the trunk immediately above the ‘basal ’ section, i.e. the section 
from, say, 36 in. to 20 in. from the ground. On all trees the tapping cut 
had advanced roughly the same distance down the section, viz. 6-8 in., at 
the beginning of the survey. 
Thus the height of the tapping cut on the trunk did not enter, as 
a disturbing factor, into the observations. 
(b) The trees were tapped by an experienced gang of tappers 1 by 
means of the simple gouge. The depth of the tapping on each tree was 
tested by a probe on each occasion that latex was collected from it. In 
the case of trees where the tapping was clearly not deep enough, the latex 
was not collected, and an instruction was issued to the tapper to go deeper 
in the future. The percentage which had thus to be dealt with was not 
very large. Finally, the records showed a small number of trees on which 
the tappers had not succeeded in going sufficiently deep, and, as already 
mentioned, these were excluded from the population surveyed. 
It may be considered that the depth of tapping on the trees surveyed 
represented the greatest accuracy in this direction which is practically 
attainable. 2 
(c) In order to avoid as far as possible the disturbing effect on the 
comparison of variation in weather conditions, not only was the figure for 
the yield from each tree based on several measurements, but the times at 
which the measurements for each tree were taken were distributed at 
different points over the period of the observations. The period of the 
year at which the trees lose their leaves was avoided for the observations, 
as trees are very unequally affected by this ‘ wintering ’. The observations 
were made over what may be regarded as four typical months in the 
1 It may fairly be mentioned that the native tappers show a degree of skill with the tapping 
gouge and an accuracy in excising thin strips of bark greater than any to which the author, 
personally, could lay claim. 
2 An attempt was made to classify the trees according to the degree of exactness with which 
the fullest possible depth of tapping consistent with avoidance of injury to the cambium had been 
attained, by estimating the extent to which further latex, if any, issued forth on probing at the 
tapping level ; but it did not prove to offer any particular advantage in regard to the main object of 
the investigation. It appeared that, excluding the cases already mentioned in which the tapping 
was palpably not deep enough, the trees, on the one . hand, which would, on the ground that 
deepening of the incision entirely failed to give more latex, be classed as being tapped fully deep 
enough were in general those giving small yields, and the trees, on the other hand, which would, on 
the ground that deepening of the incision gave somewhat more latex, be classed as not being tapped 
quite deep enough were in general those giving large yields. Thus, it may be remarked, a still 
more exact adjustment of the-tapping depth than that actually attained Avould merely have had the 
general effect of increasing the contributions from the large yielders, and hence of making still more 
clearly marked the characteristic features of the variation noted later. 
