RUBBER PLANTING 65 



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 wound- 

 ing 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 com- 

 parison with the amount present at the beginning of 

 tapping. In trees growing under more favourable con- 

 ditions 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 



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