The further remarks in this and the following chapter refer specially to Hevea brasiliensis unless otherwise stated, for this is the only species of which the physiology has been at all adequately studied. Even in the case of Hevea further observations are much needed, especially from the chemical side.
The evil effects of ringing the bark, i.e. severing it by a cut which penetrates, to the cambium and extends right round the trunk, are primarily due to the interruption of the downward food current Their food supply being cut off, the starvation and ultimate death of the roots is only a matter of time, if the ringing is complete and permanent Under these circumstances the tree may linger on for several years, but all its vital functions will be greatly impeded. Such ringing may occur more or less completely as the result of injudicious or unskilful tapping. Any cut or prick which reaches the cambium cannot fail to sever a certain number of the channels through which the sap passes down to the roots; and it is probable that the weakening effect of excessive tapping is often due as much to the starvation of the roots as to the removal of the latex. Any system of tapping which involves the cutting or pricking of the whole circumference of the tree at one time is bad from this point of view. In fact it must be considered advisable never to tap more than one-third, or at the most one-half, of the total circumference of the tree at any one time.
Special stress has been laid by Fitting on the damage caused by tapping too wide an area, and the views of this observer undoubtedly carry great weight; although some authorities consider that he has taken an unduly serious view of the effects of such a process. Microscopical examination shows that the functional conducting tubes of the bark are mostly situated very close to the cambium, inside the main layers of latex vessels. Consequently the process of paring, if carried out with sufficient skill, need scarcely affect the conducting tubes. This may account for the fact that on some estates the spiral system of tapping has been continued for years without any perceptible injury to the trees. Nevertheless, human skill is never perfect, and any false cut, even if it just misses the cambium, may sever some of the conducting tubes. This danger is clearly increased when a large part of the circumference is tapped at one and the same time. For this reason we do not recommend the spiral system of tapping to those commencing work on new estates.
Fitting has also studied the effect of tapping in depleting the food supplies stored up in the bark. He found a marked reduction in the amount of starch present in the cells around and especially immediately below the wound, after paring had been carried out for some time. This fact suggests that food materials are rapidly used up in the neighbourhood of the wounded area It may therefore be concluded that the removal of large quantities of latex will tend to produce partial starvation of other parts of the tree, an effect quite distinct from any mechanical injury.