Plant Sanitation. 



36 



[July, 1909. 



The above account is based on the 

 supposition that the original black 

 patches remain distinct. If, on the 

 other hand, each patch spreads to any 

 extent, or if the pricker-cuts into the 

 wood are close together, then the 

 adjacent patches will coalesce, and the 

 subsequent ingrowth of cambium can 

 only arise from the cambium which 

 remains healthy. In these cases the 

 patch of bark which dies off and is cut 

 out will be much larger, corresponding 

 in size, as before, to the underlying black 

 patch, and in extreme cases the tree may 

 be ringed. 



These facts bring vis in conflict with 

 another of those plausible phrases which 

 have been promulgated and accepted 

 without due consideration, viz., "inci- 

 sion, not excision." In the case where the 

 planter incises and injures the cambium, 

 the tree immediately begins to excise 

 very much more bark than was expected. 

 Even when the cambium is not injured, 

 the tree excises a small cylinder of bark 

 immediately round the incision made by 

 the pricker ; these may fall out, and 

 leave the bark pitted, or they may re- 

 main in situ and constitute so many 

 obstacles to the free flow of the latex. 

 It has been previously pointed out 

 that if the planter "incises," the tree 

 "excises "the tissues round the wound. 

 In the old system of paring and pricking 

 practically all the pricked bark was 

 subsequently scaled off. Further, it is 

 known, that the renewal of bark after 

 paring (i.e., uniform excision) is satis- 

 factory as far as regards the number 

 and arrangement of its latex tubes, while 

 on the other hand we have been warned 

 that this is not the case after pricking 

 (i.e., incision and consequent local 

 .excision). The theory of " incision, not 

 "excision," however admirable it may 

 appear, fails therefore in practice. 



Through the courtesy of the Editor of 

 the Times of Ceylon, I have received the 

 two samples of bark referred to in that 

 paper, on May 17th last. One of these was 

 pricked six months previously and the 

 other apparently more recently. Both 

 show that even if the pricker does not 

 actually penetrate to the cambium, its 

 effect is felt there in that the cambium 

 subsequently produces abnormal tissue, 

 and the sample with six months ' renewed 

 bark completely confirms the result 

 arrived at by Dr. Fitting, i.e., that the 

 renewed bark beneath the pricker-cut 

 consists of stone cells without latex 

 tubes. Now that the specimens are 

 preserved in alcohol, this difference in 

 structure is clearly indicated to the 

 naked eye. For the tissue built up of 



stone cells is denser than the normal cor- 

 tex, and therefore does not contract to 

 such an extent as the latter when placed 

 in alcohol. The tissue underneath each 

 piicker-cut forms, therefore, a small 

 elevation on the inner (cambial) side of 

 the preserved cortex. 



Another phenomenon which has caused 

 some alarm is the occurrence of pads of 

 coagulated latex between the wood and 

 the bark surrounding the pricker-cuts. 

 In some cases this will be only an exten- 

 sion of the effect noted above, i.e., the 

 occurrence of black spots in the same 

 situation. The explanations which have 

 been furnished, however, are widely at 

 variance with the known structure and 

 reactions of the Hevea tree. It may be 

 laid down as a general rule, after four 

 years' investigation of the subject, that 

 when the bark decays owing to the 

 attacks of fungi or from other causes, 

 latex does not exude from the decayed 

 tissue. The latex in the vessels is 

 coagulated by the products of decay, or 

 dries up because of the interruption of 

 the water supply. Latex can only exude 

 if the tissues are suddenly wounded, 

 For example, the whole of the bark of a 

 Hevea may be killed by Corticium 

 javanicum without the appearance of a 

 single globule of rubber on or under 

 the bark. But if any expansion or 

 contraction occurs, either in the diseased 

 tissue or the surrounding healthy bark, 

 then the diseased tissue separates 

 from the wood, and, as it does so, 

 the fracture may extend along the 

 cambium for a short distance into 

 the surrounding healthy tissue, and 

 consequently cause the flow of latex 

 from the latter into the gap between the 

 wood and the diseased bark. It has 

 been claimed that a periodic contraction 

 and expansion occurs daily in a Hevea 

 stem, though the evidence in favour of 

 the contention is scarcely satisfactory ; 

 but there is undoubtedly a difference in 

 the tension of the bark cells in the 

 morning and afternoon of a dry day, 

 owing to the removal of water from the 

 latex. This is shown by the greater 

 pei^centage of rubber in the latex of 

 afternoon tappings than of morning 

 tappings. These differences in tension 

 may be quite sufficient to produce the 

 slight splitting required, though there 

 may be other causes, e.g., direct 

 sunlight ; and given the existence of black 

 spots where the organic connection of 

 bark and wood is destroyed, we have 

 all the conditions necessary for the 

 accumulation of pads of rubber under- 

 neath the bark. The point ignored in 

 the explanations offered in the local 

 press is that there must be a split 



