80 



EEPOKT OF CHEMICAL LABORATOKY 



Increase of sap 

 as response to 

 irritation 



Mercuric 

 Chloride 

 proved to 

 inhibit gum- 

 production 



P'ormalin 

 leads to an 

 increase 



The principal results of these experiments are shown in the table on the previous page, 

 the dates being those on which the different collections of gum were made, while the figures 

 represent the total yield of gum (in grammes) from the set of six trees in each case. 



The control trees were not treated in any way either before or after tapping. The formalin 

 solution was made by diluting one volume of "Formalin" (40 per cent.) with three volumes 

 of water. The hydrochloric acid contained one volume of the strong acid to ten of water. 

 The sugar solution was used as it was thought that the bacteria might increase in number 

 much faster if the wounded branch was treated with a suitable medium for their growth. 

 The first effect noticed was that exudation of gum began much more quickly from the trees 

 which had been treated after tapping. This is probably due to the fact that these solutions 

 acted as irritants, causing an increased flow of sap to the wounded part. The gum already 

 present in the sap was there exuded within the first week after the trees were tapped, and 

 then the production of gum went on more slowly. The treatment of the bark before 

 tapping cannot be held to have had much effect on the yield of gum. This was probably 

 due to the fact that even strong antiseptics such as formalin and mercuric chloride failed to 

 kill the bacteria in the inner layers of the bark, and they also had little action on the 

 tissues of the tree when merely rubbed on the outside of the bark. The most interesting 

 results are seen in the after-effects of the treatment. Contrary to expectation, the yield of 

 gum was not increased by rubbing the wound with a solution of sugar. There were, most 

 probably, sufficient bacteria in these trees at the beginning of the experiment, or the sap 

 itself was quite suitable as a medium without the addition of any extra constituents. 

 Probably, also, the mere question of sufficient infection has not alone to be considered, as 

 the sap ceases to flow in some trees much sooner than in others, speaking at least from the 

 much longer time which some trees keep their leaves. 



It will be noticed that treatment witli mercuric chloride almost entirely prevented the 

 further production of gum. This is specially interesting in view of the bacterial origin of 

 gum. When the solution had dried up, the tissues would still remain impregnated with the 

 antiseptic, in consequence of which any bacteria brought to, that part would immediately be 

 killed, and would therefore be unable to produce any gum in spite of the lessened resistance 

 of the tissues to bacterial action through wounding Later in the season I tapped a number 

 of trees on two branches, treating one branch only with mercuric chloride. As a result the 

 branches thus treated exuded no more gum, while the control branches kept on doing 

 so. The trees treated with formalin after tapping continued to give a much larger yield of 

 gum than the control trees. In this case, while the formalin would at first kill any bacteria 

 present, this solution would soon entirely evaporate, and fresh bacteria would be brought to 

 the spot in the sap. The increased flow of sap to an injured branch probably continues 

 until the injury is repaired, which would account for the large yield of gum after formalin 

 treatment. This would account for the increase after treatment with hydrochloric acid. 

 The other solutions used would probably have less effect in injuring the branches, and 

 therefore the flow of sap to these parts would not be markedly increased, thus to some 

 extent explaining the fact that the exudation of gum was not greatly affected in those cases. 



It must be remembered that the explanations given above of the action of formalin, etc., 

 are largely tentative, owing to the difficulty of being certain that all the trees under 

 observation were exactly comparable at first, and that the flow of sap would have been the 

 same in each set of trees under ordinary conditions. 



Dr. Beam has dealt fully with the question of the yield of gum when trees are tapped 

 in various ways. At his suggestion I tapped several by making a series of cuts with an axe, 

 these cuts being arranged spirally around the tree from the ground upwards, and also along 



