EXPEBIMENTS ON UUM rUODUCTION IN KOKDOFAN 81 



the branches. Only a piece of bark about eight inches long by an incli and a lialf wide was 

 removed. These trees, however, did not exude any more gum than a simihir set tapped in 

 the ordinary way at the same time. 



Pkoduction of Gum by Bacteria 



As mentioned in the Third liepurt of the Wellcome Tropical Research Lahonilorief, 

 Khartoum, the question of the bacterial origin of gums was first investigated by Greig Smith 

 in Australia. I described briefly in that Eeport a few experiments carried out in Khartoum 

 with a view to confirming Greig Smith's conclusions in the case of Kashdh gum. Being 

 unaljle, in the time at my disposal in Khartoum, however, to reach conclusive results, 

 I took tlie opportunity of pursuing the matter further at Taiara. The procedure was the 

 same as already described, twigs from which gum was exuding being passed through 

 the flame in order to kill any bacteria accidentally adhering to the outside of the bark, 

 and from these cultures were made in tubes of glucose gelatine. After about 24 hours' Bacteriological 



,.. nr-^i 1-1 T •• • technique, and 



cultivation at 3^ G. the growths were plated on media containing cane-sugar, asparagin, discovery of 

 potassium citrate and agar. No potato extract was used in these media, as was done in '^^ probable 



... T . , . -,. .. bacterial cause 



the earlier expermients. A large number of twigs was treated in this manner at dmerent 



times of the season, and in every case the predominant bacterium showed the same habit 



of growth and the bipolar staining of the bacterium which I had already isolated in my 



experiments in Khartoum. In a number of cases, particularly where the gum had been 



exuding from a small puncture in the bark only, the original plate culture showed this 



bacterium with no contamination, while in the other cases, by sub-cultivation on the same 



medium, a pure culture was readily obtained. From these and my previous experiments, it 



seems justifiable to assume that this bacterium is always present in gum-bearing branches 



of hashub trees. I also succeeded in isolating it free from contamination from branches 



showing no wound, and from which no gum was exuding. It seems improbable, however, 



that any trees in a gum-producing district are absolutely free from gum, as it was 



noticed especially towards the latter half of the season that nearly all the trees, even those 



which had never been tapped, were exuding a small amount of gum. This was very often 



in the form of thin rods or spirals of gum, and an examination of the sijot showed the 



presence of a slight wound, caused in many cases by the accidental breaking of a small 



twig, or again, by the borings of small beetles or other insects. With respect to this latter 



2Joint it may again be stated that Zimmermann connects the formation of gum in the case of 



Acacia decurrens with the borings of Ambrosia beetles, mentioning also that in all cases 



of gum production the essential point is, in the first place, some injury to the tree. 



On tapping the trees in the early part of the season I noticed that within a few liours presence of 



a large number of ants were usually to be seen swarming over the wounded surface of the ;""* «he'e 



trees have been 

 branches, attracted, no doubt, by the sweetness of the sap. Dr. Beam also called my tapped, and 



attention to the fact that he had observed the presence of large numbers of ants after 'heir possible 



• (■ ■ • -1 11- """'^ "* 



tapping some trees in connection with a series of experiments earned out by huii up the "carriers" 



Blue Nile. Besides the ants, flies of several kinds were observed on the wounded surfaces 



soon after the trees had been tapped. 



These facts at once suggested a means by which this particular bacterium might 



readily be transmitted from one tree which had been tapped to another, or, indeed, a wound 



of any kind might serve as a jjoint from which ants or other insects could pick up infection, 



or on the other hand bacteria conveyed by insects to a wounded surface could readily 



obtain a hold in the tissues of the tree at that point. It was important, therefore, to 



