i io Butler . — A Study on Gummosis of Prunus and Citrus , with 
In pathognomonic tissues the intercellular substances and primary- 
membranes first changed over into gum, the secondary membranes then 
became affected, and finally the starch within the cells dissolved away. In 
certain cases the starch in apparently healthy tissues degenerated with the 
formation of gum while the cell-walls still remained normal ; when this 
occurred the gum appeared as irregular yellow drops, or in the form of the 
original granule. The most marked changes in the starch occurred, how- 
ever, in the cells bordering the gum pockets ; the granules lost their form, 
became irregular masses, and finally gum droplets, the transformation 
stages being readily followed by their fainter and fainter blue coloration 
when treated with iodine. 
Frank also believed that the plant sap, besides the cell-walls and 
starch, was an important constituent of gum. 
We gather further from Frank’s memoir that : (i) cherry gum was 
insoluble in cold and hot water, in which it did not even swell ; (2) when 
sections of affected tissues were boiled in a solution of caustic potash, the 
secondary membranes of healthy cells gave the cellulose reaction with iodine 
and sulphuric acid, while the gum-forming tissues were coloured yellow. 
Prillieux 1 in his study on gum formation in fruit trees confirmed in the 
main the conclusions arrived at by previous workers, notably Wigand, Frank, 
and Sorauer, but differed from them as regards the formation of the gum. 
As a result of his investigations, which were confined largely to the apricot, 
he concluded that: (1) the gum in the vessels was due to infiltration and 
not to a change in the cell-walls, as the internal spiral and punctiform 
thickenings remained unaffected even when the lumen was completely 
filled with gum ; (2) the gum filling the lumen of the wood-cells and bast 
fibre bundles was also due to infiltration or starch ; no change in the cell- 
walls was observed except sometimes a slight swelling in the outermost 
fibres of the bast fibre bundle ; (3) the first stages of gum formation in the 
abnormal wood parenchyma were a transient rift between the middle 
lamellae and the cell-walls, followed by the appearance of gum within the 
suture. Coincident with the appearance of gum, the middle lamellae began 
to lose their identity, and soon became indistinguishable from the accumu- 
lating gum ; (4) the starch grains did not gradually change over into gum. 
With the first appearance of gum within the cells the starch grains became 
agglomerated. As the production of gum increased the starch became 
resorbed, but no transitional stages were ever observed ; the starch grains 
at all stages reacted blue to iodine, the gum always yellow. 
Hydrochloric acid and chloriodide of zinc were found useful in studying 
the younger stages of gum formation. The former coloured the wood-cells 
of plum, apricot, and peach violet, and the gum yellow — recently formed 
1 Prillieux, Ed. : Etude sur la formation de la gomme dans les arbres fruitiers. Annales 
des Sciences Naturelles (Botanique), s£r. 6, vol. i, 1875, pp. 176-200. 
