The Immunity of Apple Stocks from Attacks of Woolly Aphis (Eriosoma lanigerum, Hausmann). Part II. The Causes of the Relative Resistance of the Stocks

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Abstract
1. A short account of gall formation is given.2. The stylets of Eriosoma lanigerum penetrate to within three or four cells of the cambium. Penetration is mainly intracellular, but the stylets cannot pass through the masses of sclerenchyma.3. When a mass of sclerenchyma is encountered the stylets bend round it; if they cannot bend round it, they are usually withdrawn.4. So far as can be seen at present, immunity cannot be explained by mechanical considerations.5. There is strong evidence for supposing that resistance above ground is directly proportional to the percentage of sclerenchyma round the circumference of the stem.6. The above statement must be qualified, if necessary, by a consideration of the extent to which the masses of sclerenchyma are broken up, i.e. the number of spaces that are available for the penetration of the stylets.7. The middle lamella can be dissolved by the saliva of the Aphid; so that the check formed by a mass of sclerenchyma can eventually be overcome.8. Variations in the character of the sclerenchyma and its position are shown to be of possible importance. There would also seem to be some connexion between the size of the Aphid and the size of the openings between the masses of sclerenchyma.9. The relations of the stylets and stylet track to the cells of the stock are briefly summarised.