The work of Smith on Bacterium tumefaciens was followed by a period of generalization as to the prevalence of the crown gall disease. Recent investigations have shown that a high percentage of the knots occurring on root-grafted apple trees are due to excess callus formation at the union of stock and scion. Apple stem tumors, and certain types of aerial galls on apple trees are thought to be an inherent varietal response. Certain types of hairy root on apple seedlings, previously supposed to be a manifestation of crown gall infection have been found to develop in the absence of Bacterium tumefariens. It has also been found in a majority of cases of galled trees, that there is a lack of continuity between the stock and scion. Such overgrowths produced by excess callus at the union of root-grafted trees have been shown to reduce materially the upward flow of water through the union.