The purity of the bacterial strains employed was insured, as far as possible with available technic, by employing only cultures grown from individual bacterial cells. The method employed in isolating and culturing these single bacterial cells has been described by Wright, Hendrickson and Riker (1930). The parent cultures employed in these studies were isolated from various hosts secured from widely separated regions. The crown-gall cultures used in this study were secured as follows: A black-raspberry gall grown in Iowa in 1924 was the source of culture T5 from which seven single-cell cultures were secured. An almond gall grown in California in 1913 was the source of culture T2 from which four single-cell cultures were isolated. A walnut gall grown in California in 1926 was the source of T37 from which three single-cell cultures were obtained. Culture T3 was secured in 1916 from incense cedar and was the source of four single-cell cultures used. The writers secured some of these cultures through the courtesy of C. 0. Smith and J. H. Muncie. The hairy-root cultures were all obtained from apple. Hairyroot developments in Arkansas and Michigan in 1926 were the sources of cultures T32 and T34, respectively, from which five and four single-cell cultures were isolated. Hairy-root overgrowths secured in Wisconsin in 1925, 1926, and 1926 were the sources, respectively, of cultures T38, T48, and T49 from which three, one, and five single-cell cultures were isolated. From radiobacter cultures R3, R11, and R16, secured from the United States Department of Agriculture, two, three, and two single-cell cultures respectively, were obtained. A summary of the cultures employed shows that 43 single-cell strains were obtained from 4 crown-gall, 5 hairy-root, and 3 radiobacter cultures. These single-cell cultures with their parent cultures, were employed in all the trials unless otherwise noted. Earlier trials of the pathogenicity of the parent cultures have been recorded by Riker et al. (1930). Physiological tests were employed with these cultures which showed in part their identity. Among these tests were growth

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The investigations in this paper were conducted as a part of a co6perative project between the Bureau of Plant Industry, United States Department of Agriculture, and the University of Wisconsin. This work received some support through a grant from the special research fund of the University of Wisconsin. The figures were prepared by Eugene Herrling.

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