408 PLANT GROWTH SUBSTANCES 



tions with attenuated bacteria induced large galls about the attenuated 

 bacteria. Braun and Laskaris (3) and Riker (15) repoited similar results 

 with growth substances. With this as a background, Thomas and Riker 

 (23) employed some 56 chemicals on five different kinds of plants. Each 

 plant was puncture-inoculated with attenuated bacteria, then decapi- 

 tated several inches above. Each cut stem was treated with one or another 

 of the chemicals used. Records were kept of chemically induced galls, 

 the sizes of the attenuated bacterial galls, and of growth-substance 

 effects which were induced. 



Of the 56 chemicals used 19 were found to stimulate, in varying 

 degrees, the attenuated-bacterial galls. These substances represented 

 a variety of different chemical compounds. No particular compound or 

 type of compound was found exclusively to stimulate the attenuated- 

 bacterial galls. The compounds most active in this respect were the 

 growth substances a-naphthaleneacetamide, a-naphthaleneacetic acid, 

 2,4-dichlorophenoxyacetic acid, 2,4-dichlorophenoxybutyric acid, and 

 indolebutyric acid. 



Most of the effective gall-stimulating substances also induced other 

 growth responses, such as tissue proliferation, root stimulation, bud 

 suppression, epinasty, and formative effects. However, stimulation of 

 galls about attenuated bacteria was not consistently associated with the 

 production of any particular growth response. Neither did all of the 

 active gall-stimulating substances induce all of the growth responses. 

 Consequently, stimulation of galls about attenuated bacteria appeared to 

 be a distinct type of growth response that could be induced by a variety 

 of chemical compounds. This suggests that no one of the known growth 

 substances used was, by itself, the cause of crown gall. 



Bacteria reisolated from the stimulated galls were still attenuated. 

 This fact, and the wide variety of compounds capable of inducing gall 

 stimulation, indicated that the effects of the chemicals used were 

 probably on the host cells. 



In addition to the known chemicals, a water extract of virulent 

 bacterial cells, which had been dried while frozen, had a slight gall- 

 stimulating effect. Galls produced from attenuated bacteria, which de- 

 veloped shoots in the absence of terminal and lateral shoots on the host 

 plant, were also larger than normal. 



Unbalance offactors for diseased growth. — While continuing to examine 

 individual factors, one of which might act as a trigger mechanism to set 



