GROWTH SUBSTANCE AND THE DEVELOPMENT OF CROWN GALLi

Advances made recently in the study of plant growth substances and their function in normal growth have raised the question of their possible role in atypical and pathological growth and specifically in the gall development induced by Phytomonas tumefaciens (Smith and Town.) Bergey et al. A suggestion of their presence was first given by the marked epinasty of leaf petioles in plants inoculated with the crown gall organism. Other responses, including the initiation of adventitious roots, stimulation of cambial activity, inhibition of bud development, and delayed abscission of senescent leaves, strengthened this suggestion. The relation of these responses to growth substances in general are discussed by Boysen-Jensen (S).^ Most of these phenomena have doubtless been frequently observed by those studying crown gall, but so far as the writers are aware only Nëmec (25) has interpreted any of them in relation to growth substances. Some of the literature providing a background for this problem is very briefly considered. Epinasty of leaf petioles in tomato, resulting from the rapid elongation of cells on the upper side of the petioles, has been induced by betaindole-acetic acid, and by numerous other compounds (7, ^, iO, 11), Initiation of adventitious roots in response to treatment with various growth substances has been frequently reported. Unsaturated hydrocarbon gases (44j 4^j 4^)y pure auxins {40, cf. 4^)j beta-indole-acetic acid {S9)j and certain other synthetic compounds {47) were demonstrated to stimulate the production of root initials. Initiation of roots in conjunction with crown gall was apparently observed early in the history of the disease {33) and was confirmed later (5, 25), However, this is not to be confused with the hairy root condition caused by Phytomonas rhizogenes R. B. W. K. S. Growth substances were found (37) to stimulate the cambium. Such stimulation was also obtained with beta-indole-acetic acid {36), Recently {1 ) cambial activity has been found to begin at the terminal buds and to progress basipetally following the movement of the growth substance. Stimulation of the cambium in the vicinity of crown gall tissue has also been noted {26, 32), Apical dominance and inhibition of axillary buds was at first attributed to a specific-retarding substance {34, 35) which was later considered to be identical with growth substance {41, 42). Inhibition of axillary buds has been obtained with beta-indole-acetic, beta-indole-