Experimental Induction and Inhibition 

 of Overgrowths in Plants 



R. S. DE ROPP 



OVERGROWTHS Can be induced on plants by a variety of agents, 

 including chemical substances, genetic factors, viruses, bacteria, 

 fungi, nematodes, and insects. In this paper four different kinds of over- 

 growths induced by a chemical substance, a genetic factor, a virus, and 

 a bacterium will be described. An account will also be given of the 

 experimental inhibition of the growth of one of these tumors. 



In 1936 and 1937 a series of papers (27,4,21) was published describing 

 the effect on certain plant organs of indole-3-acetic acid (lAA). This 

 substance was applied to plant organs in lanolin, the concentration of 

 lAA varying from 1.5 to 3 per cent. The stems of both bean and tomato 

 plants and the pods of the bean responded to this treatment with the 

 production of overgrowths some of which had a diameter of as much as 

 2 cm. The overgrowths were made up in part of parenchymatous tissue 

 and in part of the primordia of adventitious roots. Any parenchymatous 

 tissue of the bean stem, according to Hamner and Kraus, could be 

 rendered meristematic by this treatment. 



These studies on the action of lAA on intact plants were supplemented 

 by the investigations of Gautheret (18) on the response to this substance 

 of plant tissue cultured in vitro. Gautheret showed that the response 

 of a plant tissue to lAA depended largely on the concentration of this 

 substance in the medium. For carrot tissue he defined its action as 

 cambiogenic at a concentration of o.i mg. per liter and as rhizogenic 

 at a concentration of i mg. per hter. At a concentration of between 10 

 and 100 mg. per Hter he found that the substance ceases to be rhizogenic 

 but induces instead a disorganized type of growth composed of hyper- 

 trophied cells. 



