Histological Responses to 

 Growth-Regulating Substances 



J. M. BEAL 



BOTH gross and histological responses of plants to different growth- 

 regulating substances show a rather wide variety of patterns. Not 

 only may plants of different species, genera, and families behave dif- 

 ferently, but the age and succulence of individual plants within a 

 variety at the time of treatment and the environmental conditions 

 under which they are grown play important roles in the responses which 

 they make. It is the purpose of the present paper to review the various 

 types of responses to a number of these substances that have been re- 

 ported in recent years. 



Early attempts to analyze the responses of plant tissues to specific 

 chemical substances were made by Erwin F. Smith as far back as 19 17 

 (34). He performed certain experiments in an effort to determine the 

 mechanism of tumor formation resulting from infection with Phytomonas 

 tumefaciens, using simple inorganic compounds, mainly ammonium salts 

 dissolved in water. These were injected into the hollow pith of Ricinus 

 and similar stems where they induced cellular proliferations of a callus- 

 like character, with the differentiation of some vascular elements. 



Interest in plant hormones developed in the late 1920's and informa- 

 tion on the gross morphological responses of plants to such substances 

 accumulated rapidly. Boysen Jensen, Avery, and Burkholder (8) have 

 presented the observations recorded up to 1936. Zimmerman and 

 Wilcoxon (40), Q)oper (12), and Brown and Gardner (9) had reported 

 the production of tumors following the application of indoleacetic acid 

 to plants. Laibach and Fischnich (25) reported the effects of an indole- 

 acetic acid-lanoHn mixture on the stems of Coleus, Viciafaba, and tomato. 



