392 PLANT GROWTH SUBSTANCES 



various ones of these effects and may either stimulate or inhibit normal 

 or abnormal cell proliferation (see reviews in 10,28,26,23). A wide 

 variety of natural and synthetic growth-regulating compounds have 

 been tested on many species of whole or decapitated plants (27,34,35). 



The influence of a limited number of growth-regulating substances 

 on growth in vitro of cultures derived from normal tissue has been ex- 

 amined (18,5,6). Gautheret (5), found that indole-3-acetic acid was 

 important for unlimited growth of tissue cultures from a number of 

 species and was indispensable in some cases. Indolebutyric acid and 

 a-naphthaleneacetic acid could be substituted for indole-3-acetic acid 

 when the latter compound was essential for tissue cultures. Gautheret 

 (6,7) reviewed the growth-regulating substance requirements of his 

 tissue cultures as follows: i) cultures obtained from normal carrot and 

 endive tissue grew for limited periods in vitro without added indole-3- 

 acetic acid but growth was strikingly improved by that substance; 

 2) cultures from normal tissues of Jerusalem artichoke, black salsify, 

 and turnip did not grow without indole-3-acetic acid; 3) cultures of 

 normal tissue of black salsify having once been cultivated on media 

 with added growth-regulating substance grew in the absence of the 

 material (The compound was required to start the culture but thereafter 

 was no longer required even as an external source. Gautheret has referred 

 to such cultures as habituated); 4) cultures of normal carrot tissue 

 incubated initially on media with indole-3-acetic acid after several 

 months developed faster in the absence of the material than did normal 

 tissue without added growth-regulating substance; 5) Jerusalem arti- 

 choke tissue of crown-gall origin developed without added indoIe-3- 

 acetic acid. Such tissues were similar to those which at first required the 

 material but later grew in its absence. Thus, considerable information is 

 available about the influence of added growth-regulating substances on 

 growth and development of normal tissue. This is perhaps because 

 generally such cultures required supplements of this kind to grow for 

 unlimited periods, or at least they required an external source when 

 originally isolated. This necessitated working out the proper concentra- 

 tion requirements. 



The concentration of the growth-regulating substance is a critical 

 item in modifying the shape and kind of cells and in determining the 

 initiation or inhibition of root formation in the cultures of normal tissues 

 from a number of species. For example, Gautheret (6) noted that tissue 



