206 GROWTH OF PLANTS 



stance drained out of the block into one side of the coleoptile, retarding 

 growth and causing curvature. 



Seubert ^^ in 1925 extended the experiments of Stark by infiltrating agar 

 with substances of both plant and animal sources — diastase, malt extract, 

 saliva, etc. — demonstrating the existence of both accelerating (causing 

 curvatures) and inhibiting substances. 



The field was further advanced from 1920 to 1934 by Purdy,^^ Soding,^^ 

 Cholodny,^' •'' ^ Went,^^' ^o- ■*! Dolk,^'' Zimmerman, Crocker, and Hitch- 

 cock,^^* ^^ Boysen- Jensen,^ Laibach et al.,^^ Thimann and Went,^* and 

 Kogl et al.^'^ 



Hitchcock ^^ in 1935, experimenting with indole acids, demonstrated that 

 /3-indoleacetic acid and j8-indolepropionic acid induced curvatures, pro- 

 liferations, and adventitious roots when applied to intact plants. Hitchcock 

 also found phenylacrylic (cinnamic) and /3- (phenyl) -propionic acids to be 

 physiologically active when applied to intact plants. 



Zimmerman and Wilcoxon ^* in 1935 brought to light seven new hormone- 

 like substances, giving support to the assumption that there are many 

 physiologically active compounds, both natural and synthetic. Table 23 

 gives a list of hormone-like substances kno\vn in 1935. Two acids in this 

 list, j3-indolebutyric and a-naphthaleneacetic, were pointed out as the most 

 effective root-inducing substances. Many practical applications have been 

 made with these and they still rank among the most important. 



In 1939 Zimmerman, Hitchcock, and Wilcoxon ^^' ^^ listed a total of 

 54 different growth substances which showed activity when applied to 

 plants in the vapor form. The greatest possibility for locating large num- 

 bers of active acids came when Zimmerman and Hitchcock ^^ in 1942 

 showed that substituted phenoxy and benzoic acids were active and that 

 this activity varied wth kind, number, and location of the substituent 

 groups or atoms. ^*' ^^- ^^' ^^ This is illustrated in Table 24. Table 25 shows 

 a long list of active substituted phenoxy acids. 



Of the halogen substituents, chlorine and bromine caused approximately 

 the same degree of activation. Iodine substituents in phenoxy acids did 

 not activate to the same degree as bromine and chlorine. However, iodine 

 substituted in the ring of benzoic acids made very active molecules. ^^' *^' ^^ 

 Chlorine substituted in the ortho position to make 2-chloro-3,5-diiodo- 

 benzoic acid furnished a molecule that acted very much like 2,3,5-triiodo- 

 benzoic acid. 



Methods and applications. Physiological activity in substances or ex- 

 tracts was first detected by use of the Avena coleoptile as a test object in 

 a dark room. The method is somewhat complicated and requires a consider- 

 able amount of equipment. Since it has been described elsewhere ^^ it need 

 not be repeated. 



A simple method perfected in the Boyce Thompson Institute laboratories 

 requires only a growing plant in light or dark. The young tomato plant, 

 which has long been a standard test object for detecting the presence of 



