118 GROWTH HORMONES IN PLANTS 



each twig was connected with the lower end of a perpendicularly 

 suspended funnel 20 cm. long. When this was filled with growth- 

 substance solution, the fluid flowed slowly through the twig 

 and exuded from the apical-cut surface. These experiments were 

 carried out with different concentrations of growth substance 

 during the winter when the buds were still in a resting condition. 

 In no case was it possible to observe any ''forcing" effect as a 

 result of the treatment. 



Various investigators have studied the retarding effect which 

 the axis of the shoot has upon the development of the axillary 

 buds. According to Snow (1925a, 1929a, h, 1931a, h, 1932a), this 

 is caused by a specific retarding substance. Thimann and Skoog 

 (1933) and Skoog and Thimann (1934) have reported that this 

 substance may be identical with growth substance. When the 

 terminal buds were removed from seedlings of Vicia Faba, the 

 lateral buds developed rapidly. This activity could be retarded 

 by placing agar blocks with relatively large amounts of auxin 

 upon the cut surfaces. These investigators concluded, therefore, 

 that the growth substance formed by the terminal bud in normal, 

 nondecapitated plants is the retarding factor in the development 

 of lateral buds. When these start to develop, growth substance 

 is formed, and this has its effect upon their further development. 

 If the hypothesis of Thimann and Skoog is correct, it follows that 

 the growth substance formed by buds can both inhibit and pro- 

 mote their development. 



Uhrova (1934) found that a substance diffusible into agar or 

 gelatin was present in the leaves of Bryophyllum crenatum and 

 inhibited lateral bud development. Hormone from Avena cole- 

 optiles, diastase, and saliva, as well as acids had the same effect. 



Czaja (1934) and Soding (1935a) have demonstrated growth 

 substance in the developing buds of many woody plants, and 

 Avery (1935) has found it in the young growing leaves of Nicoti- 

 ana. The relationship between the processes involved in bud 

 development and the role of growth substance has not been 

 discovered. Further studies by Hitchcock (19356) have shown 

 that lateral bud inhibition can be brought about by the applica- 

 tion of indole-acetic and indole-propionic acids or ethylene and 

 propylene gases to decapitated tobacco plants. Bauguess 

 (1935) has found inhibition effects with /3-3-indole pyruvic, 

 /3-3-indole-oximinopropionic, and iS-3-indole acryHc acids. 



