OCCURRENCE AND FORMATION OF GROWTH SUBSTANCES 65 



oxygen in the typical oxidative deamination of the tryptophane. 

 Other amino acids which yield significant amounts of the active 

 growth substance in mold cultures can also be converted readily 

 into 3-indole acetic acid. 



Higher Plants. — Very little is known about the conditions 

 required for the formation of growth substances in higher plants. 

 In many plants the process is not influenced by unilateral light or 

 gravity (Chaps. VIII and IX). According to van Overbeek 

 (1933), light has an effect on its formation in the cotyledons of 

 Raphanus. The very young cotyledons of etiolated and of 

 illuminated plants contain the same amounts of growth sub- 

 stance, probably derived from storage in the seed; however, 

 seedling plants grown in darkness lose their ability to form growth 

 substance rather soon, while normal plants placed in the light 

 retain this ability for a long time. Navez (1933c) found that 

 illuminated Lwpinus albus seedlings contain twice as much growth 

 substance in the apical regions as darkened plants. It seems 

 probable from the observations of Cholodny (1935a, h), Laibach 

 and Meyer (1935), and Kogl, Haagen Smit, and Erxleben (1933, 

 Mitt. VII), that the growth substance stored in seeds is liberated 

 in an active form following hydrolysis and the activity of 

 enzymes. Pohl (1935) has concluded that the growth hormone in 

 Avena is moved from the endosperm up to the coleoptile tip 

 where it is activated before being moved downward. 



Avery (1935) found that a growth hormone was produced in the 

 young growing leaves of tobacco in the light ; it disappeared slowly 

 when plants were placed in the dark. Chesley (1935) observed 

 that wheat seedlings sprouted in the light were less sensitive to 

 X radiation (which, according to Skoog, 1935, destroys auxin) 

 than those germinated in darkness. This observation may be 

 explained on the supposition that a greater concentration of 

 auxin was present in the illuminated plants ; hence greater doses of 

 X rays were required to inhibit growth. Went (19356) has 

 stated that root-forming substances are produced in leaves 

 growing in the light, the red and orange wave lengths being 

 especially effective. Kiistner (1931) observed that the activity 

 of growth substance obtained from urine was increased by red 

 Ught and decreased by shorter wave lengths. Went (1928a) 

 found a decrease of 18 per cent in the amount of growth substance 

 given off by the Avena coleoptile when illuminated with 1,000 



