Occurrence and Role of Growth Hormones 



73 



tomato (Kramer and Went, 1949). Small, young apices yielded little 

 diffusible auxin and older apices yielded proportionally more, as 

 shown in figure 33. The diffusible auxin more than doubled with 

 each ten day period in the development of the plant. 



Light is an important factor in auxin formation, although not 

 a simple one. There is some confusion concerning the necessity of 

 light for auxin formation, but in general it would appear that green 

 plants do require light for this function, whereas excised plant parts 

 grown on an organic medium or seedlings with a stored food supply 

 can produce auxin in the absence of light. 



LiJ 



3 



> 



O 



LlI 



70 HRS. 



TIME IN LIGHT 



Fig. 35. The reappearance of auxin in tobacco apices when the plants were placed 

 in light (8000 ergs/cmVsec.) (Avery et al, 1937). 



An excellent quantitative study of the relationship of light to 

 auxin fonnation was carried out by Avery et al (1937) using tobacco 

 plants. They observed that the auxin diffusible from the apices of 

 tobacco plants diminished rapidly when the green plants were held in 

 darkness (figure 34). Using this information, they then developed a 

 technique for following the resurgence of auxin formation after starva- 

 tion in the dark. Exposure of the plants to ten hours of light was 

 sufficient to allow the reappearance of auxin, and a quantitative in- 

 crease with greater duration of light was observed (figure 35). This 



