30 Fundamentals of Auxin Action 



in complete darkness become much more etiolated and the epidermal 

 cells grow excessively long and poorly differentiated, resembling great 

 long cortical cells. Because the slit pea test depends on differential 

 growth in length between epidermal and cortical cells, the ability of 

 the completely etiolated and relatively undifferentiated stem to pro- 

 duce slit-stem curvatures is seriously limited or even reduced to zero. 

 In order to avoid this difficulty, a small amount of red light of the 

 same sort as is used in the Avena test should be applied to the grow- 

 ing seedlings. The quantitative requirement for red light to produce 



6 HRS. 



DURATION OF LIGHT 



Fig. 13. The effect of red light upon the subsequent sensitivity of pea stems in the 

 slit pea test. Red light of approximately 10 f.c. was applied one day before carrying 

 out the test. Each point represents the average cur\ature obtained with 10 mg./l. 

 of indoleacetic acid (Kent and Gortncr, 1951). 



optimal response in the pea test has been established by Kent and 

 Ciortner (1951). They found that i hours of light applied the day 

 before running the test were best. I he (piantitative relationship of 

 this light treatment to auxin sensitivity of the slit pea stems is shown 

 graphically in figure l.S. A convenient method for growing peas of uni- 

 formly high sensitivity is to expose the plants to .S hours of red light 

 daily by placing a time clock in the red light electrical circuit. 



When severed from the plant and .decapitated, the pea stem is 

 deprived of its natural source of auxin and of the capacity to regen- 



