Extraction and Measurement Techniques 19 



The Avena coleoptile is a cylinder of relatively uniform, elongat- 

 ing cells. Cell division takes place only until the etiolated coleoptile 

 reaches approximately 1 cm. in length. After that the growth is ac- 

 complished almost exclusively by cell elongation and it continues for 

 approximately 4 days at a maximum rate of roughly 1 mm. per hour. 



The seedlings are grown in darkness to prevent occurrence of the 

 reduction in sensitivity caused by blue light. However, oat plants 

 grown in total darkness undergo elongation of the first internode to a 

 very inconvenient length. Such elongation can be prevented by ex- 

 posure to red light for a period of 2 to 4 hours during the second day 

 after germination. 



Fig. 5. Diagrammatic representation of the steps involved in the Avena test. 



The steps involved in carrying out the Avena test are shown dia- 

 grammatically in figure 5. When the coleoptile is 15 to 30 mm. high 

 (above the glass holder or sand level) the ajMcal 1 mm. is removed in 

 order to cut off the natural source of auxin within the coleoptile. In 

 this decapitated condition the natural auxin content of the stump de- 

 creases for a period of two hours after which a regeneration of auxin- 

 forming capacity occurs at the new physiological tip. In order to pre- 

 vent the renewed formation of auxin during the test period, a second 

 decapitation is carried on three hours after the first. Removal of from 

 2 to 4 mm. in this second decapitation reduces the curvature responses 

 very little. However, if the second decapitation removes 6 mm. or more 

 the curvature obtainable is seriously reduced. The data of Went and 



