20 Fundamentals of Auxin Action 



Thimann (1937, p. 40) showing the effect of different lengths of 

 second decapitation on curvature obtained are given in figure 6. On 

 the basis of these data it is generally assumed that, if a 2 mm. section 

 is removed and the first attempt at this decapitation should fail (the 

 primary leaf may break off while being pulled out), another attempt 

 may be made without seriously interfering with the plant's sensitivity. 

 After the second decapitation, the primary leaf which has been thus 

 exposed is gently pulled up in order to break its connection with the 



2 4 6 8 10 12 14 MM 



AMOUNT OF COLEOPTILE REMOVED 



Fig. 6. The effect of removal of various amounts of coleoptile tip upon the subse- 

 quent sensitivity (in the Avena test) to a standard auxin concentration (Went and 

 Thimann, 1937, p. 40). 



growing point of the plant (figure 5 D). Now the primary leaf will 

 provide a stable brace upon which an agar block can rest. An agar 

 block is then placed on one side of the cut end of the coleoptile, and 

 auxin from the agar block, transported in a polar direction down- 

 ward, stimulates growth on the side to which the agar block has been 

 applied and curvature of the coleoptile is obtained. 



Three main environmental conditions must be maintained for 

 the Avena test. First, only red light should be used because the shorter 

 wave lengths both destroy auxin and reduce sensitivity of the coleop- 

 tile to a given supply of auxin. Ruby glass which prevents the trans- 

 mission of wave lengths shorter than 5200 Angstrom units is generally 

 used. Many types of vaporproof light fixtures and photographic safe- 

 lights are commercially available with acceptable ruby glass bowls. 

 Second, temperatures must be controlled in order to maintain constant 

 high sensitivity in the plant material. 30° C is optimal for growth of 

 the coleoptile, but temperatures below 25° C are optimal for greatest 



