AUXIN AND GROWTH 83 



consideration. Elongation of the mesocotyl has been en- 

 countered as a technical difficulty by almost everyone who 

 has worked with Avena, the very long curved mesocotyls 

 making it impossible to obtain straight plants. This can be 

 prevented by exposing the germinating seeds to light 

 (Beyer, 1927) or to heat (du Buy and Nuernbergk, 1929, 

 1929a, 1930) (see Figure 36). The mesocotyl length of 



Fig. 36. Upper row, Avena seedling.s grown in darkness: white dots indicate 

 node between mesocotyl and coleoptile; lower row, grown in yellow light. 

 (From van Overbeek, 1936.) 



4-day old Avena seedlings may reach 60 mm. if grown in 

 darkness, but is only 2 mm. if occasionally exposed to red 

 or yellow light during growth. 



An explanation of this phenomenon has been given by 

 van Overbeek (1936), according to whom it depends on a 

 decrease in the amount of auxin produced by the tip on 

 exposure to red or yellow light, or to high temperature. 

 Thus the coleoptile tips of Zea Mays plants which had been 

 kept for 45 minutes at 48°, and whose mesocotyls were 

 correspondingly greatly inhibited, gave off less auxin than 

 those of control plants. If this auxin deficiency was re- 

 moved by smearing 0.02 per cent auxin paste on the tip 



