142 



GROWTH HORMONES IN PLANTS 



candle seconds, the first positive curvature disappears entirely, 

 and a pure negative curvature appears. With still greater 

 amounts of light, the negative curvature also disappears, and a 

 positive curvature appears immediately — the so-called second 

 positive curvature. The negative curvatures appear only when 

 the tip is illuminated and cannot be definitely demonstrated 

 when the base is unilaterally illuminated. 



These results have been confirmed by other investigators. 

 Lundegardh (1922) found that the first purely positive reaction 

 appeared with light values up to 10 meter-candle seconds, the 

 maximum being at 10 meter-candle seconds. The range of the 

 negative curvature lay between 800 and 500,000 meter-candle 

 seconds, with a minimum at 4,000 to 10,000, the second maximum 

 being at about 2,000,000 meter-candle seconds. The reaction 

 decreased again with still greater amounts of light. 



The experiments of Pringsheim, Clark, Arisz, an^ Lundegardh 

 were all carried out with mixed white light. Du Buy and Nuern- 

 bergk (19296) investigated the course of phototropic response 

 using monochromatic blue light of wave length 4,360 A. Only the 

 apical 2 to 3 mm. portion of the coleoptile was illuminated. 

 The amount of light was measured in ergs per square centimeter 

 per second. For the sake of comparison with the other investi- 

 gations cited, 1 erg at wave length 4,360A. may be considered 

 as corresponding to about 10 meter-candle seconds (Table 8). 

 Du Buy and Nuernbergk believe that they are able to distinguish 



Table 8. — Positive and Negative Curvatures in the Avena Coleoptile 

 IN Response to Different Amounts of Radiation 



