174 PHYTOHORMONES 



tion (Cholodny-Went effect), and about half to this Ught- 

 growth reaction (Blaauw effect). 



With Helianthus hypocotyls Blaauw measured light- 

 growth reactions for different intensities and calculated the 

 difference between the growth rates on the two sides, hav- 

 ing previously found that the light side received 3^4 times 

 as much light as the dark side. This difference accounted 

 quantitatively for the curvature observed. Phototropism 

 in Helianthus would thus appear to be largely accounted 

 for by the Blaauw effect. 



In Avena, however, the Blaauw effect, as we have seen 

 above, plays little or no part in phototropic curvatures at 

 low light intensities. This is confirmed by experiments in 

 which a light-growth reaction was observed without any 

 accompanying phototropic curvature (Beyer, 1928) or a 

 phototropic curvature without any light-growth reaction 

 (Cholodny, 1932a, 1933a). The observed light-growth re- 

 action of Avena at these intensities is apparently due to a 

 reduction of about 20 per cent in the amount of auxin 

 diffusing out of the tip within the first hour after illumina- 

 tion (Went, 1928). This corresponds well with the observa- 

 tion that this type of growth reaction (the '4ong reaction") 

 occurs only after illumination has fallen on the tip of the 

 coleoptile. A second type of light-growth reaction (the 

 ''short reaction") reaching its maximum half an hour after 

 illumination, occurs when the basal zones are illuminated. 

 The response is not proportional to the total amount of 

 light energy, as was shown by Burkhardt (1926), but ap- 

 pears to be more of the nature of a shock reaction or typical 

 "stimulus"; it is ascribed by van Overbeek (1936a) to a 

 transient decrease in sensitivity to auxin. It was shown by 

 Cholodny (1930) that plants subjected to a gradual increase 

 of unilateral illumination did not give this reaction, although 

 their phototropic response was still normal. If, on the other 

 hand, the light were applied to them suddenly, this light- 

 growth reaction was very marked. 



In this connection an analysis of the effect of all-sided 



