TROPISMS 



167 



curvature is that which appears in ordinary dayhght (see: 

 du Buy and Nuernbergk, 1934). All these curvatures may 

 be brought about by illumination of the tip alone; illumina- 

 tion of the base gives only positive curvatures. 



That the curvature depends upon a gradient of light across 

 the coleoptile was conidncingly shown by Buder (1920) 

 and subsequently worked out for decapitated and regener- 

 ated coleoptile bases by du Buy (1934). The gradient can 



8 log. MCS 



Fig. 49. Relation between phototropic curvature (ordinate) of Avena 

 coleoptiles and the amount of unilateral light (abscissa). From 10 to 4000 

 meter-candle seconds gives the first positive curvature, 40,000 to 1,000,000 

 the second, and above 10,000,000 the third. Between 4000 and 40,000 negative 

 curvature. (After du Buy and Nuernbergk, 1934.) 



be reduced by pulling out the primary leaf, or still further 

 reduced by filUng the interior of the hollow coleoptile with 

 water; if this is done the curvature is greatly reduced. 



The extent to which the first positive and the first negative 

 curvature are related to auxin distribution may be seen 

 from Table XIV, which summarizes the results of Went 

 (1928), van Overbeek (1933), and Asana (1936,u) obtained 

 by placing unilaterally illuminated tips upon two separate 

 agar blocks as in Figure 48. It is e\ddent from these figures 

 that there is the strictest parallelism between both the 

 magnitude and the sign of curvature and the auxin distribu- 

 tion a few mm. below the tip. The extremely good agree- 

 ment in the controls and in plants in the region of indifference 

 gives additional confidence in the figures. 



