148 



GROWTH HORMONES IN PLANTS 



the convex and concave sides resulting from unilateral illumina- 

 tion with 80 meter-candle seconds, after different reaction times. 

 In this experiment, the front and back sides of the unilaterally 

 illuminated coleoptile are exposed to the same two light intensities 

 as the two coleoptiles mentioned, owing to the light gradient 

 through it. This should yield information of value as to whether 

 the differential growth occasioned by this difference in the inci- 

 dent light can account for phototropic curvature. It is clear 

 from the data (Table 9) that the growth changes in the light- 



Table 9. — Comparison of the Light-growth Reaction in the Avena 

 Coleoptile Subjected to General Illumination of Two Inten- 

 sities, and Growth on the Two Sides of a Similar Coleoptile 

 Unilaterally Illuminated So That the Lighted and 

 Shaded Sides Are Subjected to the Same Two 

 Intensities 



growth reactions under the two different intensities are not 

 sufficient to produce a curvature of the size that is actually 

 obtained in the unilaterally illuminated coleoptile. With a 

 somewhat different approach, Beyer came to the same conclu- 

 sion. In Beyer's experiments, three series of plants were illumi- 

 nated bilaterally for one hour. At the end of the hour, the 

 illumination was continued unchanged in series A; in series B, 

 both lamps were turned off; in series C, one lamp was turned off. 

 Phototropic curvature resulted in the plants of the C series by 

 decreasing and not by increasing the light intensity. According 

 to the Blaauw theory, one would expect that the lighted side 

 Ci of the curved plants should grow just as fast or certainly not 

 any more slowly than that of a plant in series A and that the 

 shaded side Cg should grow as fast as, or no faster than, a plant 

 in series B. The results may be expressed in the following 

 way: A - Ci = 0.07 and 0.08, and C, - 5 = 0.12 and 0.11. 

 Since a decrease in the rate of growth takes place on the lighted 

 side, while an increase occurs on the darkened side, the data are 

 not in accordance with Blaauw's theory. Bergann's (1930) 



