PHOTOTROPISM JOHNSTON 



319 



The question arises as to what influence light has on these growth 

 substances. By illuminating the coleoptiles with different quantities 

 of light from to 1,000,000 MCS (meter-candle seconds) and impreg- 

 nating gelatin with substances from these tips and studying the 

 curvatures produced on decapitated coleoptiles, Went shows the 

 possibility of imitating phototropic bending. In his own words: 



I first placed, as in all former experiments, on one side of the stump gela- 

 tin treated with tips to which an illumination of, e. g., 100,000 MCS had been 

 ■applied. Then on the other side a gelatin block was placed, on which the 



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4 5 6 7 8 9 



Figure 3. — (From Went). Schematic preparation of the analysis of growth substances. 

 1. Agar plate stamped from sheet of agar. 2 Coleoptile tips placed on agar plate. 

 3. Agar plate divided into twelve cubes. 4. Coleoptile tip cut on one side. 5 and 6. 

 Removal of tip. 7. Primary leaf pulled out. 8. Agar cube placed unilaterally on 

 coleoptile. 9. Curved growth response of plant. 



tips had stood that had been illuminated with 10 times less light. In this 

 way a gelatin system was placed on the stump which, according to Blaauw's 

 theory, as nearly as possible approached the unilaterally illuminated tip. The 

 plantlets indeed bent themselves in perfect accordance with the figures ob- 

 tained by Arisz. With a difference of 1,000 versus MCS a positive curvature 

 (in 7 out of 9 plants, 2 remained straight) occurred, reckoned toward the 1,000 

 MCS. With 10.000 versus 1,000 MCS the curvature was negative. With 100,000 

 versus 10.000 MCS I found a positive curvature again, i. e., towards the 100,000 

 MCS. It appears that the retardation in growth obtained by van Dillewijn 

 with an illumination of 800 MCS and the acceleration with 80,000 MCS was 

 the result of a smaller and larger formation of growth-promoting substances, 

 respectively. 



