318 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



In the upper graph of figure 2 the entire coleoptile is illuminated. 

 In the middle graph only 1.25 mm of the tip is illuminated, and in 

 the lower graph only the base, 9 mm in length, receives light. The 

 abscissa represents the time in minutes and the ordinate the growth 

 in fi per minute. The arrow represents the point of illumination for 

 5 seconds by a light intensity of 100 meter-candles. In the second 

 and lower graphs the first effect is seen to be a retardation in growth 

 followed by an acceleration. When only the base is illuminated, as 

 shown in the lower graph, minimum growth rate occurs after 16 

 minutes. When the illumination is confined to the tip, the minimum 

 occurs after about an hour. The growth responses following tip and 

 base illumination is reflected in the response of the coleoptile which is 

 illuminated along its full length (upper graph). The position of the 

 minimum depends on temperature. A somewhat lower temperature 

 delays the time of minimum growth rate. Also, the duration of the 

 illumination is a factor to be considered. 



This complex relationship existing between the plant and light is 

 further complicated by the discovery in recent years of a growth- 

 promoting substance formed in the tips of coleoptiles, which controls 

 their growth. When this substance diffuses from the tip into the 

 elongating portion of the sprout, growth is accelerated. It is pos- 

 sible that light either prevents the formation of this growtli substance, 

 destroys it, or changes its direction of diffusion. A number of in- 

 vestigators support the theory that phototropic curvatures arise as a 

 result of auxin being transported sideways from the illuminated side 

 toward the shaded side of the coleoptile. Went, of Utrecht, and his 

 school perhaps have contributed more to this phase of phototropism 

 than any other investigators. Their experiments are most interest- 

 ing. In some of this work the tips cut from a number of coleoptile? 

 were placed on a thin sheet of gelatin with the cut surface next to 

 the gelatin. After an hour these tips were removed and the gelatin 

 cut into small blocks. These blocks were then placed asymmetrically 

 on the freshly cut surfaces of decapitated coleoptiles. In 3 hours 

 these decapitated coleoptiles showed a very marked bending with 

 the blocks on the convex sides. (See fig. 3.) Control blocks not 

 previously treated with the growth substance produced either no 

 bending or very slight positive curvature. 



It has been found that under certain conditions the curvature is 

 proportional to the amount of growth substance supplied. If the 

 angle of curvature is 10°, the growth effect has been called an Avena 

 unit (AU). In order to place all data on a quantitative basis, Dolk 

 and Thimann suggest as the unit that quantity of growth substance 

 which has to be present in 1 cc of solution to give, when mixed with 

 1 cc agar, an angle of 1° after 110 minutes. 



