1082 BIOLOGICAL EFFECTS OF RADIATION 



Went's data indicate two reactions, and in this respect they agree with 

 what Sierp (38) mentioned earHer. In Graph 3 the entire coleoptile 

 is illuminated, in Graph 4 only 1.25 mm. of the tip, and in Graph 5 

 only 9 mm. of the base, while the top 3 mm. is kept in darkness. The 

 abscissa represents the time in minutes and the ordinate the growth in 

 microns per min. The arrow is the point of illumination for 5 sec. by 

 100 meter-candles. In Graphs 4 and 5 it will be seen that the first 

 effect of light is a retardation followed by an acceleration in the growth 

 rate. When only the base is illuminated (Graph 5) the minimum occurs 

 after 16 min. When the illumination is confined to the top (Graph 4) 

 the minimum occurs after approximately 60 min. When the entire 

 seedling is illuminated (Graph 3) the two separate light-growth responses 

 are distinctly seen. The position of the minimum of these curves depends 

 on temperature, a somewhat lower temperature retards the time of 

 minimum growth rate. Also the duration of the illumination enters 

 as a factor. Went points out that the responses of Koningsberger (27) 

 with continuous illumination must be considered a base response. 



Arisz's work shows that the first positive curvature occurs when 

 the tip of the coleoptile is illuminated by light less than 4000 Mcs. 

 From these facts and those furnished by Dillewijn, Went concludes 

 that, "The first positive curvature results from the different tip responses 

 of the proximal and the distal sides of the coleoptile" and that, "The 

 second positive curvature is the consequence of the base responses of 

 the proximal and the distal sides of the coleoptile," which appears after 

 more illumination. These suppositions are in agreement with Blaauw's 

 theory. Moreover, it is possible to account for the negative curvatures 

 by a combination of the tip and base responses. 



In a later paper Dillewijn (18) discusses experiments in which illumi- 

 nation was limited to three zones of coleoptiles : Zone 0, from the tip down 

 to 2 mm.; Zone II, from the 2-mm. to the 7-mm. point; and Zone VII, 

 from the 7-mm. to the 9-mm. point. It was found that the top zone 

 was by far the most important, especially from to 0.5 mm. The light 

 quantities used were 800, 8000, 80,000, and 800,000 mcs, at 20°C. and 

 80 per cent humidity. A layer of water absorbed the heat rays. 



"Between and 800 Mcs the retardation becomes continually stronger 

 by increasing quantities of light, that between 800 and 8000 mcs it 

 reaches a maximum and then diminishes and entirely disappears at 

 8000 MCS. By still increasing the light an acceleration sets in, which 

 at 80,000 MCS is fairly considerable, and with still more light also dimin- 

 ishes again so that at 800,000 Mcs, disregarding a transitory retardation 

 and acceleration, an indifferent stage is again reached. The same 

 behavior was found for the uppermost top zone of 0.5 mm., with this 

 difference that both retardation and acceleration are somewhat smaller 

 than in illuminating Zone 0. This means that the top reactions take 



