1084 BIOLOGICAL EFFECTS OF RADIATION 



(391 to 75 min.), being shorter for the higher temperatures. Light 

 exerted no influence. The time required to develop a new tip also 

 varied with the length cut off, being shorter for a short section cut off. 

 Contrary to Dolk's claim that the growth of a regenerated tip never 

 reaches the value of that before decapitation, Li found that with the 

 removal of only 1 to 2 mm., a growth substance is produced that equals 

 that of the original. Dolk cut off 4 mm. in his experiments. 



The work of earlier investigators likewise indicated that by removing 

 the tip, the properties of the tip are assumed by the uppermost zone. 

 Paal (32) assumed a growth regulator in the coleoptile of Avena. When 

 this substance diffuses from the tip into the growing region, growth is 

 accelerated. He also was of the opinion that light either prevents the 

 formation of this substance, destroys it, or impedes its diffusion. 



Went (43) has carried out a number of most interesting experiments 

 which have given much information as to the formation, character, and 

 effect of the growth-accelerating substance produced in the tips of cole- 

 optiles of Avena. He placed a number of tips cut from coleoptiles on a 

 thin sheet of gelatin with the cut surfaces next to the gelatin. After an 

 hour these tips were removed and the gelatin cut into small blocks which 

 were then placed on one side of decapitated coleoptiles. In 3 hr. these 

 decapitated coleoptiles showed very marked bending (negative) with the 

 })locks on the convex sides. Control blocks not previously treated with 

 the growth substance from tips produced either no bending, or a very 

 slight positive curvature. Likewise, gelatin on which sections below the 

 tip had stood gave no curvatures. 



Went raises the question: "What influence has light on the formation 

 of growth regulators?" By illuminating the coleoptiles with different 

 quantities of light from to 1,000,000 mcs and impregnating gelatin 

 with substances from these tips and studying the curvatures produced on 

 decapitated coleoptiles, he shows that a less amount of growth substance 

 diffuses into the gelathi from the tips irradiated with 1000 mcs than 

 from those irradiated with 100,000 mcs. He also shows the possi- 

 bility of imitating phototropic bending. Stated in his own words, "To 

 do this I first placed, as in all former experiments, on one side of the stumj) 

 gelatin, 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 tips had stood that had been illuminated with ten 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 obtained by Arisz. With a difference 

 of 1000 versus mcs a positive curvature (in 7 out of 9 plants, 2 remained 

 straight) occurred, reckoned toward the 1000 mcs. With 10,000 versus 

 1000 MCS the curvature was negative. With 100,000 versus 10,000 



