GROWTH MOVEMENTS 1083 



place almost entirely in the extreme 0.5 mm. of the top and the next 

 1.5 mm. is only very little sensitive to these top reactions. . . . All basal 

 zones lack the property of giving top reactions with any of the light- 

 quantities used." But the author found that all the zones give short 

 reactions with large light quantities (about 8000 mcs) and that the 

 top zone gives only long reactions. 



From the application which Dillewijn makes of the growth reactions 

 to phototropism it appears to him "that the first positive curvature 

 is caused by unequal growth-retardations at the front and back, while 

 the second positive curvature is caused by unequal growth accelerations 

 at the front and back. Negative responses may be caused in various 

 ways. The front may present a retardation or an indifferent reaction, 

 as well as an acceleration, while the back may also present this reaction, 

 provided that the growth rate is there smaller than at the front." He 

 therefore claims that the theories of Paal (32) and Boysen-Jensen (8) 

 are not at variance. Paal's conception that light influences the forma- 

 tion of growth-regulating substances in the tops, which are continually 

 produced in darkness, holds good for the first positive curvature, while 

 that of Boysen-Jensen, that a positive curvature is caused by growth 

 acceleration on the shaded sides, holds good for the second positive 

 curvature. 



Dillewijn concludes that, "Phototropic curvature with large light 

 quantities consists of two different components, namely of passing 

 oscillations, caused by reactions in all parts of the coleoptile, and of 

 the phototropic curvature proper, caused by the long reaction which 

 occurs in the top only and from there proceeds downward." 



GROWTH SUBSTANCE 



Dolk (19) carried out a number of experiments which showed the 

 immediate connection between the production of a growth substance 

 and the transmission of the phototropic stimulus. By cutting off the 

 sensitive tips of coleoptiles of Avena sativa he found that the upper 

 portion of the decapitated coleoptile became sensitive in 150 min. to a 

 definite amount of hght, while the corresponding zones of the intact 

 coleoptiles showed no such growth accelerations. 



Studies on the appearance of new physiological tips of decapitated 

 coleoptiles reported by Soding (39) and by Dolk (19) were extended 

 by Li (30) with special attention to the influence of environmental 

 conditions on the formation of growth-promoting substances and the 

 effect of the length of tip removed. In these experiments he considered 

 the average number of minutes between decapitation and the minimum 

 growth rate a measure of the ability to regenerate these growth sub- 

 stances. For the temperature range studied (10° to 35°C.) the time 

 required for the appearance of a new physiological tip varied considerably 



