GROWTH SUBSTANCES FOR PHOTOTROPISM 133 



light gradient and phototropic curvature in Avena coleoptiles 

 which were filled with water or with air. Curvatures were less in 

 the water-filled coleoptiles owing to a smaller light gradient. 



Distribution of Sensitivity to Light. — Early studies upon the 

 question of relative phototropic sensitivity of different zones of 

 the oat coleoptile established the fact that not all regions are 

 equally sensitive to light (Darwin, 1899; Rothert, 1892, 1894). 

 Rothert concluded from the results of his studies that "the 

 apical region of high phototropic sensitivity is not longer than 

 3 mm., and only in the extreme 1 to 1.5 mm. portion of the tip 

 is the sensitivity to light particularly great." 



Since the studies by Rothert, this same question has been 

 investigated with improved technique. Sierp and Seybold (1926) 

 used adjustable screens in order to partially darken an exactly 

 determinable portion of the tip. The presentation time was then 

 determined when 0.25, 0.50, 0.75, to 2 mm. portions of the tip 

 were darkened. From the figures obtained in this manner, a 

 curve was constructed which showed an increase in presentation 

 time with an increase in the length of the darkened apical region. 

 Sierp and Seybold stated their results as follows : 



In conclusion it can be said that the sensitivity to light in the Avena 

 saliva coleoptile is greatest in the J^ mm', at the tip, from which point 

 downward it decreases rapidly. In the zone of about K to K tarn. 

 lying directly below the tip region, the sensitivity is only J-^o of that at 

 the extreme tip. At a distance of 2 mm. from the tip, sensitivity 

 decreases to 1/36,000 of that of the uppermost region; and from this 

 point on to the bottom, it remains about constant. 



At about this time, Lange (1927) studied the distribution of 

 sensitivity to light in the coleoptile tip. He thought that the 

 method of darkening used by Sierp and Seybold could cause half- 

 shadow formations which would impair the accuracy of the 

 measurements. To avoid this, he illuminated each transverse 

 zone to be investigated by means of a slit, the width of which could 

 be diminished to 50 microns. The light value was determined 

 as the product of the intensity of light X time of illumination X 

 area of the illuminated surface. The threshold values necessary 

 to elicit phototropic response were determined for different zones 

 of the coleoptile. From the formula: 



Sensitivity to light = threshold hght value' V^^^oi^^ 



iulLIIRARY 



^ 



