144 GROWTH HORMONES IN PLANTS 



the two halves just balance, so that no change in the growth rate 

 of the whole organ takes place under conditions leading to 

 phototropic curvature. 



The course of the first positive curvature has been investigated 

 by Lundegardh (1922), Went (19286), du Buy and Nuernbergk 

 (19296), Dolk (19296), and du Buy (1933). Dolk illuminated 

 Avena coleoptiles unilaterally with 50 meter-candles for 10 

 seconds and recorded the course of the tropic curvature cinemato- 

 graphically. The radii of curvature of the individual growth 

 zones were measured on enlarged photographs. The results of 

 his investigations are given in Fig. 46. The ordinates give the 

 size of curvature, i.e., the reciprocal value of the radius of curva- 

 ture, and the abscissae represent the time in minutes. The 

 curvature begins almost simultaneously in the first three zones, 

 that is, about 40 minutes after illumination, and proceeds down- 

 ward. In the tenth zone, curvature becomes apparent only 

 after 100 minutes. In the meantime, the curvature has increased 

 continuously in the apical region, reaching its maximum in the 

 first two zones after 170 to 180 minutes. That the average rate 

 of growth is not changed during the second positive curvature 

 has been shown by the investigations of Beyer (1927c), who 

 employed three series of experimental plants: series A, illumi- 

 nated bilaterally 1 hour and bilaterally 2 hours; series B, 1 hour 

 bilaterally illuminated and 2 hours darkened; and series C, 1 hour 

 illuminated bilaterally and 2 hours unilaterally (50 candles at a 

 distance of 60 cm.). The growth of the lighted and darkened 

 sides of series C during the phototropic curvature was then com- 

 pared with that of both the other series. The linear growth in 

 series A was 0.20 cm.; and in series B, 0.23 cm.; in C on the light 

 side it was 0.11 cm., and on the dark side, 0.33 cm., which gives 

 an average of 0.22 cm. The average rate of growth during photo- 

 tropic curvature appears to be the same as it is in plants in the 

 dark or in those bilaterally illuminated. Du Buy and Nuernbergk 

 (19296) obtained similar results when they followed the course of 

 the second positive curvature under strong illumination. 

 Although the second positive curvature corresponded to the 

 course of the first positive response, a sudden increase in growth 

 could be observed during illumination. The authors attributed 

 this increase to cell-wall extension brought about by the strong 

 illumination. 



