626 



PLANT RESrONSE 



marked by 4 P.M. The next record was taken at 7 P.M., 

 when the sun had gone down, and the source of stimulus 

 was therefore removed. The dotted portion of the record 

 shows the partial recovery which had now taken place, 

 and whose extent was still further increased by 10 P.M. 



This partial undoing of the 

 induced curvature is mainly 

 due to the recovery which 

 is always observed on ces- 

 sation of stimulus. It 

 might, however, be urged 

 that geotropic action, ab- 

 sent when the plant was 

 vertical, and coming into 

 increasing effectiveness 

 with its growing horizon- 

 tality, played a large part in 

 bringing it about. In order, 

 then, to eliminate this ele- 

 ment, I next experimented 

 on a stem of Ciiciirbita. 



Response to Daily Periodic (0 Cucuvbita. — This 



Light and Darkness of P'lagiotropic plagiotropic stem waS held 

 Stem of Citciirbita vmxiina 



Fig. 252 



The specimen held horizontally, with plane 

 of dorsi-ventral division vertical, the 

 ventral surface being here represented 

 to the right. The action of gravitation 

 is thus practically eliminated. Cumula- 

 tive efiect of day's illumination is seen 

 in progressive concavity of normally 

 lower or shaded surface. Dotted figures 

 show positions of recovery after dark. 



horizontally and sideways, 

 in such a way that the 

 plane which divided its 

 dorsal and ventral halves 

 was vertical. The shaded 

 side is here represented in 

 the diagram to the right 

 (fig. 250). Owing to this particular arrangement of the 

 plant it will be seen that the responsive heliotropic move- 

 ment must take place in a horizontal plane, and that on it 

 geotropism will have no influence. The free end of the 

 plagiotropic stem had been naturally somewhat curved up- 

 wards, and this appeared, when placed sideways, as a slight 

 curve to the left, in its position at 7 A.M. Under the stimulus 



