626 



PLANT RESPONSE 



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 Cucurbita. 



(c) Cucurbita. — This 

 plagiotropic stem was held 

 horizontally and sideways, 

 in such a way that the 

 plane which divided its 

 dorsal and ventral halves 

 was vertical. The shaded 



FlG. 252. Response to Daily Periodic 

 Light and Darkness of Plagiotropic 

 Stem of Cucurbita maxima 



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 effect of day's illumination is seen 

 in progressive concavity of normally 



lower or shaded surface. Dotted figures side is here represented in 

 show positions of recovery after dark. , ,. . ■,, . . , 



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 



