PHOTOTROPISM 321 



acted on by light from all sides. A seedling of wheaf 

 was mounted on the Balanced Crescograph, and record 

 was first taken under exact balance ; this , gives a hori- 

 zontal record. The np-curve represents retardation, and 

 down-curve acceleration of rate of growth. The source 

 of light was at first placed at a distance of 50 cm. 

 from the plant, and exposure was given for 4 minutes 

 at the point marked with an arrow (Fig. 115a). We 

 shall find in the next chapter that the intensity of 

 photntropic effect is jjrojjortioaal to the quantity of incident 

 light. This quantity at the beginning proved to be sub- 

 minimal, and hence there was an acceleration at the 

 beginning. Continued action induced the normal effect of 

 retardation, as seen in the subsequent resulting up-curve. On 

 the cessation of light, the balance was upset in an oppo- 

 site direction, the resulting down-curve showing an accelera- 

 tion of the rate of growth above the normal. This acce- 

 leration persisted for a time, after which the normal rate 

 of growth was restored, as seen in the curve becoming 

 once more horizontal. IVie after-effect of light of moderate 

 intensity is thus a temporary acceleratio)i of rate of 

 groivth ahove the normal. 



Effect of strong light : Experiment 121. — The same speci- 

 men was used as in the last experiment. By bringing 

 the source of light to a distance of 25 cm. the inten- 

 sity of light was increased fourfold ; the duration of 

 exposure was kept the same as before. The record (Fig. 

 115b) shows that a retardation of rate of growth occurred 

 from the very beginning without the preliminary accelera- 

 tion. This is for two reasons : (1) the increased intensity 

 was now above the critical minimum, and (2) the tone 

 of the organ had become improved by previous stimu- 

 lation. On the cessation of light, the after-effect showed 

 no enhancement of rate of growth, the recovery from re- 

 tardation to the normal rate being gradual. In the next 



