CURVATURE OF GROWING ORGANS 



105 



follow the course of the sun in such a way that the axis of 

 the cup, formed by the terminal and lateral leaflets, is 

 coincident with the sun's rays (fig. 52). 



Positive Phototropic Curvature of Growing 



Organs 



For this I employed with success the young stems of 

 numerous plants such as Dregea voUihilis, Vicia Faha, and 



Fig. 53. Positive phototropic curvature of a growing stem. 

 Light apphed at vertical arrow and withdrawn at horizontal 

 arrow within circle. Successive dots at intervals of 10 seconds. 

 (Vicia Faba.) 



others. The reaction of some of these was relatively feeble 

 and sluggish, whereas in others it was far more energetic. 



Experiment 60. Positive phototropic curvattire of Vicia 

 Faba. — Light from a 25-candle-power Pointolite acted on 

 one side of the young stem at the most active growing 

 region, the record being taken on a smoked plate oscillating 



