Facts and Factors of Development 41 



toward or away from a source of stimulus, the former being 

 known as positive and the latter as negative tropism. When re- 

 sponses are very complex, one response calling forth another and 

 involving many complex reflexes or chains of reflexes, as is fre- 



FIG. 18. PHOTOTROPISM OF SEEDLING OF WHITE MUSTARD supported by a 

 sheet of cork (K, K) floating on the water. The direction from which 

 light comes is shown by the arrows ; the stem and leaves are turned toward 

 the light (positive phototropism), the root away from the light (negative 

 phototropism). (After Strasburger.) 



