FACTS AND FACTORS OF DEVELOPMENT 59 



the organism turns or moves toward or away 

 from a source of stimulus they are termed 

 tropisms; if the responses are very compli- 

 cated, one response calling forth another and 

 involving many reflexes, as is frequently the 

 case in animals, they are known as instincts. 

 In the embryo the rhythmic contractions of 

 heart, amnion and intestine are early manifes- 

 tations of reflex motions. These appear chiefly 

 in the involuntary muscles before nervous con- 



B 



Fig. 19. Geoteopism of Seedling Oak. After starting to 

 grow with the axis A- A in vertical position the seedling was 

 gradually turned through 90° until the axis B-B was vertical; 

 during this change of position the stem continues to grow 

 upward (negative geotropism), the root downward (positive 

 geotropism) . 



