IRRITABILITY. 



53 



The motor zone in this instance is located in the region of 

 most active growth. (See Experiment 86.) 



EXPERIMENT 63. 



PERCEPTIVE ZONE OF ROOTS. 



Repeat Experiment 59 after cutting away a portion of the tip 

 of the root i to 2 mm. in length. The root does not now respond 

 to gravity, and shows no movement until the tip is rehabilitated, when 

 it curves downward in a natural manner. 



EXPERIMENT 64. 



MOTOR ZONE OF ROOTS. 



With a fine brush carefully mark off equal spaces (2 to 3 mm.) on 



FIG. 47- 



Motor zone in roots. (Pfefter.) 



the primary root of a seed- 

 ling of Phaseolus (Bean). 

 Suspend in a horizontal 

 position in a moist chamber. 

 In a day note the region 

 in which curvature has 

 occurred, and its distance 

 from the tip. 



EXPERIMENT 65. 



MOTOR ZONES OF CULM OF 

 GRASS. 



Cut a length of 12 cm. 

 from a vigorously growing 

 culm of Grass. Place in a 



horizontal position in a moist chamber with one end imbedded 

 in sand. Six hours later note the region of curvature. The motor 

 zone will be found in the FIG. 48. 



pulvinus like internodes. (Fig. 

 48.) 



EXPERIMENT 66. 



FORCE OF CURVATURE. 



Fasten a seedling of Pea 

 with a radicle i cm. in length 

 to a piece of cork attached to 

 the side of a vessel containing 

 mercury. Place the seedling in such position that the root is 



Curvature of Culm of Grass. 

 (After Oels.) 



