212 Light 



slightly inclined (angle a), threshold discrimination is reached after 

 a smaller amount of turning, or at a smaller final angle of orientation 

 (6). When the plane is steeper, the animal continues turning to a 

 greater angle of orientation. The result of a series of trials made 

 with varying angles indicates the mechanical nature of the taxis and 

 the quantitatively exact way in which the response of the animal is 

 controlled by the magnitude of the orienting stimulus (Fig. 6.14). 



90 



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40 50 60 70 



Inclination of surface, ex, degrees 

 Fig. 6.14. Relation between the angle of inclination of plane (a) and the mean 

 angle of orientation ( ^ ) of caterpillar creeping upon it. Open circles are means 

 for one individual; black circles are means for all individuals tested. The curve 

 is that for A(9/A log sin a. z=z constant. ( Crozier, 1929, Copyright, Clark Univ. 



Press. ) 



Orientation of growth and locomotion in relation to light and other 

 factors plays a profoundly important role in the lives of plants and 

 lower animals. The responses of all species that survive must ob- 

 viously generally bring the organism into favorable surroundings, but 

 lack of flexibility in these reactions often produces harmful results 

 under extreme or unusual conditions. The rigidity of the tactic 

 responses of the tent caterpillar is often seen to lead to its destruction. 

 When the animals emerge from their nests in the crotches of trees, 

 they crawl upward in response to their negative geotaxis to the tips 



