210 



Light 



illustrate the turning reaction of phototropism by actual growth or by 

 turgor movements. 



Phototaxis, geotaxis, and other tactic reactions involve the orienta- 

 tion of locomotion toward or away from light, gravity, or some other 

 source of stimulation in the environment. In many instances the turn- 

 ing of the moving organism appears to be due to the unequal stimu- 

 lation of symmetrical sense receptors that control the tonus of the 

 body, leg, or wing musculature. When the animal moves, it is caused 

 to turn by the unequal posture of its locomotory apparatus. For 

 example, in a positively phototactic animal such as the swimming 

 insect Ranatro, if the left eye receives more light than the right eye, 

 the legs on the left side of the body will be more strongly flexed, 

 whereas the legs on the right side of the body will be more greatly 



\^ 



Fig. 6.12. (Upper) Symmetrical swimming position of positively phototactic 

 Ranatra moving toward a light source in front of it. (Lower) Position of normal 

 animal turning toward a light source at its left. This position will be produced 

 permanently if the right eye is removed; if both eyes are removed, the animal's 

 position becomes symmetrical again. (Crozier, 1929, Copyright, Clark Univ. 



Press. ) 



