36 COLLEGE ZOOLOGY 



The term " tropism " means " a turning "; it is used for purely 

 descriptive purposes. Nothing is known of the psychic phe- 

 nomena of the lower animals, and one must be cautious in at- 

 tributing to them his own mental states. The term " tropism " 

 merely describes an animal's behavior in response to stimuli. 

 The kind of stimulus employed is indicated by a prefix. The 

 principal kinds of tropisms are as follows: — 



(i) Thigmotropism = reaction to contact. 



(2) Chemotropism = reaction to a chemical. 



(3) Thermotropism = reaction to heat. 



(4) Phototropism = reaction to light* 



(5) Electrotropism = reaction to electric current. 



(6) Geotropism = reaction ta gravity. 



(7) Chromotropism = reaction to color. 



(8) Rheotropism = reaction to current. 



"Taxis" is often employed instead of "tropism," when the 

 terms read " thigmotaxis," " chemotaxis," etc. If the animal 

 reacts by a movement toward the stimulus, such as light, it is 

 said to be positively phototropic or phototactic, etc. ; if away 

 from the stimulus, negatively phototropic or phototactic, etc. 

 Ameba has been found to respond to contact with solids, to 

 chemicals, to heat, to light, to colors, and to electricity. 



Ameba exhibits negative thigmotropism when touched at any 

 point with a solid object; the part affected contracts and the 



Fig. 13. — Thigmotropism of Ameba. The animal moves away when 

 stimulated by a glass rod. (From Jennings.) 



animal moves away (Fig. 13). When, however, an Ameba is 

 floating freely in the water and a pseudopodium comes in con- 



