46 



TYPICAL ANIMAL CELL 



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sidered negative and is represented by a minus sign ( tropism). 

 The different tropisms are named in terms of the stimulus to 

 which the organism is reacting. Listed below are the tropisms 



shown by the amoeba. 



Phototropism reaction to light. 

 Chemotropism reaction to 

 chemicals. 



Thigmotropism reaction to 

 touch or contact. 



Thermotropism reaction to 

 heat. 



Galvanotropism reaction to 

 electric current. 



These reactions are largely pro- 

 tective. As a result of tropisms 

 the amoeba often escapes unfavor- 

 able conditions and gets into an 

 environment where the conditions 

 are favorable for its life and ac- 

 tivity. 



Locomotion. By means of its 

 pseudopodia, the amoeba " walks" 

 from place to place. It sends out 

 a stream of protoplasm in one 

 direction. Gradually the rest of the cytoplasm carrying the 

 nucleus flows into it ; and again a thin stream is sent out. Thus 

 the organism moves. By means of the function of locomotion it 

 can get food and escape unfavorable conditions. 



Problem. Study of a Paramecium. 



Place some hay in a beaker of distilled water. Heat in order to soften the 

 hay. Let the material stand two or three weeks. The addition of a little 

 thyroid extract will usually promote the multiplication and growth of the 

 organisms present. This is a Paramecium culture. 



When the amoeba is viewed f-rom the 

 side, pseudopodia may be seen to ex- 

 tend which result in movements like 

 walking. 



