46 



TYPICAL ANIMAL CELL 



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 bv 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. x\s 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 

 th\Toid extract will usually promote the multiplication and growth of the 

 organisms present. This is a Paramecium culture. 



When the amoeba is viewed from the 

 side, pseudopodia may be seen to ex- 

 tend which result in movements like 

 walking. 



