68 BIOLOGY 



Reaction to light (Phototropism) (Gr. photos - light -f- trope). 

 If a strong light is directed upon an Amoeba from one side, it 

 will move away from the light. A strong, white light may cause 

 the animal to stop moving. 



Reaction to electricity (Electropism) (Eng. electro -f- Gr. trope). 

 If an electric current is passed through an Amoeba, it contracts 

 on the side of the positive pole of the current and moves toward 

 the negative pole. 



In all these cases the Amceba reacts to a stimulus. But there 

 are other things which are irritable and react to a stimulus in 

 a purely mechanical fashion. Gunpowder is also irritable, since 

 it will react to heat with an explosion. A locomotive is irri- 

 table, since it will react to a touch upon its throttle valve. The 

 Amoeba certainly reacts in a more complex and more varied 

 manner, but the question inevitably arises whether the action 

 may not be simply that of a bit of machinery responding to its 

 appropriate stimulus. There is no definite answer to this ques- 

 tion that can yet be given. 



Reproduction. As the Amoeba by assimilation converts its 

 food into new protoplasm, it inevitably increases in size. If 

 this went on without interruption there would be no limit to 

 the size of the animal. But after growing for a time, a constric- 

 tion appears in the middle of the body which deepens until it 

 finally divides the animal into two parts; Fig. 19 C. Each -of 

 the resulting parts is like the other and each like the original, 

 except in size. It is the nucleus that seems to take the lead in 

 this process of division, which is one of great complexity. This 

 will be described in the next chapter, for it goes through the 

 complicated series of changes known as karyokinesis (Gr. 

 karyon = nucleus + kinesis = movement) described on page 85. 

 As a result of this division there arise two animals, evidently 

 alike, each of which now moves away and lives an independent 

 life. This method of reproduction, by which the animal divides 

 into two practically equal parts, is called fission. 



A second method of reproduction sometimes occurs in 



