AMEBA 



55 



fore, reacts negatively to a strong mechanical stimulus and posi- 

 tively to a weak one. 



Chemotropism. Ameba is sensitive to changes in the chemi- 

 cal composition of the water surrounding it. If a chemical is 



a ~b c 



FIG. 20. Method by which a floating Ameba passes to a solid. 

 (From Jennings.) 



brought into contact with one side by means of a fine capillary 

 glass rod or tube, the part affected contracts and the animal 

 moves off in some other direction (Fig. 21). "It has been shown 

 to react negatively when the following 

 substances come in contact with one side 

 of its body: methylene blue, methyl green, 

 sodium chloride, sodium carbonate, potas- 

 sium nitrate, potassium hydroxide, acetic 



acid, hydrochloric acid, cane sugar, dis- FlG ; 2I - Ne s ative 



r tion to chemical stimu- 



tillea water, tap water, and water trom 



other cultures than that in which the 

 Amceba under experimentation lives " 

 (41, p. 10). 



Thermotropism. A meba gives a nega- 

 tive response if locally affected by heat. 

 This may be accomplished by bringing 

 a heated needle in front of an Ameba 

 which is creeping along on the under surface of a cover glass. 

 When this is done, the animal moves off in another direction. 

 The degree of temperature also has an influence upon the rate 

 of the normal movements. Cold and excessive heat retard its 

 activities, while a moderate amount of heat increases the move- 



lation in Ameba. A 

 little methyl green dif- 

 fuses against the ad- 

 vancing end, causing 

 new pseudopodia to be 

 thrust out, and a change 

 in the direction of move- 

 ment. (From Jennings.) 



