58 



THE TROPISMS 



THERMOTAXIS 



Almost all organisms which are free to move go away from 

 regions in which the temperature is injuriously high, and 

 many, although a less number, withdraw from regions which 

 are colder than a certain optimum. If the water near an 

 Amoeba is heated by a hot needle the animal will contract 

 on the side nearest the needle, send out pseudopods in some 

 other direction and crawl away. Paramcecium and other 

 infusoria form groups in a region of optimum temperature by 

 the same method employed in reacting to chemicals. When 

 a Paramcecium swims into a region above the optimum it 

 gives the motor reflex and goes in another direction. When it 

 encounters a region below a certain temperature it behaves 



FIG. 6. Reactions of Paramcecia to heat and cold. One end of the 

 slide is heated to 35 C. while the other end is kept on ice. The Par- 

 amcecia gather in an intermediate zone d c. 



in the same wa}^. If Paramcecia are placed in a trough one 

 end of which is heated to 35 C. while the other is placed upon 

 ice the infusorians will form a band near the middle where 

 the temperature ranges from 24 to 28 C. Previous ex- 

 posure to higher or lower temperatures than those usually 

 experienced changes the optimum to a considerable degree. 

 Most organisms above the protozoa turn directly away 

 from hot or cold objects. Sometimes the animal may escape 

 undue stimulation by making a number of random move- 



