RESPONSES IN THE SIMPLEST ANIMALS 195 



at night and in the morning ; and the relatively rapid response 

 of the leaves of the sensitive plant to outside stimuU are aU 

 explained by changes in the water content of cells in pulvini, or by 

 rapid and temporary fluctuations in growth on opposite sides of 

 the leaves^ or by a combination of both. But other than external 

 stimuli influence and modify the growth and actions of plants. 

 We know that enzymes play an important part in the storage of 

 food in fruits and seeds, and there seem to be evidences of vitamin 

 and hormone action as well. It is probable 

 that the protoplasm of a plant is under 

 much the same control as is the protoplasm 

 of an animal. 



Responses in the Simplest Animals. — 

 We have already seen that amoebse and 

 paramecia seem to respond to the presence 

 of food. Examination of a drop of hay in- 

 fusion containing paramecia will show many 

 collected around masses of food, indicating 

 that they are attracted by it. In another 

 part of the slide we may find a number of 

 the paramecia lying close to the edge of an 

 air bubble, with the greatest possible amount 

 of their surface exposed to its surface. 

 These animals are evidently taking in oxy- 

 gen by diffusion. They are carrying on res- 

 piration. A careful inspection of the jar 

 containing paramecia shows thousands of 

 tiny whitish bodies collected near the surface 

 of the jar. Some force or forces keep them 

 close to the surface. Professor Jennings and 

 others have made careful studies of the reactions of paramecia and 

 other one-celled animals to various stimuli, and have found that 

 in general they react positively toward favorable and negatively 

 toward unfavorable conditions in their environment. For example, 

 if a slide containing paramecia is heated at one side, the animals 

 will back off from the unfavorable stimulus, then shoot forward 

 until they encounter the heat, then again back off and repeat the 

 operation until they escape from the heated area. 



Trial and error method of 

 a Paramecium. 



