RESPONSES OF THE SIMPLEST ANIMALS 



425 



contain thin- walled cells filled with water, and the position of the 

 leaf probably depends on the relative amount of water in these 

 cells. The more rapid movements of the opening and closing of 

 flower petals ; the changes in position of leaflets of the pea, clover, 

 alfalfa, oxalis, and 

 other plants at night 

 and in the morning; 

 and the relatively 

 rapid response of the 

 leaves of the sensitive 

 plant to outside stimuli 

 are all explained by 

 changes in the water 

 content of the cells in 

 the pulvini, or by rapid 

 and temporary fluc- 

 tuations in growth on 



Opposite Sides Of the Whatis thepulvinus? 



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

 stimuli may 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. 



What use is it to a leaf ? 



Demonstration 3. To show responses of Paramecium. 



Place a drop of Paramecia culture in a grooved slide. At intervals, 

 heat the water at one end of the slide by introducing a hot needle into 

 it. Note the actions of the Paramecium as the water becomes warmer. 

 Single out one Paramecium and make a diagram showing exactly how 

 it gets away from the heated area. This reaction is known as the 

 " avoiding reaction." How does a Paramecium escape from an 

 unfavorable environment ? 



Responses of the simplest animals. We have already seen 

 that amoebas and Paramecia seem to respond to the presence of 

 food. Examination of a drop of hay infusion containing Para- 

 mecia will show many collected around masses of food, indicating 



