424 MAN BECOMES THE CONQUEROR OF THE WORLD 



T. F. 6. The amoeba has no special organs of sense. 



T. F. 7. If touched, the leaves of sensitive plants show thermo- 

 tropism. 



T. F. 8. The responses of simple animals to stimuli are always 

 positive. 



PROBLEM II. HOW DO SIMPLE PLANTS AND ANIMALS 

 RESPOND TO STIMULI? 



Demonstration 2. To show the use of the pulvinus to a plant,. 



Stain a longitudinal section of a bean stem to show the pulvinus. 1 

 What might be the use of it ? How is it able to do that ? 



The mechanism of responses in plants. Some of the results 

 of responses are easily seen in plants, but the method by which 

 the responses are brought about is not so easy to see. For example, 

 we say leaves place themselves so as to get as much light as possible. 



But this movement is different 

 from that found in animals 

 which have an internal skele- 

 ton with muscles attached. 

 The changes in position in 

 parts of plants are often pro- 

 duced by a more rapid growth 

 of the cells on one side of a 

 structure than on the other, 

 this growth having been ex- 

 cited by an external stimulus, 

 such as gravity, water, light, 

 or heat. Such are the curving 

 movements of roots or stems. 

 The turning of the leaves in a horizontal position is brought about 

 by the more rapid growth of tissues on one side of the leaf stalk 

 than the other. 



Changes in the position of leaves are often brought about by 

 special structures at the base of the petiole, as may be seen in the 

 bean plant. These structures, called pulvini 1 (sing, pulvinus), 



1 Pulvinus (pul-vTn#s) : cushionlike enlargement of petiole at its point of inser- 

 tion qn the sterol 



Clover leaf. (1) In the morning. (2) In the 

 evening. Explain the difference. 



