PLANT IRRITABILITY 175 



Then place it in a dark closet for half an hour and again 

 observe it. Notice the difference. If the plant is growing 

 in the garden, study it by day and again by night, using 

 a lantern. If the plant is large-leaved, like the locust, be 

 careful to note just where the bending or folding of parts 

 occurs. 



Note. Many plants have a swelling at the leaf base or petiole 

 foot. This organ is known as a pulvinus. 



Movements of such plants are thought to be caused by variation 

 of sap pressure in the pulvini. 



Suggestion. Go into the field by night and study plants 

 by the aid of a lantern. Many of the most familiar plants 

 will be scarcely recognizable when seen in their sleeping 

 condition. 



158. RESPONSE TO CONTACT 



Object. To learn something of the behavior of sensitive 

 plants. 



Apparatus. The best plant for this purpose is the greenhouse 

 sensitive plant, Mimosa pudica, but the wild sensitive plant, Cassia 

 nictitans, and the sensitive joint vetch may be used. The sundew 

 and the Venus' s-flytrap are too rare to be available. 



Method. Stroke a leaf lightly with a brush or the 

 ringer. Wait a minute and note the result. What happens? 



If the mimosa is used, a very interesting result follows 

 if the terminal leaflet is pinched. 



Does the motion follow instantaneously like a reflex ac- 

 tion in animals, or does it spread gradually through the leaf? 

 Look for pulvini. Do you see any explanation of these 

 phenomena? 



Conclusion. State what has been learned from this 

 experiment. 



