252 TEXTBOOK OP BOTANY 



A change from light to darkness is also a stimulus. If the 

 room or box in which a sensitive plant stands is darkened, 

 the leaflets of the plant fold and its leaves droop, just as 

 though some or all of the leaflets had been struck. If light 

 is now admitted, the leaves gradually straighten up and the 

 leaflets unfold ; that is, the plant recovers from its stimulated 

 condition. If a sensitive plant is placed for some minutes 

 in an atmosphere saturated with chloroform, it loses the 

 power to respond to stimuli of any sort, just as an animal 

 does when it is chloroformed. Like the animal, the plant 

 may return in time to its normal condition if the chloroform 

 is removed ; but if the plant is exposed to chloroform too 

 long, it will die. 



We have seen that a leaf -stalk is usually somewhat thick- 

 ened at its base ; the thickening or cushion is especially 

 marked in the leaves of members of the pulse family, to which 

 the sensitive plant belongs. The cushion is usually lighter 

 green in color than the rest of the leaf-stalk. There is a 

 similar, though smaller, cushion at the base of each primary 

 leaflet, and a small one also at the base of each secondary 

 leaflet. When we touch a secondary leaflet, we may ob- 

 serve that the upward movement which brings this leaflet 

 face to face with the one opposite is due to a bending in the 

 cushion at its base ; and the drooping of the whole leaf is 

 brought about by a bending in the cushion at the base of the 

 leaf -stalk. These cushions, then, act somewhat like hinges ; 

 and the movements of the different parts of the leaf result 

 from changes that occur in the cushions when the leaf or 

 one of its parts is stimulated. The change that takes place 

 in one of these cushions at such a time results from a change 

 in the amount of water present in some of its cells. 



263. " Sleep Movements." Although other plants of 

 the pulse family have similar cushions at the bases of their 

 leaflets and of their leaves, very few of them respond to 

 stimuli by such rapid movements. However, many plants 



