IRRITABILITY. 



89 



up like a wire coil spring, thus drawing the leaf or branch from which the 

 tendril grows closer to the object of support. This coil between the object 

 of support and the plant is also very important in easing up the plant when 

 subject to violent gusts of wind which might tear the plant from its support 

 were it not for the yielding and springing motion of this coil. 



207. Sensitive plants. — These plants are remarkable for the 

 rapid response to stimuli. Mimosa pudica is an excellent plant 

 to study for this purpose. 



208. Movement in response to stimuli. — If we pinch with 

 the forceps one of the terminal leaflets, or tap it with a pencil, 

 the two end leaflets fold above the "vein" of the pinna. This 



is immediately followed 

 by the movement of the 

 next pair, and so on as 

 shown in fig. 81, until all 

 the leaflets on this pinna 

 are closed, then the stimu- 

 lus travels down the 

 other pinnae in a simi- 

 _ lar manner, and 



Fig. 80. 

 Sensitive-plant leaf 

 in normal position. 



Fig. Si. 

 Pinnse fold- 

 ing up after 

 stimulus. 



soon the pinnee approximate each other and 

 the leaf then drops downward as shown in ^^^^ ^,j ^,^^ ^.^^^ 

 fig. 82. The normal position of the leaf is fo'^ed and leaf drooped, 

 shown in fig. 80. If we jar the plant by striking it or by jarring 

 the pot in which it is grown all the leaves quickly collapse into 

 the position shown in fig. 82. If we examine the leaf now we 

 see minute cushions at the base of each leaflet, at the junction of 

 the pinnse with the petiole, and a larger one at the junction of 

 the petiole with the stem. 'We shall also note that the move- 

 ment resides in these cushions. 



Fig. S2. 



