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. 



-These plants are remarkable for the 

 Mimosa pudica is an excellent plant 



207. Sensitive plants. - 



rapid response to stimuli, 

 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- 

 v lar manner, and 



Fig. 80. 

 Sensitive-plant leaf 

 in normal position. 



Fig. 82. 



Fig. Si. 

 Pinna? fold- 

 ing up after 

 stimulus. 



soon the pinnse approximate each other and 

 the leaf then drops downward as shown in , ,, n 



fig. 82. The normal position of the leaf is *°Wed 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 

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

 junction of the pinna? with the petiole, and a larger one at the 

 junction of the petiole with the stem. We will also note that 

 the movement resides in these cushions. 



