MOVEMENTS OF VARIATION 



319 



four pinnate leaflets are palmately attached; each of these leaflets consists, in 

 turn, of a secondary petiole and rachis, which bears a large number of small 

 leaflets of the third order (Fig. 158). The main petiole bears at its base a well- 

 developed cushion or pulvinus, and organs of this kind occur also at the bases of 

 the petioles of the leaflets of the second and third orders. A very slight touch 

 upon the largest pulvinus is enough to cause the primary petiole to fall, and the 

 leaflets of the third order to become erect with the upper surfaces of each pair of 



Fig. 158. — Leaves of Mimosa pudica. A, normal position; B, after stimulation. 



Pfeffer.) 



{After 



opposite leaflets against each other (Fig. 158, B). If the stimulus is strong 

 enough it is propagated through the stem to the other leaves of the plant, both 

 above and below, and these also fall and fold together. After a time the leaves 

 gradually re-expand and regain their earlier positions. These phenomena occur 

 in completely mature leaves, and they are entirely independent of growth. 



Observations relating to details of the response here considered have shown 

 that the movements of the leaf are caused by changes in the form of the pulvinus. 



