MOVEMENTS OF VARIATION 



321 



thetizes Mimosa and causes the plant to lose its power of reaction for some 

 time. 



The leaves of many other legumes, as well as those of some species of Oxalis, 

 also respond to stimuli much as does Mimosa, but their sensitiveness is not 

 nearly so pronounced. 



Fig. 159. — Epidermal cell of staminal filament of Opuntia vulgaris, showing a contact papilla. 



(After Haberlandl.) 



Filaments of the Cynareae {Centaurea jacea, for example) and some other 

 groups of plants also respond to contact stimuli. They contract when weak 

 pressure is applied, the shortening being accompanied, as in the Mimosa pul- 

 vinus, by an extrusion of water into the intercellular spaces. The epidermal 



Fig. 160.- 



1 



-Two branches of Desmodium gyrans. A, in day position; B, in night position. 



(After Darwin.) 



cells of the filaments have specially sensitive papillae with very thin cell walls, 

 into which the protoplasm extends (Fig. 159). These papillae are the sensi- 

 tive organs that receive the stimuli. 



The fully mature leaves of many plants take a different position during the 

 day from that assumed at night. Their leaflets approach each other at night 



21 



