ADJUSTMENT TO GRAVITY, CONTACT, AND SHOCK 143 



of leaflet and leaf. The pulviniis consists chiefly of turgid paren- 

 chyma, surrounding a fibrovascular bundle sheathed in coUen- 

 chyma. In consequence of a stimulus, the protoplasm of the 

 cells upon the lower side of the leaf pulvinus contracts, forcing 

 a small amount of cell-sap out into the air spaces. This destroys 

 the turgidity of the lower cortex, and at the same time shortens 

 the cells. This releases the tension upon the cells of the upper 

 side, and allows them to expand, thus causing the leaf to bend 

 downwards. This action is emphasized by the pull of the leaf 

 itself. In the pulvini of the leaflets, the contraction must occur 

 upon the upper side, allowing the lower cortex to expand, and to 

 raise the leaflets in direct opposition to gravity. After a response, 

 the contractile cells gradually absorb the excreted water and 

 regain their turgidity, thus restoring leaf and leaflets to their 

 normal position. 



Experiment 46. Response to shock. Grow several plants of Mimosa. 

 Make various experiments to ascertain what regions are sensitive, as 

 well as the strength of stimulus necessary to produce a response. Strike 

 the end leaflets of two leaves at the same time, one forcibly, the other 

 gently, and note the time necessary for the transmission of the impulse 

 to the pulvinus of each leaf. Note when leaves regain the normal posi- 

 tion, the effect of repeated blows upon a leaf that has reacted, and 

 how long a time it requires to resume its normal condition. 



