MOVEMENT IN PLANTS. 249 



cut off the leaf and set it in a bottle of water. Place this 

 in a dish containing a little chloroform, or ether ; cover the 

 whole with a bell-glass and set it in strong light. In a 

 few minutes the leaflets expand, but they will not now 

 respond to stimuli try the effect of a lighted match, 

 striking the leaflets, etc. Remove the chloroform, and 

 note that the anaesthesia soon passes off, the leaf recover- 

 ing its power of reacting to stimuli. 



349. Effect of continued Darkness. We have seen 

 that continued stimulation causes loss of power to react 

 to stimuli. Take two pot plants of which A is to be kept 

 under normal conditions of illumination, being set in a 

 good light during the daytime. Set B on a plate of wet 

 sand or sawdust, cover it with a box to exclude light, and 

 keep it at a temperature of about 20 C. For a few days 

 the leaves perform periodic movements, expanding during 

 the day and closing at night, though in constant darkness, 

 but in about five days these movements stop. 



Now expose the plant to light for a few minutes, then 

 replace it in darkness : no movements occur the plant 

 has passed into a state of darkness-rigor, and in this 

 state it is no longer sensitive even to mechanical stimuli. 

 Now set both plants in front of a window for a few hours, 

 then place them both in darkness : the leaves of A close 

 up, those of B remain expanded. After about half an 

 hour, set both plants in the light again for the rest of the 

 day : B gradually recovers its phototonus, or power to 

 react to changes in illumination, but its sensitiveness to 

 mechanical stimulation does not return until later. 



350. Mechanism of Movement in Mimosa. Careful obser- 

 vation has shown that the upper half of the pulvinus shows a slight 

 increase in volume during the downward movement of the petiole, 

 while the lower half shows a marked decrease in volume. As the 

 movement is made just as well when the epidermis is removed, and 

 the passive veins need not be considered, this reduction in volume 

 must be due to contraction of the parenchyma on the lower side of 

 the pulvinus. 



Cut across the petiole a little above the pulvinus, and set the 

 plant in saturated air under a bell-glass to recover ; after stimula- 

 tion, liquid is seen to ooze from the cut surface, having been 



