550 LECTURE XX. 



diminished since so powerful a stimulus was needed to make 

 it close. 



Bert seems to be mistaken in asserting that under the 

 influence of chloroform a stimulated leaf of Mimosa is arrested 

 in the position, which it assumes on stimulation, for Pfeffer, 

 on repeating the experiment, found that under these circum- 

 stances, the primary petiole slowly rises although the leaf may 

 have become quite insensible to touch under the influence of 

 chloroform. 



Another means by which irritability is diminished or 

 abolished is repeated stimulation. This has been observed 

 by Bert and by Pfeffer in the case of Mimosa pudica. When 

 the primary petiole is first stimulated it sinks, and then rises, 

 although the stimulation is continued, quite as rapidly as 

 under normal conditions. The pulvinus is, however, now 

 insensible, and a fall of the petiole on stimulation can only be 

 induced after a period of rest of from five to fifteen minutes. 

 The interval between the stimuli, in order that the condition 

 of fatigue may be thus induced, must be very short, the 

 shorter the more irritable the condition of the plant: with 

 imperfectly irritable plants Pfeffer found that the interval 

 might be as long as two minutes. If, however, a sufficient 

 time be allowed to elapse between each stimulation to permit 

 the leaf to regain its irritable condition, it appears that stimu- 

 lation an indefinite number of times will be followed by 

 movement. 



The effect of anaesthetics or of repeated stimulation in 

 destroying irritability are purely local. When one leaf of a 

 Mimosa is exposed to the action of chloroform or ether, that 

 leaf alone is affected, the other leaves remaining irritable, and 

 this is true also of the repeated stimulation of one leaf. 

 Burdon-Sanderson states that when in consequence of re- 

 peated stimulation any one of the sensitive hairs on the leaf 

 of Dionaea has lost its irritability, the movements of the leaf 

 may be induced by stimulation of another hair. 



In some cases not only is irritability destroyed by these 

 conditions but, as mentioned above, the organ passes into a 

 completely immotile state under prolonged exposure to them. 



