220 PLANT RESPONSE 



remaining unaffected. And, finally, the excitability of the 

 motor region may be depressed by certain agencies, and 

 the stimulation, initiated at the receptive point, and trans- 

 mitted through the intervening conducting channels, will 

 nevertheless fail to find expression. We shall next proceed 

 to demonstrate experimentally the influence of various 

 agencies on the receptivity, on the conductivity, and on the 

 excitability of the tissue. 



Variation of motile excitability : (a) Under ancestJietics. — 

 First we shall take the variation of excitability in the motor 

 region. Let us then select a leaf of Biophytum and apply 

 ether to the two terminal pairs of leaflets beyond D. 

 Thermal stimulus is then applied at x , by touching with a 



hot wire (fig. 96). As the 

 ^^^ receptivity of the point of 

 application, and the con- 

 ductivity of the intervening 

 Fig. 96. Effect of Ether in the Abolition ..;..„„ remain iini'mnaired 

 of Motile Excitability tissue, remam unimpaired. 



Ether is applied to the two pairs of leaflets the excitatory disturbance 



to the right of D ; stimulus is applied prOCCeds ill the normal 

 at X . Excitation travels up to D, and , 



cannot pass beyond. manner to the point D, a 



fact seen by the successive 

 depressions of the leaflets. Owing, however, to the abolition 

 of their excitability, the last two pairs remain unaffected. 



A similar loss of excitability, due to the action of ether, 

 may be demonstrated in Mimosa. On taking a stem pro- 

 vided with three motile leaves. A, B, and C, the pulvinus of B 

 is touched with ether, and thermal stimulus is applied 

 between A and B. The excitation is transmitted in both 

 directions, up and down, as seen by the fall of the leaves A 

 and C. But the intermediate leaf B fails to respond, showing 

 that its excitability has been abolished by the ether. 



[b) By effect of cold. — The prolonged application of 

 cold, also, will produce, as would be expected, molecular 

 sluggishness, with consequent loss of motor excitability. 

 This may be shown by touching the small pulvinus of a 

 leaflet of Biophytum with ice- water. If stimulus now be 



