CHARACTER OF TRANSMITTED IMPULSE 165 



are fulfilled by thermic and electric modes of stimulation. 

 I will now describe two typical experiments on electrotonic 

 block, in Biophytum under thermal stimulation and in 

 Mimosa under electric stimulation. 



Biophytum. — In this plant we have a whorl of leaves 

 bearing sensitive leaflets. Stimulus is applied on the 

 stem by means of the electro-thermic stimulator. The 

 intensity of stimulus is so graduated as to cause an excitatory 

 impulse to traverse the petioles, effecting the fall of the 

 leaflets in a centrifugal order. Selecting a leaf, electrotonic 

 block is applied in the middle part of the petiole. When 

 the anode A is to the left, the 

 excitatory impulse is found ar- 

 rested at A ; when the current 

 is reversed, the arrest is found to 

 take place at the new anode a' 

 to the right. On the stoppage of 

 the blocking current the excitatory '^ ir p 



impulse is observed to traverse the 



I. 1 1 it, r 4.U 1 x Fig. 91.— Arrangement of 



whole length of the leaf. the electrotonic block. 



Mimosa. — In the next series Polarising circuit p inter- 



of experiments a different species P°^^^^ ^"^^^'^ .r''*'"f 



^ ^ secondary circuit s and 



of plant and a different testing- responding puivinus. 



stimulus is selected. The employ- The polarising current 



, r 1 i • 1 T r acts as a block, whether 



ment of an electrical mode of ascending or descending. 



stimulation completely obviates 



the difliculty of securing a stimulus the intensity of 

 which may be either maintained constant or increased 

 in a known manner. With the help of a sliding induc- 

 tion-coil it is easy to arrive at an intensity of stimulus 

 which is always effective in normal circumstances. The 

 proximal of the two exciting electrodes was placed at a 

 distance of 30 mm. from the primary puivinus. Half-way 

 between the point of excitation and the puivinus were 

 placed two polarising electrodes, 5 mm. apart, through which 

 a constant current could be maintained, for the purpose 

 of serving as a block (fig. 91). The first and uppermost 



