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 

 impulse is observed to traverse the 

 whole length of the leaf. 



Mimosa. — In the next series 

 of experiments a different species 

 of plant and a different testing- 

 stimulus is selected. The employ- 

 ment of an electrical mode of 

 stimulation completely obviates 

 the difficulty 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 pulvinus. Half-way 

 between the point of excitation and the pulvinus 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 



Fig. 91. — Arrangement of 

 the electrotonic block. 

 Polarising circuit p inter- 

 posed between exciting 

 secondary circuit s and 

 responding pulvinus. 

 The polarising current 

 acts as a block, whether 

 ascending or descending. 



