transmission of impulse 121 



Transverse Transmission of Impulse 



Of greater importance is the transverse transmission 

 of impulse from the proximal to the distal side. It is 

 obvious that the conductivity across the stem must be very 

 much less than that along its length. Hence application of 

 stimulus to the stem of Mimosa diametrically opposite to the 

 indicating leaf, is equivalent to indirect stimulation of that 

 leaf. I will describe the effects of photic, electric, and 

 radio-thermal stimulation which result from the transmission 

 of impulse across the semi-conducting stem. 



Experiment 70. Transverse transmission of impulse 

 under photic stimulation. — A narrow beam from a small arc- 

 lamp was made to fall on the stem, at a point diametrically 

 opposite to a motile leaf which was attached to the recording 

 lever, the successive dots in the record being at intervals 

 of a second. Stimulation by light caused a positive or 

 erectile movement of the leaf indicator within 5 seconds of 

 apphcation. The positive response affords conclusive proof 

 of the induction of increase of turgor at the distal point of 

 the stem to which the leaf was attached. When the stimulus 

 is moderate or of short duration the response remains 

 positive. But under strong or prolonged stimulation the 

 leaf falls, showing that the slower excitatory negative 

 impulse is conducted to the distal point (fig. 65). It will 

 be noted that the fall is slow at the beginning ; it then 

 becomes suddenly rapid, as indicated by a scratch-line 

 instead of a dot in the record. 



The excitatory impulse reached the motile pulvinus 

 35 seconds after the initiation of the positive response. The 

 velocity of the negative impulse, it should be remembered, 

 depends on the intensity of the stimulus ; the stimulus 

 was moderately strong and the stem was thin, only 2 mm. 

 in diameter. The velocity of the excitatory impulse in the 

 transverse direction in this case was 0-05 mm. per second. 



Experiment 71. Transverse transmission under electric 

 stimulation. —In order to show that the effects described 



