TRANSMISSION OF EXCITATORY WAVES IN PLANTS 253 



(c) Ordinary Plants 



It will thus be seen that the velocity of transmission in 

 conducting plant-tissue is not very different from that in the 

 conducting tissue of certain animals. 



Summary 



Successive determinations of velocity of transmission are 

 consistent when the stimuli are uniform, and when inter- 

 vening periods of rest, sufficient for complete protoplasmic 

 recovery, are allowed. 



Velocities of transmission are not the same in centripetal 

 and centrifugal directions. In Biophytum, for example, the 

 centrifugal velocity is greater than the centripetal. 



When a given point in a plant-tissue is gradually raised 

 in excitability, the consequent excitatory discharge takes 

 place preferentially in one direction. 



Conductivity and excitability are both diminished by the 

 increasing fatigue consequent on shortened intervals of rest. 

 When the resting-period is shortened below a certain critical 

 interval, the motile organ proves ' refractory ' to further 

 stimulus. 



The velocity of transmission is not the same for all 

 intensities, but increases with increasing stimulation. 



The velocity of transmission is diminished by lowering, 

 and increased by raising, of temperature. 



The fibro-vascular elements are the best channels for con- 

 duction of stimulus : in them, the transmission lengthwise is 

 greater than crosswise. Indifferent parenchymatous tissue 

 has little or no power of conducting stimulus. 



The velocity of transmission in plants is not of an 

 altogether different order of magnitude from that in certain 

 animal tissues. 



