222 



PLANT RESPONSE 



Fig. 97. Experimental Demonstration 

 of Effects of Cold and Anaesthetics 

 in Abolishing Conductivity 



Cold or ether applied at E ; stimulus 

 at x cannot be transmitted across E, 

 and there is no effect on the 

 motile leaflets. 



velocity of transmission was reduced to 1*3 mm. per 

 second, or nearly to one-third of its original value (p. 249) ; 

 a still greater application of cold produces a temporary 

 abolition of conductivity. This may be shown by touching 



a given portion, E, of the pe- 

 tiole with ice, when moderate 

 stimulus applied below such 

 a point will not be transmitted 

 across the lethargic area, and 

 the motile leaflets beyond will 

 remain unaffected. The nor- 

 mal conductivity will, how- 

 ever, be restored when the 

 tissue regains the temperature 

 of the surrounding atmo- 

 sphere, and a second similar 

 application of stimulus will then be found to be conducted 

 to the motile leaflets, producing successive depressions 



(fig. 97)- 



(b) By rise of temperature. — We have seen how, in 

 consequence of the molecular sluggishness induced by cold, 

 the conductivity of the tissue is lowered. A rise of tempera- 

 ture might therefore be expected, by increasing molecular 

 mobility, to enhance the conducting power. That this is 

 the case is shown in detail in Chapter XX. In a leaf of 

 Biofikytum, for instance, it was found that a velocity of 37 

 mm. per second at 30 C. was increased at 35 C. to 7^4 mm., 

 and at 37 C. to 9*1 mm. per second. Thus, by a rise of 

 temperature of from 30 C. to 37 C. the conductivity of the 

 tissue was increased to nearly three times its initial value. 



(c) By effect of fatigue. — We have already seen (p. in) 

 that motile response, and the transmission of excitation, are 

 both alike expressions of the protoplasmic changes induced 

 by stimulus. We there saw also that just as fatigue of 

 motile excitability was exhibited by diminished motile 

 response, so too a diminished speed of transmission exhibits 

 fatigue of conductivity. An experiment will be described 



