112 PLANT RESPONSE 



It will there be seen that, as the resting period was gradually 

 shortened to half a minute, the conductivity underwent di- 

 minution from the normal r88 mm. to i 54 mm. per second, 

 that is to say, by 18 per cent. The reduction of motile 

 excitability was found, however, to be still more marked, 

 the height of response being reduced from the normal thirty- 

 four to one division, that is to say, by as much as 97 per cent. 

 On still further reducing the period of rest, from half a minute 

 to ten seconds, it was found that there was no motile response 

 whatever. The tissue is thus seen to be altogether incapable 

 of response during a certain refractory period. The time- 

 value of this refractory period not only differs in different 

 plants, but also varies with the physiological condition. In 

 BiopJiytum, normally speaking, it is ten seconds, and in 

 Mimosa about one minute. 



Earlier abolition of motile excitability than of con- 

 ductivity. — We have seen in the last experiment, that while 

 the conductivity of the petiole was reduced by 18 per cent, 

 the motile excitability of the attached leaflet underwent a 

 diminution of 97 per cent. This shows that motile excitability 

 disappears earlier than conductivity. The reason will be 

 apparent if we consider the difference between the two 

 expressions of protoplasmic excitation. We have, in the 

 conduction of the state of excitation from point to point, 

 a direct expression of the transmission of the molecular 

 change initiated by stimulus. The motile response, however, 

 is a somewhat remote consequence of the series of events 

 which follows on the fundamental molecular change. Inter- 

 mediate occurrences are the permeability variation and the 

 contraction, in this case differential, which it produces. The 

 movement of the leaf is a result of all of these, and depends 

 for its complete fulfilment on certain favourable circumstances. 

 In any case, there are mechanical obstacles which have to 

 be overcome in forcing the expelled water through channels 

 of escape. That this must involve some degree of waste of 

 force is partly seen in the fact that all excitations do not 

 produce response, it being necessary that the stimulus should 



