228 PLANT RESPONSE 



temperatures. This distinction between the effects of conduc- 

 tivity and of excitability is especially important, since by its 

 means we are enabled to explain certain facts apparently 

 anomalous, which seem at first sight to lend support to the 

 hydro -mechanical theory of excitation. I have shown that, 

 under normal conditions, the intensity of excitation must 

 exceed a certain value before it can be manifested as 

 mechanical response. I have also shown that under un- 

 favourable circumstances, motile excitability is abolished 

 earlier than conductivity. An excited tissue may thus 

 conduct stimulus, without itself exhibiting any motile indica- 

 tion. Numerous examples of such a state of things may be 

 cited. It must be borne in mind that the mechanical 

 indication of the state of excitation can be afforded by a 

 pulvinated organ, only when there is some difference of 

 excitability as between its upper and lower halves. If this 

 difference of excitability be in any manner reduced or 

 diminished, there will be a failure of the mechanical response. 

 In old leaves of BiopJiytum, for example, not only is the 

 general excitability diminished, but the differential excitability 

 also has disappeared. Hence, excitation of such leaves gives 

 rise to no local excitatory response of the leaflets. But that 

 the leaf is still nevertheless excitable, and can transmit that 

 state of excitation, is shown by the fact that on stimulating 

 it strongly, the leaflets of younger leaves at a distance are, 

 after a time, seen to be depressed in serial succession. This 

 proves that, though unable itself to give the motile indication, 

 the leaf was capable of receiving and transmitting the state 

 of excitation. Similarly, it may be shown that a tissue whose 

 motile excitability is temporarily abolished, by, say, the 

 application of ether, may, nevertheless, be the conductor of 

 stimulation. 



In order to demonstrate this, let us take a plant of 

 Hiopliytum, and expose some of the leaflets of a particular 

 leaf to ether-vapour. Strong stimulation of that portion of 

 the petiole which bears them, will now fail to induce move- 

 ment of the leaflet in the etherised region ; but the excitation 



