PULSATORY RESPONSE 69 1 



apparently autonomous, the application of excessive stimulus 

 of strong light often brings about fatigue, and under these cir- 

 cumstances I have obtained two different types of response, 

 according to the particular condition of the tissue. In the 

 first of these, progressive fatigue, under the continued action 

 of light, is shown by the gradual diminution of amplitude of 

 pulsation. If the strong stimulus of light, which causes this 

 fatigue, be applied for a short time only, the pulsation whose 

 amplitude was diminished, again on the cessation of stimulus 

 recovers its first amplitude in a staircase manner ; but if the 

 light be long continued, fatigue becomes so great that the 

 pulsatory movements are brought to a stop, at least for a 

 considerable period. We have seen again that an organ 

 sometimes exhibits fatigue somewhat differently from this, 

 that is to say in a periodic manner. In accordance with 

 this fact the leaflets of Desmodium are sometimes seen 

 to display periodic fatigue — that is to say, the pulses wax 

 and wane in groups, an example of which is seen in 

 fig. 276. 



Thus we see that in a leaflet at standstill, light, by 

 supplying energy, initiates autonomous movement ; while in 

 an actively pulsating leaflet, strong illumination may bring 

 about such fatigue as to cause cessation of movement At 

 still other times, again, owing to the favourable condition of 

 the tissue, fatigue is slight, and there is no arrest of activity. 

 These "facts are sufficient to explain the discrepancy already 

 mentioned, in the observations made by Strasburger and by 

 Pfeffer, on the action of the leaflets of Trifolium pratense 

 under light. 



Changes induced in existing anisotropy of Desmodium 

 leaflets. — A very interesting consideration arises at this 

 point as to the differential fatigue caused in the anisotropic 

 organ by strong stimulus. We have seen that the lower half 

 of such an organ is the more excitable, hence we can see the 

 probability of fatigue being relatively greater there. Now, it 

 is the naturally greater excitatory contraction of the lower 

 half of the organ which causes the downstroke of the leaflet 



