320 



PLANT RESPONSE 



of internal hydrostatic pressure in shifting the vibration-Hmits 

 of the Desjnodhun leaflet. 



It will be remembered that in the case of Mimosa the 

 effect of this increased internal hydrostatic pressure was to 

 cause the erection of the leaf above its normal position. If, 

 then, the same result should follow in the case of Desuwdiujti, 

 we should be justified in inferring that the mechanics of the 

 motile organ were similar in the 

 two cases. The increase of in- 

 ternal hydrostatic pressure was 

 in this case effected by the same 

 method as was employed in that 

 of Mimosa — that is to say, the 

 cut end of the petiole of Des- 

 modiiim was placed in one limb 

 of a U-tube, filled with water, and 

 the pressure was increased by 

 adding water, so as to raise the 

 level at the free end of the tube. 

 As the leaflet of Desmodium, how- 

 ever, is in constant oscillation, 

 we must regard the mean of its 

 vibration-limits as the normal 

 mean, and the extent of the 

 leaflet's erection under increase 

 of hydrostatic pressure must be 

 measured by the shifting upwards 

 of this mean. 



In carrying out this experi- 

 ment, I found, on applying the 

 increase of internal pressure exerted by a water-column of 

 lo cm., that, as will be seen in fig. 128, the lower limit 

 of oscillation was displaced upwards in the record by 

 11-5 mm. and the upper limit by 3'5 mm. The mean 

 position was thus raised by 7-5 mm. As the magnification 

 of the record was in this particular case five times, it will 

 be seen that the pressure exerted by a height of 10 cm. of 



Fig. 128. Displacement of Mean 

 Position of Vibration of Des- 

 7nodium Leaflet by Increased 

 Internal Hydrostatic Pressure 



U, upper limit ; 1,, lower limit ; 

 M, mean position under normal 

 conditions ; u', l', m', corre- 

 sponding positions under in- 

 creased internal hydrostatic 

 pressure ; M m' is the extent 

 of displacement upwards. In- 

 creased internal pressure, gene- 

 rally speaking, produces a 

 movement towards expansion, 

 and tends to diminish the 

 amplitude of the pulse, wliile 

 increasing the frequency. 



