RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 575 



Phenomenon of oscillation under long-continued 

 stimulation.— We have seen, in Chapter XXIV., that 

 when a tissue is subjected to continuous stimulation, so 

 that it becomes possessed of excess of energy, its response 

 becomes pulsatory in consequence of periodic variations of 

 its excitability. The exhibition of this variation of excita- 

 bility we have, for the sake of convenience, designated as 

 periodic fatigue. The occurrence of autonomous response is 

 intimately connected with this periodic variation. In some 

 tissues it may be exhibited only once, while in others it may 

 be repeated indefinitely. For example, in Mimosa^ under 

 continuous stimulation, we see a single complete pulsation, 

 consisting of the contractile response, 

 and the subsequent fatigue-relaxation, 

 which looks like recovery (fig. 59). 

 But, in the longitudinal response of 

 the style of Uriclis Lily to continuous 

 stimulus, we observed two pulsations 

 (fig. 58). And in Biophylum and 

 Desmodium we see these repeated 

 indefinitely, constituting what we Fig. 236. Oscillatory Re- 



sponse of Arsenic acted 

 know as multiple or autonomous on Continuously by 



response. Hertzian Radiation 



It is very interesting to note, in ^^^^^ 

 connection with this, that even in- 

 organic substances, under continuous stimulation, exhibit 

 this pulsatory response. I give here a record (fig. 236) 

 of the oscillatory response of arsenic when acted on con- 

 tinuously by Hertzian radiation. 



Or we may view this induction of multiple response in a 

 radial vegetable organ, again, from a different standpoint. 

 By the action of stimulus, it is easy to see that antagonistic 

 hydrostatic actions may be set up as between the excited 

 organ and the rest of the plant. Thus, the direct effect of 

 continuous stimulation is contraction, with increasing expul- 

 sion of water from the excited organ into the rest of the 

 plant. The hydrostatic reaction which this will induce in the 



