PULSATORY RESPONSE 693 



we have seen that such an organ, when acted on unilaterally 

 by strong light, often exhibits to-and-fro oscillations, due to 

 alternate fatigue of the two sides. 



We thus find, starting with a Desmodium leaflet in a 

 state of standstill, that moderate intensity of light initiates 

 normal pulsatory movements, in which the downstroke is 

 quicker than the upstroke ; but, in a pulsating leaflet, under 

 intense or long-continued light, these beats are reversed, that 

 is to say the upstroke becomes the quicker. Under the 

 action of continuous light, again, these reversals themselves 

 may become periodic or recurrent. 



Reversals under intense stimulation seen in all forms 

 of response. — We have already seen that autonomous pulsa- 

 tion is simply ordinary response repeated, owing to excess of 

 energy ; and that the Desmodium leaflet is an ordinary aniso- 

 tropic organ, in which response may be initiated by any form 

 of stimulation — thermal, photic, chemical, or electrical. We 

 have just seen, further, in the case of photic stimulus, that 

 the character of the response may be reversed by the in- 

 tensity of the stimulation. Thus the normal response, in 

 which the downstroke is more energetic than the upstroke, 

 may be exchanged for a type of response in which the up- 

 stroke is quicker than the down. This law of reversal of 

 response with varying intensity of stimulation has already 

 been shown to be illustrated in different types of response. 

 For example, in the case of chemical stimulation, we found 

 that the leaf of Mimosa when subjected to the action of 

 a dilute solution of sodium chloride gave an erectile response, 

 whereas when the solution was stronger it responded by 

 depression (pp. 551, 552). In the response of growth, again, 

 very dilute and very strong solutions of a given reagent were 

 shown to produce opposite effects. Thus, while dilute solu- 

 tion of sugar accelerated, a very strong solution retarded, the 

 rate of growth (p. 488). 



In the case, then, of an organ which is capable of multiple 

 response, we find that any form of stimulus is competent to 

 initiate such response, and that, with regard to certain charac- 



