EFFECT OF TONIC CONDITION ON RESPONSE 



57 



in the chamber, the normal irritability was found to be 

 gradually restored. 



In regard to the action 

 of poisons, it is often found 

 that while a moderate dose 

 abolishes all response, a 

 minute dose of the same 

 poison causes an enhance- 

 ment of physiological 

 activity. 



Fig. 13. Depression of Mechanical 

 Response of Mimosa under Ether, 

 and Gradual Recovery on Removal 

 of the Anaesthetic 



Effect of Tonic Con- 

 dition on Response 



We come last of all to 

 the very intricate internal 

 factor which will be de- 

 scribed as the tonic condition of the plant. Under optimum 

 tonic condition, all modes of response exhibit heightened 

 activity, as shown in the mechanical response of Mimosa, in 

 the autonomous pulsations of Desmodium, in the movement 

 of growth and in the rate of photosynthesis. In the sub- 

 tonic condition the various responses are found to exhibit a 

 great depression. 



In regard to this intricate question of the effect of varying 

 tonic conditions on response, a brief summary of the modi- 

 fying effects of age, season and unfavourable condition on 

 different modes of response is given below. 



(a) Effect of age. — In Mimosa the middle-aged leaf 

 counted from the top is found to be most irritable to mechan- 

 ical and to geotropic stimulation. The very young leaf 

 near the apex or the old leaves at the lower end of the stem 

 are relatively inexcitable. Similar differences in activity 

 as modified by age are also observed in the pulsating leaflet 

 of Desmodium. 



(b) Effect after flowering. — The physiological vigour of 

 the plant is greatly lowered after flowering. The mechanical 



