STIMULATION BY LIGHT 4OI 



Experiment 220. — The next experiment was undertaken 

 with Thunbergia, and stronger and more prolonged friction 

 was appHed. The normal rate was 38, which became 

 reversed into — 22 after stimulation. The recovery was 

 nearly complete in the course of an hour. 



Effect of Direct and Indirect Stimulation 



BY Light 



\\ ith regard to the unilateral action of light, Pfeffer 

 summarises the results as follows : ' According to Mohl, 

 Dutrochet, Darwin, and Baranetzky, the circumnutating 

 shoots of climbers are positively heliotropic, but this 

 irritability is so weak as merely to somewhat accelerate 

 circumnutation when the stimulus is applied so as to aid 

 the autonomic movement, and shghtly to retard the latter 

 when acting against it.' ^ The relation to light is, how- 

 ever, more complex than has been supposed. I will first 

 describe the effect of diffuse light acting on all sides of a 

 twisting stem. Two mirrors, suitably inclined, were placed 

 behind the stem, so that the incident sunlight acted on all 

 sides of it. The plants had been previously kept in dark- 

 ness for a short while. 



Experiment 221. Effect of diffuse sunlight. — I describe 

 experiments with two different plants — Porana and Thun- 

 bergia. The former was highly excitable, and the latter 

 slightly subtonic ; Porana was subjected to the stimulus 

 of light for 5 minutes, and Thunbergia for 4 minutes. 



The normal rate of Porana was 41 ; it exhibited an 

 immediate diminution, which reached a minimum of 4, 

 8 minutes after the cessation of exposure to light. The 

 recovery was practically complete after 50 minutes. 



In Thunbergia the normal rate was 34 ; the rate ex- 

 hibited a preliminary enhancement which persisted till 

 the fourth minute, after which retardation set in, the 

 maximum retardation being at the tenth minute when the 

 rate was reduced to 2. After this there was slow recovery, 



^ Pfefier, ibid. p. 41. 



2 D 



