84 CHAP. IX. GROWTH IN SUBTONIC PLANTS 



a moving plate. Exposure to strong light from an arc- 

 lamp for 4 minutes was, however, effective in reviving the 

 growth, as indicated by the erection of the curve (fig. 40). 

 In a vigorous specimen of Allium, on the other hand, the 

 effect of light has been shown to be a retardation of growth 

 Icf. fig. Z7)- ^^^^ results offer conclusive evidence that the 

 sign of response, negative or positive, is dependent on the 

 tonic condition of the organ. 



Fig. 40. Stimulus reviving growth at standstill (Allium). 



The next problem to be considered is whether the 

 acceleration of growth in a subtonic organ under light is 

 due to its action as a stimulus, or to its possible photo- 

 synthetic effect. The experimental specimens usually 

 employed were flower-stalks or stems in which chlorophyll 

 was absent. The question can, however, be finally settled 

 by finding whether a different form of stimulus, such as 

 a tetanising electric current, which cannot possibly exert 

 any photosynthetic action, also induces acceleration of 

 growth in a subtonic organ. 



