82 CHAP. IX. GROWTH IN SUBTONIC PLANTS 



the rate of growth is very rapid, while in the subtonic 

 growth is feeble or even arrested. There are all possible 

 gradations between the two extremes. 



The responsive reaction depends then on two important 

 factors : the tonic condition of the organ and the effective 

 intensity of stimulation. The effectiveness of stimulation 

 is determined, not only by its intensity, but also by its 

 duration. 



These theoretical conclusions mav be summarised thus : 



1. A subminimal stimulus is gradually transformed into 

 a minimal and then to a maximal, under prolonged duration 

 of stimulation. 



2. A strong stimulus may prove to be subminimal for 

 a short time at the moment of its application, specialty 

 when the organ is in a slightly subtonic condition. 



The possible combinations of the effects of these varying 

 factors are very numerous, and it is therefore necessary to 

 confine attention to the typical cases which, from the 

 theoretical point of view, may be expected to give the 

 positive response of enhanced rate of growth : 



First, a normal organ under short exposure to a sub- 

 minimal stimulus ; and 



Secondly, a subtonic organ under moderate and not 

 excessively prolonged stimulation. 



Enhanced Rate of Growth under Subminimal 



Stimulation 



Experiment 47. — The abnormal positive response was 

 obtained even with a moderately vigorous specimen, when 

 the intensity of the incident light was feeble, as seen in the 

 record of growth taken on a moving plate (fig. 39). The 

 slope of the first part of the curve show^s the normal rate ; 

 stimulus was applied for a short time at the fifth dot, 

 and the sudden erection of the curve demonstrates the 

 enhanced rate of growth. This persisted for a time, after 

 which the rate returned to the normal. Continued exposure 



