*7 
The Analysis of Geotropism. 
to be freed from further geotropic action. This is in fact the 
method for determining the presentation time, i.e., the period of 
excitation needed to produce by after-effect a distinct curve. 
It was found that plant A had a much longer presentation 
time than B, that is to say the intermittent stimulation had to be 
kept going for a time longer than the exposure to continuous 
stimulus. But when the individual periods during which A had 
been stimulated were added together their sum was found to equal 
the presentation time of B. Thus if the presentation time of B 
was twenty minutes, then A would have to remain for forty minutes 
on an intermittent klinostat which gave equal intervals of rest and 
stimulation. Only one half of the forty minutes is made up of 
actual stimulation, so that the presentation time is really twenty 
minutes. It may also be made up of two exposures of ten minutes 
each by using a klinostat rotating once in 20°, or of twenty 
exposures of 1° each with a klinostat rotating in two minutes. 
The fact that, in the case of geotropism, intermittent stimulation 
is a simple case of addition, has interesting bearings. If a series 
of short exposures is equal to the effect of a single long exposure, 
it is certain that each of the short exposures must have set going 
part of the motor machinery,—a conclusion which has already been 
referred to, but without the experimental evidence here given. 
Fitting gives an interesting discussion on the relation between 
the times of reaction and relaxation. This is a point well worthy 
of attention, but it is one on which want of time forbids our 
entering. 
