462 
PHYSIOLOGY 
10 minutes and 48 minutes. The longer the stimulation, other things 
being equal, ,the more marked the curvature ; from which it is evident 
that there is an increase of the excitation with continued stimulation, 
and thereby the end reaction becomes more marked. 
Summation. Contrariwise, it should be expected that stimulation 
too short to result in curvature would not be without effect. That it 
does produce excitation is shown by the fac~ that if a plant be placed alter- 
nately horizontal and erect, each period of stimulation being shorter 
than the presentation time for that particular plant, and the interval of 
rest shorter than is needed for recovery, curvature will finally occur. 
Evidently this is a cumulative effect; yet it is not a summation of the 
total successive excitations that occur during the times of horizontality, 
A but only of the re- 
, - -^ ^ -~ - ^ sidual excitation. 
For, if a suitable 
plant be placed 
horizontal for 30 
minutes continu- 
ously, the reaction 
curvature is more 
pronounced than 
if it be so placed 
for ten 3-minute periods at lo-minute intervals. Clearly, while erect, 
the preceding excitation is slowly disappearing, and if the interval before 
the next stimulation is too long, recovery will be complete and no 
evidence of the excitation will appear in the form of curvature. 1 In 
such experiments, therefore, it is necessary to apportion properly the 
intervals of rest and stimulation. 
Rotation. From the above considerations it will be evident that when 
a plant is rotated in the horizontal plane' on a clinostat, its failure to exe- 
cute any curvature is not at all due to a lack of excitation, for while the 
side a of the stem is passing through quadrant A of its rotation (fig. 694), 
quadrants a and c are under stimulation almost as though for a corre- 
sponding time the stem were at rest. But these sides remain under stimu- 
lation for less than the presentation time and so the excitation does not 
suffice for the end reaction. When side a has passed into quadrant C 
1 It has been suggested that during the periods of no stimulation a counter-excitation is 
set up; but simple recovery from excitation seems sufficient to account for all the facts 
known. The process is apparently analogous to recovery from fatigue. 
FIG. 694. Diagram: for explanation, see text. 
